Lists

[Raj Joe]

List lst =new ArrayList()
ArrayList lst=new ArrayList()

What is difference b/w the above two statements.

[Chengwei Lee]

Hi,

The key is polymorphism. In the first line of code, we can only have access to the methods defined in the List interface. Additional methods defined in ArrayList which implements List, cannot be used by the first line of code.

For e.g.,

  
List listA = new ArrayList();
ArrayList listB  = new ArrayList();
 
listA.trimToSize();  // compiler error
listB.trimToSize();

HTH.

[Nigel Browne]

Originally posted by Cheng Wei Lee:
Hi,

The key is polymorphism. In the first line of code, we can only have access to the methods defined in the List interface. Additional methods defined in ArrayList which implements List, cannot be used by the first line of code.

This is not exactly true, in the first line of code we can only access the methods defined in the List interface, unless we explicitly downcast the instance to an ArrayList.
For e.g.

List listA = new ArrayList();
ArrayList listB  = new ArrayList();
 
listA.trimToSize();  // compiler error
((ArrayList)listA).trimToSize(); //Runtime type identification.
listB.trimToSize();

It should be noted however that downcasting is not recommended unless, one is sure that the superclass object is a subclass object.

[David Harkness]

Originally posted by Nigel Browne:
It should be noted however that downcasting is not recommended unless, one is sure that the superclass object is a subclass object.

And you can do that with

List list = new ArrayList();
...
if ( list instanceof ArrayList )
{
    ((ArrayList) list).trimToSize();  // Safe due to if test
}

[Raj Joe]

Originally posted by Cheng Wei Lee:
Hi,

The key is polymorphism. In the first line of code, we can only have access to the methods defined in the List interface. Additional methods defined in ArrayList which implements List, cannot be used by the first line of code.

For e.g.,

  
List listA = new ArrayList();
ArrayList listB  = new ArrayList();
 
listA.trimToSize();  // compiler error
listB.trimToSize();

HTH.

I have seen that people recommend the below style
List listA = new ArrayList();
Is there any advantage?

[Ernest Friedman-Hill]

Originally posted by Raj Joe:

I have seen that people recommend the below style

List listA = new ArrayList();

Is there any advantage?

Yes. It you write code this way, then if you later decide that a LinkedList would work better in your application, then you can just change this one line of code, and everything is guaranteed to still work. In general, using the least specific possible reference types makes your application easier to modify.