core java programming doubts

[p hasini]

Please clarify some doubts:

1. When I execute this class

class A
{
	public void add(E e,S s)
	{
	     System.out.println("In add in class A");
	}
}
class B extends A
{
	public void add(X x,Y y)
	{
	      System.out.println("In add in class B");
	}
}
class E
{
}
class S
{
}
class X extends E
{
}
class Y extends S
{
}
public class Test
{
    public static void main(String args[])
    {
                               A a=new B();
		
		 X x=new X();
		 Y y=new Y();
		 E e=new E();
		 S s=new S();

                                a.add(x,y);
		 
		 a.add(e,s);
		    }
}

I get the output : In add in class A
In add in class A

Please explain what is happening here.Is it overloading or overriding.


2.I have an ArrayList

ArrayList<String> list=new ArrayList<String>();
list.add("a");
list.add("b");

Can I make the list readonly ,no other values must be added to the List.

[Pramod P Deore]

Please use code tags, without code tags it is difficult to read the program.

[Pramod P Deore]

Is it overloading or overriding

Because overloaded methods must differ in the type and/or number of their parameters. Therefore it is method overloading.

[p hasini]

How do I set the code tags

[Christophe Verré]

Can I make the list readonly ,no other values must be added to the List.

The Collections class has some utility methods to wrap collections and make them unmodifiable. For example, Collections#unmodifiableList.

[Christophe Verré]

p hasini wrote:How do I set the code tags

UseCodeTags

[Pramod P Deore]

This time someone (bartender) set it for you. But please use code tags in future. using code button before posting your post, or click on edit button after posting your post and set them

[Jesper de Jong]

1. The add() method in class B does not override the add() method in class A, because the argument types are not the same. X and Y are subclasses of E and S, but they are not the same. Because of this, the method to call is determined at compile time, by looking at the type of the variable a, which is A (there is no dynamic dispatch, as you would have when there was overriding).

2. You can effectively make the ArrayList read-only by wrapping it like this:

// Note, you'll need to make list a List<String>, not an ArrayList<String>
// Program to an interface, not to an implementation
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");

list = Collections.unmodifiableList(list);

[p hasini]

Thank you

[Campbell Ritchie]

When it says unmodifiableList, that means the List returned cannot be modified, but it remains a copy of the original List. Any changes to the original List are reflected in the unmodifiable copy, so I think it might have been better to call the method readOnlyList.