Why does an iterator need a cast for a generically defined list

[O. Ziggy]

class ZiggyTest{

	public static void main(String[] args){
	
		List<Integer> list = new LinkedList<Integer>();
		list.add(4);
		list.add(5);
		list.add(-5);
		
		for(Iterator i = list.iterator(); i.hasNext();){
			Integer in = i.next();
			System.out.println(in);
		}
	}
		
}

The above code example results in the following compiler error:

ZiggyTest.java:17: incompatible types
found   : java.lang.Object
required: java.lang.Integer
			Integer in = i.next();
			                   ^
1 error

Why does the Iterator.next() method need a cast when the List associated with the Iterator is declared generically?

[Henry Wong]

O. Ziggy wrote:

class ZiggyTest{

	public static void main(String[] args){
	
		List<Integer> list = new LinkedList<Integer>();
		list.add(4);
		list.add(5);
		list.add(-5);
		
		for(Iterator i = list.iterator(); i.hasNext();){
			Integer in = i.next();
			System.out.println(in);
		}
	}
		
}

The above code example results in the following compiler error:

ZiggyTest.java:17: incompatible types
found   : java.lang.Object
required: java.lang.Integer
			Integer in = i.next();
			                   ^
1 error

Why does the Iterator.next() method need a cast when the List associated with the Iterator is declared generically?

It needs a cast, because you are not using generics for the iterator. Try...

        for(Iterator<Integer> i = list.iterator(); i.hasNext();){  
            Integer in = i.next();  
            System.out.println(in);  
        }  

Henry

[O. Ziggy]

Thanks Henry. I thought that since the List provides the Iterator it will returned a typed iterator.

[Henry Wong]

O. Ziggy wrote:Thanks Henry. I thought that since the List provides the Iterator it will returned a typed iterator.

The iterator() method returns an object. It is you who declared the iterator reference and assigned it. There is no way to create a class, with a method, that returns an object, that can control how it will be assigned. If you want to create an iterator reference, that doesn't use generics, the compiler can't stop you. It just does what you want.

Henry

[Sebanti Sanyal]

Because of 'Type Erasure',JVM does not know at runtime that 'list' refers to a LinkedList of "Integer objects".

[O. Ziggy]

Ok here is one that has confused me again:

class Olives {
	public static void main(String[] args) {
		Set<Integer> s = new TreeSet<Integer>();
		s.add(23); s.add(42); s.add(new Integer(5));
		Iterator i = s.iterator();
		while(i.hasNext()) System.out.print(i.next());
	}
}

This works fine and produces the output 5234252342. How come i didnt have to cast the i.next() call?

[O. Ziggy]

O. Ziggy wrote:Ok here is one that has confused me again:

class Olives {
	public static void main(String[] args) {
		Set<Integer> s = new TreeSet<Integer>();
		s.add(23); s.add(42); s.add(new Integer(5));
		Iterator i = s.iterator();
		while(i.hasNext()) System.out.print(i.next());
	}
}

This works fine and produces the output 5234252342. How come i didnt have to cast the i.next() call?

Ok i think in the compiler is not asking for a cast in the above example because the value is not being assigned to another reference. It just calls the toString() which will call the overriden toString() method of what ever instance the returned object is. right?

[Sebanti Sanyal]

Yes you are right.'Integer val=i.next();' doesn't compile without the cast, but you may have 'Object val=i.next();'