Is the keyword 'this' needed when instantiating a new inner class?

Question

Another example from the Oracle Java SE tutorials. It works fine, but I'm not sure if/why 'this' is necessary when creating an instance of the inner class. The result seems to be same regardless of whether I take it out or not.

To be clear, I am referring to: InnerEvenIterator iterator = this.new InnerEvenIterator(); // not sure why using 'this'

public class DataStructure {
    // create an array

    private final static int SIZE = 15;
    private int[] arrayOfInts = new int[SIZE];

    public DataStructure() {
        // fill the array with ascending integer values
        for (int i = 0; i < SIZE; i++) {
            arrayOfInts[i] = i;
        }
    }

    public void printEven() {
        // prints out the value of even indices in the array
        InnerEvenIterator iterator = this.new InnerEvenIterator(); // not sure why using 'this'
        while (iterator.hasNext()) {
            System.out.println(iterator.getNext() + " ");
        }
    }

    // inner class implements the Iterator pattern
    private class InnerEvenIterator {
        // start stepping through the array from the beginning

        private int next = 0;

        public boolean hasNext() {
            // check if a current element is the last in the array
            return (next <= SIZE - 1); // -1 b/c dealing with array's length.
        }

        public int getNext() {
            // record a value of an even index of the array
            int retValue = arrayOfInts[next];
            // get the next even element
            next += 2;
            return retValue;
        }
    }

    public static void main(String s[]) {
        // fill the array with integer values and print out only
        // values of even indices
        DataStructure ds = new DataStructure();
        ds.printEven();
    }
}

Answer 1

You don't need a this., in this case. However, an inner class must always be constructed using an enclosing instance, so the this. simply makes this clear to the reader.

If you're not inside of a non-static method of DataStructure (i.e., if this does not exist, or else isn't an instance of DataStructure), then you must actually specify an instance of DataStructure when creating an InnerEvenIterator:

InnerEvenIterator iterator = dataStructure.new InnerEvenIterator();

Answer 2

Just to add a point to correct answers mentioned above:

When you have a function, let's say setInnerEvenIterator(InnerEvenIterator), as follows

class DataStructure {

    InnerEvenIterator iterator;

    public void setInnerEvenIterator(InnerEvenIterator iterator) {
        this.iterator = iterator;
    }
}

then you need to specify this keyword to rectify the possible conflict that may arise due to same argument and instance name. So this.someThing represents instance variable of the class. If you do not explicitly provide one it is OK as long as there is no conflicting situation arising out of it.