Generic type for Iterator is also required ?

[Harikrishna Gorrepati]

Hi, This is from Whizlabs and question is given in the comments

public class Question14_iteraor_ask {
    public static void main(String[] args) {
	List<Integer> list = new LinkedList<Integer>();
	list.add(1);
	list.add(4);
	list.add(-4);

	for (Iterator i = list.iterator(); i.hasNext();) {
	    Integer in = i.next(); // Compiler Error: cannot convert from Object to Integer
	    // Why iterator doesn't know that, list contains only Integer values even though we specified list contains only Integer types??
	    System.out.println(in);
	}
    }
}    

[Abimaran Kugathasan]

The iterator method returns generically, but, you assigned it to a non-generic collection. So, you break the generics there, so, you need a cast!

import java.util.*;

public class Question14_iteraor_ask {
    public static void main(String[] args) {
	List<Integer> list = new LinkedList<Integer>();
	list.add(1);
	list.add(4);
	list.add(-4);

	for (Iterator<Integer> i = list.iterator(); i.hasNext();) {// i should be a generic collection, otherwise, you'll mix both.
	    Integer in = i.next();
	    System.out.println(in);
	}
    }
}

[Wouter Oet]

Abimaran Kugathasan wrote:The iterator method returns generically, but, you assigned it to a non-generic collection. So, you break the generics there, so, you need a cast!

The typed collection that the topic starter is using returns an typed Iterator. However he is then assigning it to a non-typed Iterator so the type information is lost. And because of that the cast is needed.

[Abimaran Kugathasan]

Wouter Oet wrote:
The typed collection that the topic starter is using returns an typed Iterator. However he is then assigning it to a non-typed Iterator so the type information is lost. And because of that the cast is needed.

This is what I intended to mean, but, may be interpreted slightly different. Does it?

[Harikrishna Gorrepati]

Thanks Abi and Oet

[Stephan van Hulst]

Because it gets erased during compilation. There's no way for the JVM to know what kind of Iterator i is on line 9. It may have changed in the mean time. The compiler recognizes this and forces you to cast the object reference.

This isn't necessary if the Iterator has a generic type, because even though the JVM still won't know what kind of iterator i is at line 9, the compiler sees that it can only return references of that particular type, so you don't have to cast.

Get into the habit of never using raw types. Always parameterize generic types.

[edit]

The question in the last post has been edited away.