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Tags/Keywords : Using-Java-Technology - Object-Oriented-concepts - Coding-Tips-on-Java Example
Author : Amit
Date (Year/Month/Date): 2009-01-27
Object Oriented concepts using Java Technology

 



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Overriding concepts in Java technology
(to the best of my understanding/knowledge):

Explaination:
1. Overriding occurs if a class A
extends class B and class A and B has
same method defined/implemented.

like:
 class A
 {
  public void print()
   {
    System.out.println("inside A print");
   }
 }
 
 
 class B extends A
 {
  public void print()
  {
    System.out.println("inside B print");
  }
 }
2. Access privileges of child class B
should be same or stronger than that of the
parent class A. in this example,

2.1 Access privileges of print method in both
the child (B) is same as parent (A), so it is allowed.
Compilation OK
Runtime OK

2.2 Access privileges of print method
in A is 'public' and in B it is
'protected/private', it is not allowed.

  class A
  {
  public void print()
  {
    System.out.println("inside A print");
  }
  }

  class B extends A
  {
    protected void print()
    {
      System.out.println("inside B print");
    }
  }
Compilation ERROR
No Runtime
  class A
  {
  public void print()
  {
   System.out.println("inside A print");
  }
  }

  class B extends A
  {
    private void print()
    {
      System.out.println("inside B print");
    }
  }
Compilation ERROR
No Runtime

3. If the overridden method (in this example,
 it is method 'print') throws
 Exception in parent class, then the child class method
 (overridden) method should either throw an exception
 that is sub class of the parent exception class,
 or no throws clause in child class overridden method.
 3.1.
	class A
	{
		public void print() throws Exception
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
		public void print() throws java.io.IOException
		{
			System.out.println("inside B print");
		}
	}
 Compilation OK
 Runtime OK

 3.2.
	class A
	{
		public void print() throws Exception
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
		public void print()
		{
			System.out.println("inside B print");
		}
	}
 Compilation OK
 Runtime OK

 3.3.
	class A
	{
		public void print() throws java.io.Exception
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
		public void print() throws java.lang.Exception
		{
			System.out.println("inside B print");
		}
	}
 Compilation ERROR

 3.4.
	class A
	{
		public void print()
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
		public void print() throws java.lang.Exception
		{
			System.out.println("inside B print");
		}
	}
 Compilation ERROR

4. If the parent class method is declared final,
  then compilation error.
 As final keyword restricts overriding.

 4.1.
	class A
	{
		public final void print()
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
		public void print()
		{
			System.out.println("inside B print");
		}
	}
 Compilation ERROR
5. In case of instance variables, be it primitive type (int, float,...) or
 wrapper class (Integer, String, Date, ...), when present in parent class A, and in
 child class B, it is not overriding.

 5.1.
	class A
	{
	 int i=10;
		public void print()
		{
			System.out.println("inside A print");
		}
	}
	class B extends A
	{
	 int i=5;
		public void print()
		{
			System.out.println("inside B print");
		}
	}
 Compilation OK

 In the client code, in case of the following lines of code:

 A a = new B();
 System.out.println(a.i);

 prints on the console as : 10

 And
 a.print();

 prints on the console as : inside B print

6. If super class is not having a default constructor, as shown below, then there
 is compilation error.

	class A
	{
	 public A(String test)
	 {
	 System.out.println("Constructor A");
	 }
	 public void print()
	 {
	 System.out.println("inside A print");
	 }
	}
 class B extends A
 {
	public B()
	{
		System.out.println("Constructor B");
	}
	public void print()
	{
		System.out.println("inside B print");
	}
 }

 Compilation ERROR


Please try this by some small sample program, and
this concept is really interesting to explore.

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