Re: iterator over superclass of collection

From:
Daniel Pitts <googlegroupie@coloraura.com>
Newsgroups:
comp.lang.java.programmer
Date:
17 Apr 2007 17:10:19 -0700
Message-ID:
<1176855019.535644.40730@q75g2000hsh.googlegroups.com>
On Apr 17, 2:22 pm, Frank Fredstone <n...@not.no> wrote:

I want to expose an iterator over a collection, where the iterator
returns elements that are a super class of the actual
objects. Essentially, so I can implement interfaces.

Or, another way to do what I think looks like the intention of this
illegal code:

given: Collection<PrivateAye> c;
Iterator<Aye> it = c.iterator();

PrivateAye below is the internal implementation of the Aye interface
that also extends another class.

Is there a simpler way than what I have in the iterator method below:

public interface Aye {
    String aye();

}

public class A {
    private String a;
    public A(String eh) { a = eh; }
    String a() { return a; }

}

import java.util.Iterator;
import java.util.Vector;

public class X implements Iterable<Aye> {
    private class PrivateAye extends A implements Aye {
        private int code = 0;
        public PrivateAye(String eh, int n) {
            super(eh);
            setCode(n);
        }
        public int getCode() { return code; }
        public void setCode(int n) { code = n; }
        public String aye() { return a(); }
    }

    private Vector<PrivateAye> ayes;

    public static void main(String[] args) throws Exception {
        X x = new X();
        x.go();
    }

    public void go() throws Exception {
        ayes = new Vector<PrivateAye>();
        ayes.add(new PrivateAye("a", 0));
        ayes.add(new PrivateAye("b", 1));
        for (Aye a : this) {
            System.out.println(a.aye());
        }
    }

    public Iterator<Aye> iterator() {
        return new Iterator<Aye>() {
            private Vector<? extends Aye> vec = ayes;
            private Iterator<? extends Aye> it = vec.iterator();
            public boolean hasNext() { return it.hasNext(); }
            public Aye next() { return it.next(); }
            public void remove() { it.remove(); }
        };
    }

}


Your concept is wrong.

Consider if you COULD do what you ask.

Collection<PrivateAye> ayes = new ArrayList<PrivateAye>();
Iterator<Aye> iterator = ayes.iterator();
iterator.next();
iterator.set(new SomeOtherAye());

Whoops, your PrivateAye array contains something that isn't a private
aye!!!!

Now, what you might be trying to solve:

public void seeAyes(Iteratble<Aye> ayes) {
    // do something with the ayes
}
seeAyes(new ArrayList<PrivateAye>()); // compile error.
--- Instead, try this:
public <T extends Aye> void seeAyes(Iteratble<T> ayes) {
    // do something with the ayes
}

seeAyes(new ArrayList<PrivateAye>()); // works fine!

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