Question

Is the interface Iterator already defined somewhere in java library (mind the terminology).
i.e.

What i am asking is that, say i have an arraylist, now i write.

Iterator itr= new Iterator();

but i never define anything like

public interface Iterator{  // all the methods };

Do i need to import some package where this iterator is already defined?

Let me take an example here:

class BOX implements Comparable {

    private double length;
    private double width;
    private double height;

    BOX(double l, double b, double h) {
        length = l;
        width = b;
        height = h;
    }

    public double getLength() {
        return length;
    }

    public double getWidth() {
        return width;
    }

    public double getHeight() {
        return height;
    }

    public double getArea() {
        return 2 * (length * width + width * height + height * length);
    }

    public double getVolume() {
        return length * width * height;
    }

    public int compareTo(Object other) {
        BOX b1 = (BOX) other;
        if (this.getVolume() > b1.getVolume()) {
            return 1;
        }
        if (this.getVolume() < b1.getVolume()) {
            return -1;
        }
        return 0;
    }

    public String toString() {
        return 
        “Length:
        ”+length +
        ” Width:
        ”+width +
        ” Height:
        ”+height;
    }
} // End of BOX class

And here is my test class.

import java.util.*;

class ComparableTest {

    public static void main(String[] args) {
        ArrayList box = new ArrayList();
        box.add(new BOX(10, 8, 6));
        box.add(new BOX(5, 10, 5));
        box.add(new BOX(8, 8, 8));
        box.add(new BOX(10, 20, 30));
        box.add(new BOX(1, 2, 3));
        Collections.sort(box);
        Iterator itr = ar.iterator();
        while (itr.hasNext()) {
            BOX b = (BOX) itr.next();
            System.out.println(b);
        }
    }
}// End of class

Now in class ComparableTest should not it implement interface iterator also, shall i not define an interface iterator that will contain all the methods. Also, where is the implementation of the iterator methods are?

I maybe confused alot, but kindly help!
thanks.

Answer 1

I suspect you mean java.util.Iterator<E>.

And no, you don’t write:

Iterator itr= new Iterator();

… that could never work, given that it’s an interface. Also, it’s Iterator, not iterator, and your code should use import, not Import – Java is case-sensitive.

Instead, you write:

Iterator<Foo> iterator = list.iterator();

But no, ComparableTest doesn’t need to implement Iterator<E> – why would it? It uses the Iterator interface, but it doesn’t implement it.