Why is Java's Iterator not an Iterable?

Question

Why does the Iterator interface not extend extend Iterable?

The iterator() method could simply return 'this'.

Is it on purpose or just an oversight of Java's designers?

It would be convenient to be able to use a for-each loop with iterators like this:

for(Object o : someContainer.listSomObjects()) {
....
}

where listSomeObject returns an iterator.

Answer 1

Because an iterator generally points to a single instance in a collection. Iterable implies that one may obtain an iterator from an object to traverse over its elements - and there's no need to iterate over a single instance, which is what an iterator represents.

Answer 2

An iterator is stateful. The idea is that if you call Iterable.iterator() twice you'll get independent iterators. That clearly wouldn't be the case in your scenario.

For example, I can usually write:

public void iterateOver(Iterable<String> strings)
{
    for (String x : strings)
    {
         System.out.println(x);
    }
    for (String x : strings)
    {
         System.out.println(x);
    }
}

That should print the collection twice - but with your scheme the second loop would always terminate instantly.

Answer 3

An Iterable is a thing from which you obtain an Iterator.

Answer 4

For the sake of simplicity, Iterator and Iterable are two distinct concepts, Iterable is simply a shorthand for "I can return an Iterator". I think that your code should be:

for(Object o : someContainer) {
}

with someContainer instanceof SomeContainer extends Iterable<Object>

Answer 5

Visage allready said that these are both different concepts introduced in different versions of Java. Iterator was introduced first, that's only one of the reasons it doesn't extend Iterable.

But nothing can keep you from implementing both interfaces in one class. In fact this is the solution I see programmers use very often.

public class MyClass implements Iterator, Iterable {

    // methods of Iterable

    public Iterator iterator() {
        return this;
    }

    // methods of Iterator

    public boolean hasNext() {
        // ...
    }

    public Object next() {
        // ...
    }

    public void remove() {
        // ...
    }

}

Answer 6

I also see many doing this:

    public Iterator iterator() {
        return this;
    }
}

But that does not make it right! This method would not be what you want!

The method iterator() is supposed to return a new iterator starting from scratch. So one need to do something like this:

public class IterableIterator implements Iterator, Iterable {

  //Constructor
  IterableIterator(IterableIterator iter)
  {
    this.initdata = iter.initdata;
  }
  // methods of Iterable

  public Iterator iterator() {
    return new MyClass(this.somedata);
  }

  // methods of Iterator

  public boolean hasNext() {
    // ...
  }

  public Object next() {
    // ...
  }

  public void remove() {
    // ...
  }
}

The question is: would there be any way to make an abstract class performing this? So that to get an IterableIterator one only need to implement the two classes next() and hasNext()