This implementation of hasNext calls the iterator's next method. This is wrong because hasNext must not cause the state of the iterator to be modified, while next usually does modify iterator state.

This will also result in subtle errors caused by missed items (hasNext calls will consume items of the iterator) and premature exhaustion of the iterator due to too many calls to hasNext.

Bad Practice

class MyIterator extends Iterator<Integer> {

    int i = 1;

    @Override
    boolean hasNext() {
        return next() < Integer.MAX_VALUE;
    }

    @Override
    Integer next() {
        return ++i;
    }
}

Recommended

Change the implementation to perform the hasNext check in a manner that does not modify iterator state.

class MyIterator extends Iterator<Integer> {

    int i = 1;

    @Override
    boolean hasNext() {
        // We now check the state of the iterator instead of the value returned by next.
        return i < Integer.MAX_VALUE;
    }

    @Override
    Integer next() {
        return ++i;
    }
}

References

• SpotBugs - DMI_CALLING_NEXT_FROM_HASNEXT