Java Iterator vs Iterable – Why Iterator is Not an Iterable

iterableiteratorjava

Why does the Iterator interface not 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.listSomeObjects()) {
    ....
}

where listSomeObjects() returns an iterator.

Best Answer

An iterator is stateful. The idea is that if you call Iterable.iterator() twice you'll get independent iterators - for most iterables, anyway. 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.

Related Solutions

Java Iterator – Why Can’t Iterate Over an Iterator?

Most likely the reason for this is because iterators are not reusable; you need to get a fresh Iterator from the Iterable collection each time you want to iterate over the elements. However, as a quick fix:

private static <T> Iterable<T> iterable(final Iterator<T> it){
     return new Iterable<T>(){ public Iterator<T> iterator(){ return it; } };
}

//....
{
     // ...
     // Now we can use:
     for ( X x : iterable(it) ){
        // do something with x
     }
     // ...
}
//....

That said, the best thing to do is simply pass around the Iterable<T> interface instead of Iterator<T>

Java – Difference Between Iterator and Iterable

An Iterable is a simple representation of a series of elements that can be iterated over. It does not have any iteration state such as a "current element". Instead, it has one method that produces an Iterator.

An Iterator is the object with iteration state. It lets you check if it has more elements using hasNext() and move to the next element (if any) using next().

Typically, an Iterable should be able to produce any number of valid Iterators.

Best Answer

Related Solutions

Java Iterator – Why Can’t Iterate Over an Iterator?

Java – Difference Between Iterator and Iterable

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