Re: Foreach and Iterators

Roedy Green wrote:


Ok, here is self contained example, directly compilable:

----
import java.util.Hashtable;
import java.util.Iterator;

public class MyIter implements Iterator<Hashtable<String,Object>>,
Iterable {

    public static void main(String[] args) {

        /* The following code doesn't work. */
        /*
        for (Hashtable<String,Object> h : new MyIter())
            System.out.println(h);
        */

        /* The following code does. */
        for (Object oh : new MyIter()) {
            Hashtable<String,Object> h = (Hashtable<String, Object>) oh;
            System.out.println(h);
        }
    }

    private int i;

    public Iterator iterator() {
        i = 0;
        return this;
    }

    public boolean hasNext() {
        return i < 3;
    }

    public void remove() {
        throw new UnsupportedOperationException();
    }

    public Hashtable<String,Object> next() {
        i++;
        Hashtable<String,Object> h = new Hashtable<String,Object>();
        h.put(""+i, new Integer(i));
        return h;
    }
}
----

The expected result of running this example are three lines on stdout:

----
{1=1}
{2=2}
{3=3}
----

It should go twice if the first, commented out attempt at foreach worked.

To be specific, my question is: why won't the following line compile:

        for (Hashtable<String,Object> h : new MyIter())

but this one will:

        for (Object oh : new MyIter())

?

The error compiler complains about is:

MyIter.java:13: incompatible types
found : java.lang.Object
required: java.util.Hashtable<java.lang.String,java.lang.Object>
        for (Hashtable<String,Object> h : new MyIter())

Of course, with the code that *does* work, the cast from Object to
Hashtable<String,Object> gives an "unchecked operation" warning:

MyIter.java:19: warning: [unchecked] unchecked cast
found : java.lang.Object
required: java.util.Hashtable<java.lang.String,java.lang.Object>
            Hashtable<String,Object> h = (Hashtable<String, Object>) oh;


Note that I'm not implementing or overloading a collection. I'm
iterating over something abstract and the result of each iteration of my
iterator is a collection (which is a detail that should be inconsequential).


The println() only serves as an example. It will print out a Hashtable's
contents cast to string.