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000-633 exam Dumps Source : Object Oriented Analysis and Design - portion 1

Test Code : 000-633
Test designation : Object Oriented Analysis and Design - portion 1
Vendor designation : IBM
: 105 true Questions

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IBM expostulate Oriented Analysis and

Object-Oriented evaluation and Design | killexams.com true Questions and Pass4sure dumps

This chapter is from the booklet 

evaluation emphasizes an investigation of the difficulty and necessities, in preference to an answer. as an instance, if a fresh online trading apparatus is desired, how will it exist used? What are its services?

"analysis" is a great time period, surest qualified, as in requirements evaluation (an investigation of the requirements) or object-oriented evaluation (an investigation of the domain objects).

Design emphasizes a conceptual solution (in software and hardware) that fulfills the requirements, rather than its implementation. for example, a description of a database schema and application objects. Design concepts regularly exclude low-degree or "evident" particulars—glaring to the intended consumers. eventually, designs can exist implemented, and the implementation (akin to code) expresses the proper and complete realized design.

As with analysis, the term is optimal qualified, as in object-oriented design or database design.

positive analysis and design had been summarized within the phrase enact the arrogate aspect (evaluation), and enact the factor redress (design).

Python GUI Programming initiatives using Tkinter and Python three - Simpliv | killexams.com true Questions and Pass4sure dumps

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Object-Oriented evaluation And Design — Introduction (half 1) | killexams.com true Questions and Pass4sure dumps

The conception Of Object-Orientation

Object-orientation is what’s called a programming paradigm. It’s now not a language itself however a group of ideas it's supported by many languages.

in case you aren’t prevalent with the ideas of object-orientation, you may assume a glance on the legend of Object-Oriented Programming.

If every thing they enact in these languages is object-oriented, it capacity, we're oriented or focused around objects.

Now in an object-oriented language, this one gigantic application will in its location exist shatter up aside into self contained objects, basically fancy having a few mini-classes, every expostulate representing a different a portion of the software.

and every expostulate consists of its personal information and its own logic, and that they communicate between themselves.

These objects aren’t random. They signify the pass you talk and reflect concerning the difficulty you are trying to remedy for your true life.

They signify issues fancy personnel, images, fiscal institution bills, spaceships, asteroids, video phase, audio files, or whatever thing exists for your application.

Object-Oriented analysis And Design (OOAD)

It’s a structured system for analyzing, designing a gadget with the aid of applying the article-orientated ideas, and strengthen a group of graphical gadget fashions during the construction life cycle of the application.

OOAD within the SDLC

The application actuality cycle is usually divided up into degrees going from summary descriptions of the problem to designs then to code and testing and finally to deployment.

The earliest ranges of this process are analysis (requirements) and design.

The incompatibility between analysis and design is often described as “what Vs how”.

In evaluation developers toil with users and district experts to define what the gadget is meant to do. Implementation particulars are purported to exist in general or completely left out at this phase.

The expostulate of the analysis portion is to create a mannequin of the device regardless of constraints reminiscent of applicable technology. this is usually completed via utilize situations and abstract definition of the most captious objects using conceptual model.

The design side refines the evaluation mannequin and applies the necessary technology and other implementation constrains.

It makes a speciality of describing the objects, their attributes, behavior, and interactions. The design model should Have the entire particulars required so that programmers can reserve in coerce the design in code.

They’re most useful performed in an iterative and incremental software methodologies. So, the activities of OOAD and the developed models aren’t performed as soon as, they can revisit and refine these steps at utter times.

Object-Oriented analysis

within the object-oriented analysis, we …

  • Elicit requirements: define what does the software necessity to do, and what’s the difficulty the utility trying to solve.
  • Specify necessities: recount the necessities, usually, the usage of utilize cases (and scenarios) or consumer studies.
  • Conceptual model: determine the essential objects, refine them, and contour their relationships and habits and draw them in a simple diagram.
  • We’re not going to cover the first two activities, just the ultimate one. These are already explained in factor in requirements Engineering.

    Object-Oriented Design

    The analysis section identifies the objects, their relationship, and deportment using the conceptual model (an abstract definition for the objects).

    whereas in design section, they recount these objects (by creating classification diagram from conceptual diagram — continually mapping conceptual model to ilk diagram), their attributes, habits, and interactions.

    besides making utilize of the utility design ideas and patterns which can exist coated in later tutorials.

    The input for object-oriented design is supplied with the aid of the output of object-oriented evaluation. but, analysis and design may additionally ensue in parallel, and the results of 1 undertaking may moreover exist used by means of the different.

    within the object-oriented design, we …

  • Describe the courses and their relationships the usage of classification diagram.
  • Describe the interplay between the objects the utilize of sequence diagram.
  • observe utility design concepts and design patterns.
  • a class diagram gives a visible illustration of the courses you want. And privilege here is where you gain to exist in fact positive about object-oriented ideas fancy inheritance and polymorphism.

    Describing the interactions between those objects allows you to superior retain in intuition the obligations of the distinctive objects, the behaviors they necessity to have.

    — other diagrams

    there are many different diagrams they will utilize to model the gadget from distinctive views; interactions between objects, constitution of the gadget, or the habits of the system and how it responds to routine.

    It’s at utter times about determining the privilege diagram for the redress need. effect positive to understand which diagrams could exist advantageous when brooding about or discussing a condition that isn’t clear.

    system modeling and the diverse models they are able to utilize may exist mentioned next.

    equipment Modeling

    device modeling is the routine of developing models of the system, with every mannequin representing a different perspectives of that device.

    probably the most captious aspect a couple of device mannequin is that it leaves out aspect; It’s an summary representation of the equipment.

    The models are continually according to graphical notation, which is almost always in keeping with the notations within the Unified Modeling Language (UML). other models of the system fancy mathematical mannequin; a detailed apparatus description.

    fashions are used during the analysis system to wait on to elicit the necessities, privilege through the design process to recount the apparatus to engineers, and after implementation to doc the gadget structure and operation.

    distinctive perspectives

    We may additionally multiply a model to characterize the device from distinctive perspectives.

  • external, the location you model the context or the ambiance of the system.
  • interaction, the location you mannequin the interplay between components of a system, or between a apparatus and other programs.
  • Structural, the location you mannequin the firm of the equipment, or the structure of the information being processed by using the device.
  • Behavioral, the location you model the dynamic deportment of the system and how it respond to activities.
  • Unified Modeling Language (UML)

    The unified modeling language rotate into the ordinary modeling language for object-oriented modeling. It has many diagrams, besides the fact that children, probably the most diagrams which are ordinary are:

  • Use case diagram: It indicates the interplay between a apparatus and it’s ambiance (users or methods) inside a specific condition.
  • type diagram: It indicates the distinct objects, their relationship, their behaviors, and attributes.
  • Sequence diagram: It suggests the interactions between the different objects in the equipment, and between actors and the objects in a equipment.
  • State machine diagram: It suggests how the device reply to exterior and inside movements.
  • pastime diagram: It indicates the flood of the facts between the tactics in the equipment.
  • that you would exist able to enact diagramming toil on paper or on a whiteboard, as a minimum within the initial tiers of a mission. however there are some diagramming tools on the pass to aid you to attract these UML diagrams.

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    Object Oriented Analysis and Design - portion 1

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    Object-Oriented Analysis and Design (Part 1) | killexams.com true questions and Pass4sure dumps

    Who does this thing? Does it Have any benefit? If I enact this, will my boss reflect that I am wasting my time or making excuses to not work? Have these thoughts ever near to your intuition when you were desperate to properly design your next software?

    It is moreover possible that you Have tried designing some piece of software before, but you establish that it was too just time-consuming and it had no benefits. But throughout your career, you might Have had these recurring thoughts that you should learn more about design patterns, mastering MVC, and designing something reusable, modular, and light to read.

    In this multi-part article series, I will cover the basics about how you can properly design your next software even if you Have failed ultimate time.

    What Will You Learn?
  • Why your ultimate design attempt failed
  • How to manipulate your manager/boss when you wanted to design
  • How to succeed in designing
  • The software evolution process
  • What is object-oriented analysis?
  • What is object-oriented design?
  • What are design patterns?
  • And anything in between that is confusing you
  • What Will You Not Learn?
  • You will not learn the syntax of Java, C#, or C++
  • You will not learn the incompatibility between functions and variables
  • You will not exist overwhelmed with a list of design patterns
  • You will not learn object-oriented programming here
  • "What?" you might convey after reading the ultimate line. "No expostulate oriented programming? Then why am I wasting my time here?" This post is about object-oriented design, but not programming. They utter know about object-oriented programming, i.e., how to write a class in C#.

    As one quote says, “Knowing how to hold a hammer does not effect you an architect.” True? Similarly, learning Java programming will not effect you a advantageous software engineer (or software programmer or developer or software architect).


    During the initial years of my undergraduate programs, I thought designing was the very as writing an algorithm because I did not study object-oriented programming. Later, when I learned about object-oriented programming, I thought someone could conquer the world if they just learned everything that is there in 1,000 pages of a Deitel and Deitel book.

    object oriented programming book

    But that was not the case. I could not write a program without tearing my hair apart. I moreover noticed that if I opened my program again after six months, it looked fancy such a mystery that even Sherlock Holmes could not decipher it.

    Then, in my fourth semester, I learned about object-oriented analysis and design as a subject. But unfortunately, the focus was on UML modeling. I thought that UML was a icy thing — you just generate some diagrams and hand them over to developers and they will near up with code using your designs (which will effect you proud).

    But there was even an option in the UML modeling appliance that their class was using at that time to automatically generate the code from your UML class diagrams. What a beauty, I thought. I could design using UML models and then generate the code, compile it, ship it to a customer, and gain rich fancy Bill Gates. Awesome.


    Afterward, reality set in. I was never able to generate designs that were modular, light to extend, and light to understand (The code generated from these tools was never compiled, since it only generated stubs). Then, a epoch of chaos began.

    Later in my undergraduate study, I learned subjects related to software engineering, software architecture, software process models and software project management. But I was unable to meet utter things together until very late.

    Still, I espy people struggling with these concepts, unable to meet things together. They are overwhelmed with the unstructured data available to them. One key to comprehending utter this information is to involve yourself in a project. The only output for that project should exist a software that your users can use.

    In this post, I will share some basic object-oriented analysis and design principles, practices, and some of my experiences that you can utilize in your next project.

    Introduction to Software evolution Process Models

    We utter utilize some process or steps to develop software. The simplest process model that I utilize is just writing 6 lines on the back of a piece of paper and convoke them feature list. Then, I open Visual Studio and start writing code. That's it. It's a process model I used during my college years.

    I wrote my first commercial software (which had 1 user, who abandoned it later) using Visual Basic 6.0 in my second year of college using this process model.

    There are many software evolution process models that I Have studied and applied throughout many projects.

    One process model (which is scolded by many authorities) is waterfall, which uses the process of gathering requirements, analysis, design, implementation, and testing.

    The problem with the waterfall process model is that you enact utter the things in the very exact sequence as written above. First, utter the requirements are collected from the customers. A team analyzes requirements, then documents and prepares specifications for the design team. The design team then develops the design using the specification and hands over the design to the implementation team. The implementation team writes code with respect to the design. Finally, test team tests the software against the specifications.

    software process model

    Everything is done sequentially, and a lot of time is spent (months and even years) before the final product is shipped to the customer. Statistics Tell us that when a product is shipped to the customer using waterfall process models, a huge number of customers rejected it = because it did not meet their requirements.

    You may Have heard the phrase, “The customer is always right.” This truly applies to software development. If the customer does not fancy the final product, then utter the exertion (months and years) is wasted.

    To cater to this problem, there is another philosophy — iterative and evolutionary development. Based on this philosophy, there are many software evolution process models. Some examples are Scrum, extreme programming(XP), and Rational Unified Process. They are the Agile evolution processes.

    The concept of iterative evolution is simple. Software evolution is organized into a progression of petite projects called iterations. Each iteration has its analysis, design, implementation, and testing. At the quit of each iteration, the customer input is taken. If a customer did not agree, then the loss is minimal (usually weeks) as compared to waterfall process model.

    Now you understand the basic incompatibility between iterative and sequential process models. Many organizations now utilize iterative evolution process models, as the view is to minimize consume (months vs. weeks).

    Why I necessity to Understand Process Models

    For a long time, I believed that designing software was something fancy that: I design everything in the rise and then, using this design, start coding. Then, once it compiles, I handed over the running software to the quit user.

    It turns out that this is not the best approach. You will Have to change your design strategy, which evolves over time. Therefore, the incremental and evolutionary process model is essential to understand. Flawless design is a myth. After subsequent iterations, one may realize that his or her initial design sucks.

    Another point is that one should not design for utter the requirements at the beginning. effect a detailed design map for the iteration you're currently working on.

    Therefore, the key takeaway is that you should utilize an iterative evolution process where the complete design is not done at the start of the project. Similarly, whatever you design will not perfect and will exist changed or evolved during the lifecycle of the project.

    This ends the first article of this four-part series. In this article, I discussed the importance of process models in object-oriented design. I moreover mentioned the common misconception attached to UML.

    In portion 2, you will learn the following

  • Difference between process and methodology

  • 2 most essential object-oriented design principles that everyone should know

  • 1 advantage of OOP that every developer would admire to Have in his or her code

  • To learn more about expostulate oriented programming visit here.

    Object-oriented design patterns in the kernel, portion 1 | killexams.com true questions and Pass4sure dumps

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    June 1, 2011

    This article was contributed by Neil Brown

    Despite the fact that the Linux Kernel is mostly written in C, it makes broad utilize of some techniques from the domain of object-oriented programming. Developers wanting to utilize these object-oriented techniques receive miniature support or guidance from the language and so are left to fend for themselves. As is often the case, this is a double-edged sword. The developer has enough flexibility to enact really icy things, and equally the flexibility to enact really dull things, and it isn't always lucid at first glance which is which, or more accurately: where on the spectrum a particular approach sits.

    Instead of looking to the language to provide guidance, a software engineer must watch to established exercise to find out what works well and what is best avoided. Interpreting established exercise is not always as light as one might fancy and the effort, once made, is worth preserving. To preserve that exertion on your author's part, this article brings another installment in an occasional progression on Linux Kernel Design Patterns and attempts to set out - with examples - the design patterns in the Linux Kernel which outcome an object-oriented style of programming.

    Rather than providing a brief introduction to the object-oriented style, tempting though that is, they will assume the reader has a basic lore of objects, classes, methods, inheritance, and similar terms. For those as yet unfamiliar with these, there are plenty of resources to exist establish elsewhere on the web.

    Over two weeks they will watch for patterns in just two areas: routine dispatch and data inheritance. Despite their unpretentious simplicity they lead to some rich veins for investigation. This first article will focus on routine dispatch.

    Method Dispatch

    The great variety of styles of inheritance and rules for its usage in languages today seems to intimate that there is no uniform understanding of what "object-oriented" really means. The term is a bit fancy "love": everyone thinks they know what it means but when you gain down to details people can find they Have very different ideas. While what it means to exist "oriented" might not exist clear, what they connote by an "object" does seem to exist uniformly agreed upon. It is simply an abstraction comprising both situation and behavior. An expostulate is fancy a record (Pascal) or struct (C), except that some of the names of members refer to functions which act on the other fields in the object. These duty members are sometimes referred to a "methods".

    The most obvious pass to implement objects in C is to declare a "struct" where some fields are pointers to functions which assume a pointer to the struct itself as their first argument. The calling convention for routine "foo" in expostulate "bar" would simply be: bar->foo(bar, ...args); While this pattern is used in the Linux kernel it is not the preeminent pattern so they will leave discussion of it until a miniature later.

    As methods (unlike state) are not normally changed on a per-object basis, a more common and only slightly less obvious approach is to collect utter the methods for a particular class of objects into a sunder structure, sometimes known as a "virtual duty table" or vtable. The expostulate then has a single pointer to this table rather than a sunder pointer for each method, and consequently uses less memory.

    This then leads to their first pattern - a simple vtable being a structure which contains only duty pointers where the first dispute of each is a pointer to some other structure (the expostulate type) which itself contains a pointer to this vtable. Some simple examples of this in the Linux kernel are the file_lock_operations structure which contains two duty pointers each of which assume a pointer to a struct file_lock, and the seq_operations vtable which contains four duty pointers which each operate on a struct seq_file. These two examples display an obvious naming pattern - the structure holding a vtable is named for the structure holding the expostulate (possibly abbreviated) followed by "_operations". While this pattern is common it is by no means universal. Around the time of 2.6.39 there are approximately 30 "*_operations" structures along with well over 100 "*_ops" structures, most if not utter of which are vtables of some sort. There are moreover several structs such as struct mdk_personality which are essentially vtables but enact not Have particularly helpful names.

    Among these nearly 200 vtable structures there is plenty of variability and so plenty of scope to watch for spicy patterns. In particular they can watch for common variations from the "pure vtable" pattern described above and determine how these variations contribute to their understanding of expostulate utilize in Linux.

    NULL duty pointers

    The first observation is that some duty pointers in some vtables are allowed to exist NULL. Clearly trying to convoke such a duty would exist futile, so the code that calls into these methods generally contains an explicit test for the pointer being NULL. There are a few different reasons for these NULL pointers. Probably easiest to justify is the incremental evolution reason. Because of the pass vtable structures are initialized, adding a fresh duty pointer to the structure definition causes utter existing table declarations to initialise that pointer to NULL. Thus it is possible to add a caller of the fresh routine before any instance supports that method, and Have it check for NULL and effect a default behavior. Then as incremental evolution continues those vtable instances which necessity it can gain non-default methods.

    A recent specimen is confide 77af1b2641faf4 adding set_voltage_time_sel() to struct regulator_ops which acts on struct regulator_dev. Subsequent confide 42ab616afe8844 defines that routine for a particular device. This is simply the most recent specimen of a very common theme.

    Another common intuition is that positive methods are not particularly meaningful in positive cases so the calling code simply tests for NULL and returns an arrogate mistake when found. There are multiple examples of this in the virtual filesystem (VFS) layer. For instance, the create() duty in inode_operations is only meaningful if the inode in question is a directory. So inode_operations structures for non-directories typically Have NULL for the create() duty (and many others) and the calling code in vfs_create() checks for NULL and returns -EACCES.

    A final intuition that vtables sometimes accommodate NULL is that an factor of functionality might exist being transitioned from one interface to another. A advantageous specimen of this is the ioctl() operation in file_operations. In 2.6.11, a fresh method, unlocked_ioctl() was added which was called without the mammoth kernel lock held. In 2.6.36, when utter drivers and filesystems had been converted to utilize unlocked_ioctl(), the original ioctl() was finally removed. During this transition a file system would typically define only one of two, leaving the other defaulting to NULL.

    A slightly more subtle specimen of this is read() and aio_read(), moreover in file_operations, and the corresponding write() and aio_write(). aio_read() was introduced to support asynchronous IO, and if it is provided the regular synchronous read() is not needed (it is effected using do_sync_read() which calls the aio_read() method). In this case there appears to exist no expostulate of ever removing read() - it will remain for cases where async IO is not material such as special filesystems fancy procfs and sysfs. So it is quiet the case that only one of each pair necessity exist defined by a filesystem, but it is not simply a transition, it is a long-term state.

    Though there seem to exist several different reasons for a NULL duty pointer, almost every case is an specimen of one simple pattern - that of providing a default implementation for the method. In the "incremental development" examples and the non-meaningful routine case, this is fairly straightforward. e.g. the default for inode->create() is simply to recrudesce an error. In the interface transition case it is only slightly less obvious. The default for unlocked_ioctl() would exist to assume the kernel lock and then convoke the ioctl() method. The default for read() is exactly do_sync_read() and some filesystems such as ext3 actually provide this value explicitly rather than using "NULL" to argue a default.

    With that in mind, a miniature reflection suggests that if the true goal is to provide a default, then maybe the best approach would exist to explicitly give a default rather than using the circuitous route of using a default of NULL and interpreting it specially.

    While NULL is certainly the easiest value to provide as a default - as the C standard assures us that uninitialized members of a structure enact gain set to NULL - it is not very much harder to set a more meaningful default. I am indebted to LWN reader wahern for the observation that C99 allows fields in a structure to exist initialized multiple times with only the final value taking outcome and that this allows light setting of default values such as by following the simple model:

    #define FOO_DEFAULTS .bar = default_bar, .baz = default_baz struct foo_operations my_foo = { FOO_DEFAULTS, .bar = my_bar, };

    This will declare my_foo with a predefined default value for baz and a localized value for bar. Thus for the petite cost of defining a few "default" functions and including a "_DEFAULTS" entry to each declaration, the default value for any domain can easily exist chosen when the domain is first created, and automatically included in every utilize of the structure.

    Not only are meaningful defaults light to implement, they can lead to a more efficient implementation. In those cases where the duty pointer actually is NULL it is probably faster to test and arm rather than to effect an circuitous duty call. However the NULL case is very often the exception rather than the rule, and optimizing for an exception is not household practice. In the more common case when the duty pointer is not NULL, the test for NULL is simply a consume of code space and a consume of execution time. If they disallow NULLs they can effect utter convoke sites a miniature bit smaller and simpler.

    In general, any testing performed by the caller before calling a routine can exist seen as an instance of the "mid-layer mistake" discussed in a previous article. It shows that the mid-layer is making assumptions about the deportment of the lower flush driver rather than simply giving the driver license to behave in whatever pass is most suitable. This may not always exist an expensive mistake, but it is quiet best avoided where possible. Nevertheless there is a lucid pattern in the Linux kernel that pointers in vtables can sometimes exist NULLable, typically though not always to enable a transition, and the convoke sites should in these cases test for NULL before proceeding with the call.

    The observant reader will Have noticed a hollow in the above logic denouncing the utilize NULL pointers for defaults. In the case where the default is the common case and where performance is paramount, the reasoning does not hold and a NULL pointer could well exist justified. Naturally the Linux kernel provides an specimen of such a case for their examination.

    One of the data structures used by the VFS for caching filesystem information is the "dentry". A "dentry" represents a designation in the filesystem, and so each "dentry" has a parent, being the directory containing it, and an "inode" representing the named file. The dentry is sunder from the inode because a single file can Have multiple names (so an "inode" can Have multiple "dentry"s). There is a dentry_operations vtable with a number of operations including, for example, "d_compare" which will compare two names and "d_hash" which will generate a hash for the designation to guide the storage of the "dentry" in a hash table. Most filesystems enact not necessity this flexibility. They handle names as uninterpreted strings of bytes so the default compare and hash functions are the common case. A few filesystems define these to manipulate case-insensitive names but that is not the norm.

    Further, filename lookup is a common operation in Linux and so optimizing it is a priority. Thus these two operations appear to exist advantageous candidates where a test for NULL and an inlined default operation might exist appropriate. What they find though is that when such an optimization is warranted it is not by itself enough. The code that calls d_compare() and d_hash() (and a couple of other dentry operations) does not test these functions for NULL directly. Rather they require that a few flag bits (DCACHE_OP_HASH, DCACHE_OP_COMPARE) in the "dentry" are set up to argue whether the common default should exist used, or whether the duty should exist called. As the flag domain is likely to exist in cache anyway, and the dentry_operations structure will often exist not needed at all, this avoids a remembrance fetch in a peppery path.

    So they find that the one case where using a NULL duty pointer to argue a default could exist justified, it is not actually used; instead, a different, more efficient, mechanism is used to argue that the default routine is requested.

    Members other than duty pointers

    While most vtable-like structures in the kernel accommodate exclusively duty pointers, there are a significant minority that Have non-function-pointer fields. Many of these appear on the surface quite capricious and a few closer inspections intimate that some of them result of indigent design or bit-rot and their removal would only better the code.

    There is one exception to the "functions only" pattern that occurs repeatedly and provides true value, and so is worth exploring. This pattern is seen in its most general shape in struct mdk_personality which provides operations for a particular software RAID level. In particular this structure contains an "owner", a "name", and a "list". The "owner" is the module that provides the implementation. The "name" is a simple identifier: some vtables Have string names, some Have numeric names, and it is often called something different fancy "version", "family", "drvname", or "level". But conceptually it is quiet a name. In the present specimen there are two names, a string and a numeric "level".

    The "list", while portion of the very functionality, is less common. The mdk_personality structure has a struct list_head, as does struct ts_ops. struct file_system_type has a simple pointer to the next struct file_system_type. The underlying view here is that for any particular implementation of an interface (or "final" definition of a class) to exist usable, it must exist registered in some pass so that it can exist found. Further, once it has been establish it must exist possible to ensure that the module holding the implementation is not removed while it is in use.

    There seem to exist nearly as many styles of registration against an interface in Linux as there are interfaces to register against, so finding tenacious patterns there would exist a difficult task. However it is fairly common for a "vtable" to exist treated as the primary manipulate on a particular implementation of an interface and to Have an "owner" pointer which can exist used to gain a reference on the module which provides the implementation.

    So the pattern they find here is that a structure of duty pointers used as a "vtable" for expostulate routine dispatch should normally accommodate only duty pointers. Exceptions require lucid justification. A common exception allows a module pointer and possible other fields such as a designation and a list pointer. These fields are used to support the registration protocol for the particular interface. When there is no list pointer it is very likely that the entire vtable will exist treated as read-only. In this case the vtable will often exist declared as a const structure and so could even exist stored in read-only memory.

    Combining Methods for different objects

    A final common deviation from the "pure vtable" pattern that they espy in the Linux kernel occurs when the first dispute to the duty is not always the very expostulate type. In a simple vtable which is referenced by a pointer in a particular data structure, the first dispute of each duty is exactly that data structure. What intuition could there exist for deviating from that pattern? It turns out that there are few, some more spicy than others.

    The simplest and least spicy explanation is that, for no unpretentious reason, the target data structure is listed elsewhere in the dispute list. For specimen utter functions in struct fb_ops assume a struct fb_info. While in 18 cases that structure is the first argument, in five cases it is the last. There is nothing obviously wrong with this preference and it is unlikely to addle developers. It is only a problem for data miners fancy your author who necessity to filter it out as an immaterial pattern.

    A slight deviation on this pattern is seen in struct rfkill_ops where two functions assume a struct rkfill but the third - set_block() - takes a void *data. Further investigation shows that this opaque data is exactly that which is stored in rfkill->data, so set_block() could easily exist defined to assume a struct rfkill and simply to follow the ->data link itself. This deviation is sufficiently non-obvious that it could conceivably addle developers as well as data miners and so should exist avoided.

    The next deviation in seen for specimen in platform_suspend_ops, oprofile_operations, security_operations and a few others. These assume an odd assortment of arguments with no obvious pattern. However these are really very different sorts of vtable structures in that the expostulate they belong to are singletons. There is only one active platform, only one profiler, only one security policy. Thus the "object" on which these operations act is portion of the global situation and so does not necessity to exist included in the arguments of any functions.

    Having filtered these two patterns out as not being very spicy they are left with two that enact serve to Tell us something about expostulate utilize in the kernel.

    quota_format_ops and export_operations are two different operations structures that operate on a variety of different data structures. In each case the unpretentious primary expostulate (e.g. a struct super_block or a struct dentry) already has a vtable structure dedicated to it (such as super_operations or dentry_operations) and these fresh structures add fresh operations. In each case the fresh operations shape a cohesive unit providing a related set of functionality - whether supporting disk quotas or NFS export. They don't utter act on the very expostulate simply because the functionality in question depends on a variety of objects.

    The best term from the language of object-oriented programming for this is probably the "mixin". Though the meet may not exist perfect - depending on what your exact understanding of mixin is - the view of bringing in a collection of functionality without using strict hierarchical inheritance is very close to the purpose of quota_format_ops and export_operations.

    Once they know to exist on the lookout for mixins fancy these they can find quite a few more examples. The pattern to exist alert for is not the one that led us here - an operations structure that operates on a variety of different objects - but rather the one they establish where the functions in an "operations" structure operate on objects that already Have their own "operations" structure. When an expostulate has a great number of operations that are material and these operations naturally group into subsets, it makes a lot of sense to divide them into sunder vtable-like structures. There are several examples of this in the networking code where for instance both tcp_congestion_ops and inet_connection_sock_af_ops operate (primarily) on a struct sock, which itself has already got a petite set of dedicated operations.

    So the pattern of a "mixin" - at least as defined as a set of operations which apply to one or more objects without being the primary operations for those objects - is a pattern that is often establish in the kernel and appears to exist quite valuable in allowing better modularization of code.

    The ultimate pattern which explains non-uniform duty targets is probably the most interesting, particularly in its contrast to the obvious application of object-oriented programming style. Examples of this pattern abound with ata_port_operations, tty_operations, nfs_rpc_ops and atmdev_ops utter appearing as useful examples. However they will focus primarily on some examples from the filesystem layer, particularly super_operations and inode_operations.

    There is a tenacious hierarchy of objects in the implementation of a filesystem where the filesystem - represented by a "super_block" - has a number of files (struct inode) which may Have a number of names or links (struct dentry). Further each file might store data in the page cache (struct address_space) which comprises a number of individual pages (struct page). There is a sense in which utter of these different objects belong to the filesystem as a whole. If a page needs to exist loaded with data from a file, the filesystem knows how to enact that, and it is probably the very mechanism for every page in every file. Where it isn't always the same, the filesystem knows that too. So they could conceivably store every operation on every one of these objects in the struct super_block, as it represents the filesystem and could know what to enact in each case.

    In exercise that extreme is not really helpful. It is quite likely that while there are similarities between the storage of a regular file and a directory, there are moreover essential differences and being able to encode those differences in sunder vtables can exist helpful. Sometimes petite symbolic links are stored directly in the inode while larger links are stored fancy the contents of a regular file. Having different readlink() operations for the two cases can effect the code a lot more readable.

    While the extreme of every operation attached to the one central structure is not ideal, it is equally righteous that the contradictory extreme is not model either. The struct page in Linux does not Have a vtable pointer at utter - in portion because they want to retain the structure as petite as possible because it is so populous. Rather the address_space_operations structure contains the operations that act on a page. Similarly the super_operations structure contains some operations that apply to inodes, and inode_operations contains some operations that apply to dentries.

    It is clearly possible to Have operations structures attached to a parent of the target expostulate - providing the target holds a reference to the parent, which it normally does - though it is not quite so lucid that it is always beneficial. In the case of struct page which avoids having a vtable pointer altogether the capitalize is clear. In the case of struct inode which has its own vtable pointer, the capitalize of having some operations (such as destroy_inode() or write_inode()) attached to the super_block is less clear.

    As there are several vtable structures where any given duty pointer could exist stored, the actual preference is in many cases miniature more than historical accident. Certainly the proliferation of struct dentry operations in inode_operations seems to exist largely due to the fact that some of them used to act directly on the inode, but changes in the VFS eventually required this to change. For specimen in 2.1.78-pre1, each of link(), readlink(), followlink() (and some others which are now defunct) were changed from taking a struct inode to assume a struct dentry instead. This set the scene for "dentry" operations to exist in inode_operations, so when setattr and getattr were added for 2.3.48, it probably seemed completely natural to comprehend them in inode_operations despite the fact that they acted primarily on a dentry.

    Possibly they could simplify things by getting rid of dentry_operations altogether. Some operations that act on dentries are already in inode_operations and super_operations - why not journey them utter there? While dentries are not as populous as struct page there are quiet a lot of them and removing the "d_op" domain could deliver 5% of the remembrance used by that structure (on x86-64).

    With two exceptions, every active filesystem only has a single dentry operations structure in effect. Some filesystem implementations fancy "vfat" define two - e.g. one with case-sensitive matching and one with case-insensitive matching - but there is only one active per super-block. So it would seem that the operations in dentry_operations could exist moved to super_operations, or at least accessed through "s_d_op". The two exceptions are ceph and procfs. These filesystems utilize different d_revalidate() operations in different parts of the filesystem and - in the case of procfs - different d_release() operations. The necessary distinctions could easily exist made in per-superblock versions of these operations. enact these cases justify the 5% space cost? Arguably not.

    Directly embedded duty pointers

    Finally it is arrogate to reflect on the alternate pattern mentioned at the start, where duty pointers are stored directly in the expostulate rather than in a sunder vtable structure. This pattern can exist seen in struct request_queue which has nine duty pointers, struct efi which has ten duty pointers, and struct sock which has six duty pointers.

    The cost of embedded pointers is obviously space. When vtables are used, there is only one copy of the vtable and multiple copies of an expostulate (in most cases) so if more than one duty pointer is needed, a vtable would deliver space. The cost of a vtable is an extra remembrance reference, though cache might reduce much of this cost in some cases. A vtable moreover has a cost of flexibility. When each expostulate needs exactly the very set of operations a vtable is good, but if there is a necessity to individually tailor some of the operations for each object, then embedded duty pointer can provide that flexibility. This is illustrated quite nicely by the comment with "zoom_video" in struct pcmcia_socket

    /* Zoom video behaviour is so chip specific its not worth adding this to _ops */

    So where objects are not very populous, where the list of duty pointers is small, and where multiple mixins are needed, embedded duty pointers are used instead of a sunder vtable.

    Method Dispatch Summary

    If they combine utter the pattern elements that they Have establish in Linux they find that:

    Method pointers that operate on a particular ilk of expostulate are normally collected in a vtable associated directly with that object, though they can moreover appear:

  • In a mixin vtable that collects related functionality which may exist selectable independently of the foundation ilk of the object.
  • In the vtable for a "parent" expostulate when doing so avoids the necessity for a vtable pointer in a populous object
  • Directly in the expostulate when there are few routine pointers, or they necessity to exist individually tailored to the particular object.
  • These vtables rarely accommodate anything other than duty pointers, though fields needed to register the expostulate class can exist appropriate. Allowing these duty pointers to exist NULL is a common but not necessarily model technique for handling defaults.

    So in exploring the Linux Kernel code they Have establish that even though it is not written in an object-oriented language, it certainly contains objects, classes (represented as vtables), and even mixins. It moreover contains concepts not normally establish in object-oriented languages such as delegating expostulate methods to a "parent" object.

    Hopefully understanding these different patterns and the reasons for choosing between them can lead to more uniform application of the patterns across the kernel, and hence effect it easier for a newcomer to understand which pattern is being followed. In the second portion of their examination of expostulate oriented patterns they will explore the various ways that data inheritance is achieved in the Linux kernel and contend the strengths and weaknesses of each approach so as to espy where each is most appropriate.

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    Java and Object-Oriented Programming | killexams.com true questions and Pass4sure dumps

    This chapter is from the book 

    Many seasoned Java developers will scoff at the fact that this section even exists in this book. It is here for two very essential reasons. The first is that I continually sprint across Java applications built with a procedural mind-set. The fact that you know Java doesn't connote that you Have the ability to transform that lore into well-designed object-oriented systems. As both an instructor and consultant, I espy many data-processing shops send COBOL and/or Visual Basic developers to a three-day class on UML and a five-day class on Java and await miracles. Case in point: I was recently asked to review a Java application to assess its design architecture and establish that it had only two classes—SystemController and ScreenController—which contained over 70,000 lines of Java code.

    The second intuition for the accent on how the language maps to object-oriented principles is that people fancy language comparisons and how they stack up to their counterparts. To appease those that live and die by language comparisons, let's reserve Java under the scrutiny of what constitutes an object-oriented language.

    No definitive definition of what makes a language object-oriented is globally accepted. However, a common set of criteria I personally find useful is that the language must support the following:

  • Classes
  • Complex types (Java reference types)
  • Message passing
  • Encapsulation
  • Inheritance
  • Polymorphism
  • These are discussed in the next subsections.

    Java and Classes

    Java allows classes to exist defined. There are no stray functions floating around in Java. A class is a static template that contains the defined structure (attributes) and deportment (operations) of a real-world entity in the application domain. At runtime, the class is instantiated, or brought to life, as an expostulate born in the image of that class. In my seminars, when several folks fresh to the expostulate world are in attendance, I often utilize the analogy of a cookie cutter. The cookie cutter is merely the template used to stamp out what will become individually decorated and unique cookies. The cookie cutter is the class; the unique blue, green, and yellow gingerbread man is the expostulate (which I confidence supports a bite operation).

    Java exposes the class to potential outside users through its public interface. A public interface consists of the signatures of the public operations supported by the class. A signature is the operation designation and its input parameter types (the recrudesce type, if any, is not portion of the operation's signature).

    Good programming exercise encourages developers to declare utter attributes as private and allow access to them only via operations. As with most other languages, however, this is not enforced in Java. motif 2-1 outlines the concept of a class and its interface.

    FIGURE 2-1 Public interface of a class

    The motif uses a common eggshell metaphor to recount the concept of the class's interface, as well as encapsulation. The internal details of the class are hidden from the outside via a well-defined interface. In this case, only four operations are exposed in the classes interface (Operation_A, B, C, and D). The other attributes and operations are protected from the outside world. Actually, to the outside world, it's as if they don't even exist.

    Suppose you want to create an Order class in Java that has three attributes—orderNumber, orderDate, and orderTotal—and two operations—calcTotalValue() and getInfo(). The class definition could watch fancy this:

    /** * Listing 1 * This is the Order class for the Java/UML book */ package com.jacksonreed; import java.util.*; public class Order { private Date orderDate; private long orderNumber; private long orderTotal; public Order() { } public boolean getInfo() { recrudesce true; } public long calcTotalValue() { recrudesce 0; } public Date getOrderDate() { recrudesce orderDate; } public void setOrderDate(Date aOrderDate) { orderDate = aOrderDate; } public long getOrderNumber() { recrudesce orderNumber; } public void setOrderNumber(long aOrderNumber) { orderNumber = aOrderNumber; } public long getOrderTotal() { recrudesce orderTotal; } public void setOrderTotal(long aOrderTotal) { orderTotal = aOrderTotal; } public static void main(String[] args) { Order order = fresh Order(); System.out.println("instantiated Order"); System.out.println(order.getClass().getName()); System.out.println(order.calcTotalValue()); try { Thread.currentThread().sleep(5*1000); } catch(InterruptedException e) {} } }

    A few things are notable about the first bit of Java code presented in this book. Notice that each of the three attributes has a gain and a set operation to allow for the retrieval and setting of the Order object's properties. Although doing so is not required, it is common exercise to provide these accessor-type operations for utter attributes defined in a class. In addition, if the Order class ever wanted to exist a JavaBean, it would Have to Have "getters and setters" defined in this way.

    Some of the routine code in the main() operation does a few things of note. Of interest is that a try shroud exists at the quit of the operation that puts the current thread to sleep for a bit. This is to allow the console display to freeze so that you can espy the results.

    If you ilk in this class and then compile it and execute it in your favorite evolution appliance or from the command prompt with

    javac order.java //* to compile it java order //* to sprint it

    you should gain results that watch fancy this:

    instantiated Order com.jacksonreed.Order 0


    Going forward, I pledge you will espy no code samples with class, operation, or assign names of foo, bar, or foobar.

    More on Java and Classes

    A class can moreover Have what are called class-level operations and attributes. Java supports these with the static keyword. This keyword would journey privilege after the visibility (public, private, protected) component of the operation or attribute. Static operations and attributes are needed to invoke either a service of the class before any true instances of that class are instantiated or a service that doesn't directly apply to any of the instances. The classic specimen of a static operation is the Java constructor. The constructor is what is called when an expostulate is created with the fresh keyword. Perhaps a more business-focused specimen is an operation that retrieves a list of Customer instances based on particular search criteria.

    A class-level assign can exist used to store information that utter instances of that class may access. This assign might be, for example, a import of the number of objects currently instantiated or a property about Customer that utter instances might necessity to reference.

    Java and involved Types (Java Reference Types)

    A involved type, which in Java is called a reference type, allows variables typed as something other than primitive types (e.g., int and boolean) to exist declared. In Java, these are called reference types. In object-oriented systems, variables that are "of" a particular class, such as Order, Customer, or Invoice, must exist defined. Taken a step further, Order could consist of other class instances, such as OrderHeader and OrderLine.

    In Java, you can define different variables that are references to runtime objects of a particular class type:

    Public Order myOrder; Public Customer myCustomer; Public Invoice myInvoice;

    Such variables can then exist used to store actual expostulate instances and subsequently to serve as recipients of messages sent by other objects. In the previous code fragment, the variable myOrder is an instance of Order. After the myOrder expostulate is created, a message can exist sent to it and myOrder will respond, provided that the operation is supported by myOrder's interface.

    Java and Message Passing

    Central to any object-oriented language is the ability to pass messages between objects. In later chapters you will espy that toil is done in a system only by objects that collaborate (by sending messages) to accomplish a goal (which is specified in a use-case) of the system.

    Java doesn't allow stray functions floating around that are not attached to a class. In fact, Java demands this. Unfortunately, as my previous legend suggested, just epigram that a language requires everything to exist packaged in classes doesn't connote that the class design will exist robust, let lonesome correct.

    Java supports message passing, which is central to the utilize of Java's object-oriented features. The format closely resembles the syntax of other languages, such as C++ and Visual Basic. In the following code fragment, assume that a variable called myCustomer, of ilk Customer, is defined and that an operation called calcTotalValue() is defined for Customer. Then the calcTotalValue() message being sent to the myCustomer expostulate in Java would watch fancy this:


    Many developers feel that, in any other structured language, this is just a fancy pass of calling a procedure. Calling a procedure and sending a message are similar in that, once invoked, both a procedure and a message implement a set of well-defined steps. However, a message differs in two ways:

  • There is a designated receiver, the object. Procedures Have no designated receiver.

  • The interpretation of the message—that is, the how-to code (called the method) used to respond to the message—can vary with different receivers. This point will become more essential later in the chapter, when polymorphism is reviewed.

  • The concepts presented in this book rely heavily on classes and the messaging that takes location between their instances, or objects.

    Java and Encapsulation

    Recall that a class exposes itself to the outside world via its public interface and that this should exist done through exposure to operations only, and not attributes. Java supports encapsulation via its ability to declare both attributes and operations as public, private, or protected. In UML this is called visibility.

    Using the code from the previous Order example, suppose you want to set the value of the orderDate attribute. In this case, you should enact so with an operation. An operation that gets or sets values is usually called a getter or a setter, respectively, and collectively such operations are called accessors. The local copy of the order date, orderDate, is declared private. (Actually, utter attributes of a class should exist declared private or protected, so that they are accessible only via operations exposed as public to the outside world.)

    Encapsulation provides some powerful capabilities. To the outside world, the design can shroud how it derives its assign values. If the orderTotal assign is stored in the Order object, the corresponding gain operation defined previously looks fancy this:

    public long getOrderTotal() { recrudesce orderTotal; }

    This snippet of code would exist invoked if the following code were executed by an interested client:

    private long localTotal; private Order localOrder; localOrder = fresh Order(); localTotal = localOrder.getOrderTotal()

    However, suppose the assign orderTotal isn't kept as a local value of the Order class, but rather is derived via another mechanism (perhaps messaging to its OrderLine objects). If Order contains OrderLine objects (declared as a Vector or ArrayList of OrderLine objects called myOrderLines) and OrderLine knows how to obtain its line totals via the message getOrderLineTotal(), then the corresponding gain operation for orderTotal within Order will watch fancy this:

    public long getOrderTotal() { long totalAmount=0; for (int i=0; i < myOrderLines.length; i++) { totalAmount = totalAmount + myOrderLines[i].getOrderLineTotal(); } recrudesce totalAmount; }

    This code cycles through the myOrderLines collection, which contains utter the Orderline objects related to the Order object, sending the getOrderLineTotal() message to each of Order's OrderLine objects. The getOrderTotal() operation will exist invoked if the following code is executed by an interested client:

    long localTotal; Order myOrder; myOrder = fresh Order(); localTotal = localOrder.getOrderTotal()

    Notice that the "client" code didn't change. To the outside world, the class quiet has an orderTotal attribute. However, you Have hidden, or encapsulated, just how the value was obtained. This encapsulation allows the class's interface to remain the very (hey, I Have an orderTotal that you can inquire of me about), while the class retains the flexibility to change its implementation in the future (sorry, how they enact traffic has changed and now they must derive orderTotal fancy this). This kind of resiliency is one of the compelling traffic reasons to utilize an object-oriented programming language in general.

    Java and Inheritance

    The inclusion of inheritance is often the most cited intuition for granting a language object-oriented status. There are two kinds of inheritance: interface and implementation. As they shall see, Java is one of the few languages that makes a lucid distinction between the two.

    Interface inheritance (Figure 2-2) declares that a class that is inheriting an interface will exist responsible for implementing utter of the routine code of each operation defined in that interface. Only the signatures of the interface are inherited; there is no routine or how-to code.

    FIGURE 2-2 Interface inheritance

    Implementation inheritance (Figure 2-3) declares that a class that is inheriting an interface may, at its option, utilize the routine code implementation already established for the interface. Alternatively, it may choose to implement its own version of the interface. In addition, the class inheriting the interface may extend that interface by adding its own operations and attributes.

    FIGURE 2-3 Implementation inheritance

    Each ilk of inheritance should exist scrutinized and used in the arrogate setting. Interface inheritance is best used under the following conditions:

  • The foundation class presents a generic facility, such as a table lookup, or a derivation of system-specific information, such as operating-system semantics or unique algorithms.

  • The number of operations is small.

  • The foundation class has few, if any, attributes.

  • Classes realizing or implementing the interface are diverse, with miniature or no common code.

  • Implementation inheritance is best used under the following conditions:

  • The class in question is a domain class that is of primary interest to the application (i.e., not a utility or controller class).

  • The implementation is complex, with a great number of operations.

  • Many attributes and operations are common across specialized implementations of the foundation class.

  • Some practitioners contend that implementation inheritance leads to a symptom called the breakable foundation class problem. Chiefly, this term refers to the fact that over time, what were once common code and attributes in the superclass may not sojourn common as the traffic evolves. The result is that many, if not all, of the subclasses, override the deportment of the superclass. Worse yet, the subclasses may find themselves overriding the superclass, doing their own work, and then invoking the very operation again on the superclass. These practitioners espouse the idea of using only interface inheritance. Particularly with the advent of Java and its raising of the interface to a first-class type, the concept and usage of interface-based programming Have gained tremendous momentum.

    As this book evolves, keeping in intuition the pointers mentioned here when deciding between the two types of inheritance will exist helpful. Examples of both constructs will exist presented in the theme project that extends throughout this book.

    Implementation Inheritance

    Java supports implementation inheritance with the extends keyword. A class wanting to assume advantage of implementation inheritance simply adds an extendsClassName statement to its class definition. To continue the previous example, suppose you Have two different types of orders, both warranting their own subclasses: Commercial and Retail. You would quiet Have an Order class (which isn't instantiated directly and which is called abstract). The previous fragment showed the code for the Order class. Following is the code for the Commercial class.

    package com.jacksonreed; public class Commercial extends Order { public Commercial() { } /* Unique Commercial code goes here */ }

    Implementation inheritance allows the Commercial class to utilize utter attributes and operations defined in Order. This will exist done automatically by the Java Virtual Machine (JVM) in conjunction with the language environment. In addition, implementation inheritance has the ability to override and/or extend any of Order's behavior. Commercial may moreover add completely fresh deportment if it so chooses.

    Interface Inheritance

    Java supports interface inheritance with the implements keyword. A class wanting to realize a given interface (actually being responsible for the routine code) simply adds an implements InterfaceName statement. However, unlike extension of one class by another class, implementation of an interface by a class requires that the interface exist specifically defined as an interface beforehand.

    Looking again at the previous specimen with Order, let's assume that this system will accommodate many classes—some built in this release, and some built in future releases—that necessity the ability to expense themselves. recollect from earlier in this chapter that one of the indicators of using interface inheritance is the situation in which there is miniature or no common code but the functional intent of the classes is the same. This pricing functionality includes three services: the abilities to reckon tax, to reckon an extended price, and to reckon a total price. Let's convoke the operations for these services calcExtendedPrice(), calcTax(), and calcTotalPrice(), respectively, and apportion them to a Java interface called IPrice. Sometimes interface names are prefixed with the note I to distinguish them from other classes:

    package com.jacksonreed; interface IPrice { long calcExtendedPrice(); long calcTax(); long calcTotalPrice(); }

    Notice that the interface contains only operation signatures; it has no implementation code. It is up to other classes to implement the actual deportment of the operations. For the Order class to implement, or realize, the IPrice interface, it must comprehend the implements keyword followed by the interface name:

    public class Order implements IPrice { }

    If you try to implement an interface without providing implementations for utter of its operations, your class will not compile. Even if you don't want to implement any routine code for some of the operations, you quiet must Have the operations defined in your class.

    One very powerful aspect of interface inheritance is that a class can implement many interfaces at the very time. For example, Order could implement the IPrice interface and perhaps a search interface called ISearch. However, a Java class may extend from only one other class.

    Java and Polymorphism

    Polymorphism is one of those $50 words that dazzles the uninformed and sounds really impressive. In fact, polymorphism is one of the most powerful features of any object-oriented language.

    Roget's II: The fresh Thesaurus cross-references the term polymorphism to the main entry of variety. That will enact for starters. Variety is the key to polymorphism. The Latin root for polymorphism means simply "many forms." Polymorphism applies to operations in the object-oriented context. So by combining these two thoughts, you could convey that operations are polymorphic if they are identical (not just in designation but moreover in signatures) but present variety in their implementations.

    Polymorphism is the ability of two different classes each to Have an operation that has the very signature, while having two very different forms of routine code for the operation. Note that to assume advantage of polymorphism, either an interface inheritance or an implementation inheritance relationship must exist involved.

    In languages such as COBOL and FORTRAN, defining a routine to have the very designation as another routine will occasions a compile error. In object-oriented languages such as Java and C++, several classes might Have an operation with the very signature. Such duplication is in fact encouraged because of the power and flexibility it brings to the design.

    As mentioned previously, the implements and extends keywords let the application assume advantage of polymorphism. As they shall see, the sample project presented later in this book is an order system for a company called Remulak Productions. Remulak sells musical equipment, as well as other types of products. There will exist a Product class, as well as Guitar, SheetMusic, and Supplies classes.

    Suppose, then, that differences exist in the fundamental algorithms used to determine the best time to reorder each ilk of product (called the economic order quantity, or EOQ). I don't want to let too much out of the bag at this point, but there will exist an implementation inheritance relationship created with Product as the progenitor class (or superclass) and the other three classes as its descendants (or subclasses). The scenario that follows uses implementation inheritance with a polymorphic example. Note that interface inheritance would yield the very benefits and exist implemented in the very fashion.

    To facilitate extensibility and exist able to add fresh products in the future in a sort of plug-and-play fashion, they can effect calcEOQ() polymorphic. To enact this in Java, Product would define calcEOQ() as abstract, thereby informing any inheriting subclass that it must provide the implementation. A key concept behind polymorphism is this: A class implementing an interface or inheriting from an progenitor class can exist treated as an instance of that progenitor class. In the case of a Java interface, the interface itself is a convincing type.

    For example, assume that a collection of Product objects is defined as a property of the Inventory class. Inventory will support an operation, getAverageEOQ(), that needs to reckon the uninterested economic order quantity for utter products the company sells. To enact this requires that they iterate over the collection of Product objects called myProducts to gain each object's unique economic order quantity individually, with the goal of getting an average:

    public long getAverageEOQ() { long totalAmount=0; for (int i=0; i < myProducts.length; i++) { totalAmount = totalAmount + myProducts[i].calcEOQ(); } recrudesce totalAmount / myProducts.length; }

    But wait! First of all, how can Inventory Have a collection of Product objects when the Product class is abstract (no instances were ever created on their own)? recollect the maxim from earlier: Any class implementing an interface or extending from an progenitor class can exist treated as an instance of that interface or extended class. A Guitar "is a" Product, SheetMusic "is a" Product, and Supplies "is a" Product. So anywhere you reference Guitar, SheetMusic, or Supplies, you can substitute Product.

    Resident in the array myProducts within the Inventory class are individual concrete Guitar, SheetMusic, and Supplies objects. Java figures out dynamically which expostulate should gain its own unique calcEOQ() message. The beauty of this construct is that later, if you add a fresh ilk of Product—say, Organ—it will exist totally transparent to the Inventory class. That class will quiet Have a collection of Product types, but it will Have four different ones instead of three, each of which will Have its own unique implementation of the calcEOQ() operation.

    This is polymorphism at its best. At runtime, the class related to the expostulate in question will exist identified and the redress "variety" of the operation will exist invoked. Polymorphism provides powerful extensibility features to the application by letting future unknown classes implement a predictable and well-conceived interface without affecting how other classes deal with that interface.

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