CODEWITHSUNDEEP
javaobjectiteratoriterable
Miguel Ruivo
Miguel Ruivo

Reputation: 17746

How to create a custom Iterator in Java?

I've this problem:

Given the iterable class Foo, which keeps always only one int value which is set on its constructor, make an iterator so it respects all of its restrictions which are: you can't change the int value after its initialization. You should include only the required exceptions to be thrown.

Ok, so far, if I understood the question the right way, I should create an iterator for that Foo class, however I've never done this before and it seems to be that the question itself is a bit misleading. Is it a list? Or shouldn't it be? Anyway, despite that, all I want to know is how to create it.

So now I've this:

public class Foo implements Iterable<Foo> {
    @Override
    public Iterator<Foo> iterator() {   
        throw new UnsupportedOperationException("Not supported yet."); 
    }
}

But I don't even know if this is the right way to do so. I'd be very appreciated if someone could help me out with this.

Thank you in advance.

Upvotes: 3

Views: 11640

Answers (1)

xiaofeng.li
xiaofeng.li

Reputation: 8587

A minimal example would be to return an empty iterator, whose hasNext() always returns false and next() will throw NoSuchElementException. 

public Iterator<Foo> iterator() {
    return new Iterator<Foo>() {
        public boolean hasNext() { 
            return false;
        }
        public Foo next() {
            throw new NoSuchElementException();
        }
    };
}

Of course most iterators have states. For example you can iterate from 0 to the integer value the Foo instance holds.

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Foo implements Iterable<Foo> {
    private final int value;

    public Foo(final int value) {
        this.value = value;
    }


    @Override
    public Iterator<Foo> iterator() {
        return new Iterator<Foo>() {
            private Foo foo = new Foo(0);

            @Override
            public boolean hasNext() {
                return foo.value < Foo.this.value;
            }

            @Override
            public Foo next() {
                if (!hasNext()) throw new NoSuchElementException();

                Foo cur = foo;
                foo = new Foo(cur.value+1);
                return cur;
            }
        };
    }

    public static void main(String[] args) {
        Foo foo = new Foo(10);
        for (Foo f: foo) {
            System.out.println(f.value);
        }
    }
}

Upvotes: 3

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