Interface Objects

[Phillipe Rodrigues]

I am a bit confused on the following:

Collection is an interface and ArrayList is a concrete class.Then

Collection c =new ArrayList(); ......................(1)

How above(1) is posible as interface objects cannot be created.What could be the idea behind the above instruction(1)

[Ernest Friedman-Hill]

You're creating an instance of the class ArrayList, and you're holding it in a variable of type "Collection". This is like buying a pet of type "Goldfish" and putting it in a fish tank. You can't buy a "Fish" -- you can only buy an Angelfish, a Goldfish, a Guppy, etc. But the tank can hold any kind of Fish.

A Collection variable can hold any kind of Collection -- and ArrayList, which implements List which extends Collection, is a kind of Collection.

[Phillipe Rodrigues]

What about the below line numbers for the below code.When do we use them.What could be the possible cause for assiging class objects to an interface reference?

interface IStack{
public void push();
public Object pop();
 
}
 
class StackImpl implements Istack{
 
public void push(){
some code written
}
public Object pop(){
return Object;
}
interface ISafeStack extends IStack{
boolean isEmpty();
}
 
class IStackImpl extends StackImpl implements ISafeStack{
some code written
}
 
class Main_impl{
public staticc void main(String[] args){
IStackImpl one =new IStackImpl();
StackImpl two =one;....................(1)
ISafeStack three =one;.................(2)
IStack four =one;......................(3)
}
}