OOP Basics Classes

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OOP Basics Classes Methods

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OOP

• In many OOP courses and books, as soon as they
  start OOP discussion, they get out of EDP world
  and jump to some real world!
• - Sending flowers to another person (Budd
  2000), taking an elevator (Java in 21 Days)...
• And you see a variety of confusing definitions
  right from Booch (1994)
• We stick to the EDP world and terms!

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OOP

• OOP is the design and implementation of programs
  in terms of classes and objects.
• A class defines the methods and the types of
  data (variables) associated with the object
• An object is an instance of a class
• From a programmers viewpoint, what we get is an
  abstract data type
• ie class can be used as if it is a datatype!
  This new datatype has methods and variables
  associated with it!

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Class

• A class consists of
• a collection of methods representing the
  behavior of an object
• a collection of instance variables to represent
  the state of an object
• scope class ClassName extends OldClass //
  Class implementation
• eg - public class Greeting . . .
• - public class HelloWorld2 extends
  Applet . . .

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Methods

• A class contains methods
• All methods follow similar syntax
• scope return-type method-name(arguments)
• statements
• eg public static void main (String args) ...
  
• public float FindArea(int radius)
• return 3.14fradiusradius

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Methods . . .scope

• - public - the method is accessible by any
  system object.
• - protected - is only accessible by subclasses
  and the class in which it is declared.
• - private - accessible only within current
  class.
• - fina1 - cannot be overridden by any subclass.
• - static - is shared by all instances of the
  class.
• If a method is not given a scope, it is only
  accessible within the scope of the current file.

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Variables... a recap

• double salary 2534.50 // instance variable
• unique to each object created
• usage is obj1.name syntax
• static int counter1 // class variable
• usage is class.name syntax

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Variables of class type. . .

• A variable of a class type holds a reference to
  an object. (unlike a variable of primitive
  datatype)
• It doesnt hold the object itself.
• Instance objects can be created using
• MyClass myObj1 new MyClass()
• Assignment means storing a reference to a
  different object.
• MyClass myObj2 myObj1

An instance of MyClass
myObj1
myObj2
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Object as a parameter

• -Parameters to methods are primitive datatypes
  ...
• Parameters can also be objects
• Suppose you want to pass a MyClass object as a
  parameter to a method void
  someMethod(MyClass obj1)
• ........
• A parameter variable can be declared as normal.

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OO Language must support

• the three very important concepts
• - encapsulation,
• - inheritance,
• - polymorphism.

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Encapsulation

• Encapsulation is the process of defining the
  class and its implementation using methods
• Data in an object must be accessed via methods
  defined on that object
• Interface is the external view of a method
• Actual implementation is hidden and can be
  changed, but the interface is unchanged
• From the external view, an object is an
  encapsulated

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Encapsulation . .

• The access modifiers public and private are how
  encapsulation is enforced.
• Good practice states
• Instance variables are always private.
• Only a minimal number of methods should be made
  public.

Some documents use the term visibility
modifiers
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Inheritance

• A class can normally inherit the state and
  behavior of another class
• The original class is often called the
  superclass, and the new class is often called
  the subclass. Inheritance is often referred to as
  extending the superclass.
• In Java all classes inherit from a root class
  called Object. Methods defined on this Object
  class can be used or be overridden by user
  classes.

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Inheritance tree

• Inheritance is created by the keyword extends..
• class HelloWorld
• really means
• class HelloWorld
• extends java.lang.Object
• (default inheritance)
• class Snowman extends Applet

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Polymorphism

• polymorphic means having many forms.
• Polymorphism will be discussed later in the course

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Simple Application- Recap




/ HelloWorld.java / public class
HelloWorld public static void main(String
arguments) // Where the program starts
System.out.println(Hello World") // Print
out message
Class name (MUST match file name)
Main method (starting point)
Braces start end of class and methods
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Where is the object here?

• The above code is procedural ... Top to bottom
  ... We dont get to see any instances of objects
  here...
• Java HelloWorld - will start executing the main
  method always.
• To see the real objects we need to split the
  HelloWorld into 2 programs!

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Modified Program

• We create two programs 1) Greeting 2)TestGreeting
• public class Greeting
• Greeting() // default constructor, does
  nothing here
• public void greet(String whom)
• System.out.println("Hello " " " whom)
• Note - there is no main method here
• This method has to be called from another program
• It takes one argument whom to greet
• You cannot run this as Java Greeting
• So lets have another main class

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The Main ..

• public class TestGreeting
• / the main method follows /
• public static void main(String args)
• Greeting hello1 new Greeting() //
  instantiate
• hello1.greet("World") // call the method
  with an argument //end of method
• Note that several important concepts are present
  here
• Try Java TestGreeting

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The main method

• The main method gets control when we start the
  program
• Every main program must have a main method...
  (callable from JVM)
• The main method is declared static means it
  belongs to the class and no instance of the class
  is needed to execute this.
• arguments to main is String array
• main method is usually short (as a sort of
  hoved section in our COBOL ..)

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Greeting hello1 new Greeting()

• What happens here?
• Tells the JVM to construct an object hello1 of
  the class Greeting.
• Greeting hello1 // just like defining
  variables
• hello1 new Greeting() // this is also a valid
  way
• It will execute the default code in Greeting to
  do any initialization needed
• It will not execute any method within Greeting
• Greeting could have taken some parameters (we
  have none)

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Constructor

• Constructor has the same name as the Class
• But is defined like a method ... Well, almost
• Constructor returns nothing, not even void
• If not coded, a default constructor is assumed
• Constructor code is executed when an object is
  instantiated...
• ie when we say Greeting x1 new Greeting()
• A class can have more than one Constructor

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Constructor

• public class Greeting
• Greeting() // default constructor
• Lets modify the code and put a proper
  constructor with parameters

NB Same name !
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Complete Code with some extras

• public class TestGreeting
• / the main method follows /
• public static void main(String args)
• Greeting hello1 new Greeting("Hello") //
  instantiate
• hello1.greet("World") // call the method
  with an argument
• Greeting hello2 new Greeting("God Dag")
• hello2.greet("Verden")
• // definition of the other class follows
• class Greeting
• private String hilsen // instance variable is
  declared private
• Greeting(String arg1)
• hilsen arg1
• //System.out.println( "in Constructor ")
• // real method follows
• public void greet(String whom)
• System.out.println (hilsen " " whom)

Note One can put the source in 2 files or 1 file
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A simpler example of true oop

• public class Greeting3
• String hilsen "Hello"
• public void greet(String whom)
• System.out.println (hilsen " " whom)
• / the main method follows /
• public static void main(String args)
• Greeting3 hello1 new Greeting3() //
  instantiate
• hello1.greet("World") // call the method