Principles of programming
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Principles of Programming Achieving an Object-Oriented Design Abstraction and Information Hiding Object-Oriented Design Functional Decomposition Using UML to Model Designs Advantages of an Object-Oriented Approach Abstraction and Information Hiding Procedural Abstraction Data Abstraction

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Achieving an Object-Oriented Design

  • Abstraction and Information Hiding

  • Object-Oriented Design

  • Functional Decomposition

  • Using UML to Model Designs

  • Advantages of an Object-Oriented Approach

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Abstraction and Information Hiding

  • Procedural Abstraction

  • Data Abstraction

  • Information Hiding

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Procedural Abstraction

  • Separates the purpose of a method from its implementation.

  • Specifies what a method assumes and what it does, but not how it does it.

  • You can change how a method works without having to change the methods that use it.

  • Allows you to use code written by others without having to study the code.

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Data Abstraction

  • A collection of data and a set of operations on the data (abstract data type).

  • You can use the operations, if you know their specifications, without knowing how they are implemented or how the data is stored.

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Information Hiding

  • The hiding of implementation details, especially relating to how data is stored, from external access.

  • As a user of a module, you do not need to worry about the details of implementation, and as an implementer of a module, you do not need to worry about its uses.

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Object-Oriented Design

  • Classes, Objects, and Instances

  • Encapsulation

  • Inheritance

  • Polymorphism

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Objects, Classes, and Instances

  • The objects in a problem environment are the things that get used, created, and/or manipulated.

  • A class is a template for a collection of objects that are similar or of the same type.

  • An instance is a specific member of a class.

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  • It is the combining of data and operations on that data.

  • It is a mechanism that helps with information hiding because it hides the inner details of the implementation from external modules.

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  • The properties of a class can be inherited from another class.

  • This allows you to reuse classes defined earlier for different purposes with appropriate modification.

  • In the new class some of the methods can be modified and new ones can be added.

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  • This is a mechanism that enables determination to be made at execution time as to which method is the right one to use based on the type of objects operated on.

  • For example, the + operator between two numbers means addition, but between two strings it means concatenation.

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Functional Decomposition

  • The successive refinement of function into more specific functional elements.

  • It is the breaking down of a complex task into more manageable single-purpose tasks and subtasks.

  • This is also referred to as top down design.

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Using UML to Model Designs

Class Diagram Example:

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Advantages of an Object-Oriented Approach

  • Although extra time and effort is required when using object-oriented programming (OOP), it is usually worth it.

  • After you identify the classes needed for your problem and how they interact with each other, you can focus on one class at a time which simplifies implementation.

  • Once you have implemented a class, it is much easier to implement similar classes since you can inherit properties. (reuse)

  • You can make a change to an ancestor class and affect all descendant classes.

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Key Issues in Programming

  • Modularity

  • Modifiability

  • Ease of Use

  • Fail-safe Programming

  • Style

  • Debugging

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  • Modularity has a favorable impact on:

    • Constructing the program

    • Debugging the program

    • Reading the program

    • Modifying the program

    • Eliminating redundant code

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Constructing the Program

  • Because the modules are independent writing one large modular program is not much different from writing many small, independent programs.

  • Modularity permits team programming where several programmers work independently on their own modules before combining them into one program.

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Debugging the Program

  • The task of debugging a large program is reduced to the task of debugging many small programs.

  • If you thoroughly test modules as you go along, you can be almost sure that newly found bugs are in the newest module introduced.

  • Modular programs are more amenable to formal methods of verification.

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Reading the Program

  • Just as modularity helps the programmer to deal with the complexity of the problem, it also helps the reader to do the same.

  • A modular program is easy to follow because the reader can get a good idea of what is going on without reading any code.

  • A well-written can be understood fairly well just from its name.

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Modifying the Program

  • A small change in requirements will likely require modification of only one or a small number of modules.

  • With modularity you can reduce a large modification into a set of small and relatively simple modifications.

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Eliminating Redundant Code

  • You can identify a computation that occurs in many places in your program and implement it as a method.

  • This improves both readability and modiyfiability.

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  • The use of methods, e.g., creating a sort method rather than embedding the code in the main body of the program, makes a program easier to modify.

  • Using named constants, e.g., for the number of students in the class, can make a program that processes information about members of the class easier to modify if that number changes.

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Ease of Use

  • In an interactive environment, the program should always prompt the user for input in a manner that makes it clear what is expected.

  • A program should always echo its input.

  • The output should be well labeled and easy to read.

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Fail-Safe Programming

  • A fail-safe program is one that will perform reasonably well no matter how anyone uses it.

  • You need to try to anticipate how people might misuse the program and guard against these abuses.

  • There are two main types of errors to guard against:

    • Errors in input data

    • Errors in logic

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  • Five issues of style

    • Extensive use of methods

    • Use of private data fields

    • Error handling

    • Readability

    • Documentation

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Extensive Use of Methods

  • If a set of statements performs an identifiable, recurring task, it should be a method.

  • The use of methods is cost effective even if it adversely affects performance because of the human effort saved.

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Use of Private Data Fields

  • You should hide the exact representation of data fields of an object by making them private.

  • This supports the principle of information hiding.

  • Use accessor and mutator methods to access and change the data values of an object.

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Error Handling

  • In most cases a method should return a value or throw and exception instead of displaying an error message when a problem is encountered.

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  • For a program to be easy to follow, it should have:

    • A good structure and design

    • A good choice of identifiers

    • Good indentation and use of white space

    • Good documentation

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Identifier Conventions

User-defined identifiers are both upper- and lowercase letters as follows:

  • Class names are nouns, with each word in the identifier capitalized.

  • Method names are verbs, with the first letter lowercase and subsequent internal words capitalized.

  • Variable names begin with a lowercase letter and subsequent internal words capitalized.

  • Named constants are entirely uppercase and underscores are used to separate words.

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Indentation Style

  • Use blank lines to offset each method.

  • Indent individual blocks of code visibly and offset them with blank lines.

  • Indentation should be consistent.

  • Try to avoid rightward drift.

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Indentation Style

  • With regard to braces ({, }) there are two acceptable styles.

    • Put the opening brace at the end of the line that begins the compound statement and the closing brace should be on a line by itself, lined up with the with the beginning of the line that begins the compound statement.

    • Put the opening brace on a line by itself immediately following and lined up with the beginning of the line that begins the compound statement. The closing brace is as above.

  • Braces should be used around all statements, even single statements, when they are part of a control structure.

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  • A program should be well documented (commented) so that others can read, use, and modify it easily.

  • You should comment your code as you go along, not just as a last step.

  • You may be the one who benefits most from your own documentation.

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Essential Features of Program Documentation

  • An initial comment for the program that includes:

    • Author and date

    • Description of the program’s input and output

    • Description of how to use the program

    • Assumptions such as the type of data expected

    • Statement of purpose

    • Statement of exceptions, that is, what could go wrong

    • Brief description of the major classes

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Essential Features of Program Documentation (Cont’d)

  • Initial comments in each class that state its purpose and describe the data contained in the class (constants and variables).

  • Initial comments in each method that state its purpose, preconditions, postconditions, and methods called.

  • Comments in the body of each method to explain important features or subtle logic.

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  • Programs that are modular, clear and well documented are generally easier to debug.

  • Fail-safe techniques are also a great aid to debugging.

  • Use System.out.println statements as necessary.

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Debugging (Cont’d)

  • Debugging methods:

    • You should examine the values of a method’s arguments at its beginning and end.

  • Debugging loops:

    • You should examine the values of key variables at the beginnings and ends of loops,

  • Debugging if statements:

    • Just before the if statement you should examine the values of the variables within its expression.

  • Remember to either comment out or remove debugging code inserted.