java implementing an interface with additional methods

Question

I have a class MyDoublyIterator<E> that implements the Iterator<E> interface. Also, it has a couple other methods not part of the interface (for example public void boolean hasPrev()). When I try to call that method in my main class my IDE can't resolve that method. Here's the code I'm using to call it

Iterator<String> di = new MyDoublyIterator<String>();
di.hasPrev();

So I guess my question why can't it find that method. Do I have to build an abstract class that implements the interface and extend it?

You do know that ListIterator<T> provides a bidirectional iterator, right? — c00kiemon5ter
– c00kiemon5ter, Commented Jun 15, 2011 at 1:09
I didn't know that, but either way I'm just experimenting to learn. — jhlu87
– jhlu87, Commented Jun 15, 2011 at 1:11

Marten · Accepted Answer · 2011-06-15 01:11:17Z

3

The Problem is that the di-variable is of the type Iterator. So it could contain any implementation of this interface. And therefore the IDE cannot be sure that it contains an instance of MyDoublyIterator. So to make it work like you requested, you have to change the type of the variable to MyDoublyIterator.

answered Jun 15, 2011 at 1:11

Marten

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Comments

anubhava · Accepted Answer · 2011-06-15 01:08:54Z

1

It is better to first extend the original interface Iterator<E> with your own interface eg: DoubleIterator<E> with additional methods like hasPrev() and then implement the newly created interface DoubleIterator<E> in your class MyDoublyIterator<E>.

Otherwise you can instantiate your class like this:

MyDoublyIterator<String> di = new MyDoublyIterator<String>();
di.hasPrev();

Then compiler won't complain since you are creating a type of MyDoublyIterator<E>.

answered Jun 15, 2011 at 1:08

anubhava

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Alex Gitelman · Accepted Answer · 2011-06-15 01:13:40Z

1

When compiler sees di.hasPrev(); It only knows that di is Iterator<String> and has no clue that it's your own type. So it is not aware of your custom methods.

answered Jun 15, 2011 at 1:13

Alex Gitelman

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Saurabh Gokhale · Accepted Answer · 2011-06-15 01:16:00Z

0

Iterator<String> di = new MyDoublyIterator<String>(); 
di.hasPrev();

It won't help you. Compiler needs a way to know that your own interface has that method, and not the built-in interface Iterator interface i.e Iterator<String>. Here, you are creating a reference for the built-in Iterator interface i.e Iterator<String>.

Hence, if you have your own interface named MyDoublyIterator<E> and this interface has the method named hasPrev(); you need to do this :

 MyDoublyInterface<String> di = new MyDoublyIterator<String>(); 
 di.hasPrev();

and then it will resolve the method call, since it knows that you have created a reference of your own MyDoubleInterface<T> and it should solve the problem now. :-)

answered Jun 15, 2011 at 1:16

Saurabh Gokhale

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Bohemian · Accepted Answer · 2011-06-15 01:21:31Z

0

Other answers are correct, but here's another spin - create an interface that extends Iterator:

interface DoublyIterator<E> extends Iterator<E> {
    boolean hasPrev();
}

class MyDoublyIterator<E> implements DoublyIterator<E> {
    // ... implement methods
}

DoublyIterator<String> di = new MyDoublyIterator<String>();
di.hasPrev();

answered Jun 15, 2011 at 1:21

Bohemian♦

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