Re: Foreach and Iterators

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.help

Date:

Thu, 20 Aug 2009 16:18:25 -0700

Message-ID:

<dRkjm.175283$3m2.60736@newsfe06.iad>

Ivan Voras wrote:

Roedy Green wrote:

On Thu, 20 Aug 2009 17:22:26 +0200, Ivan Voras <ivoras@fer.hr> wrote,
quoted or indirectly quoted someone who said :

The problem is that the following (simplified) code works:

for (Object orow : new MyIter()) {
Hashtable<String, Object> row = (Hashtable<String, Object>) orow;
System.out.println(row);
}

I suspect this example is too far away from your real code. Try
composing an SSCCE. see http://mindprod.com/jgloss/sscce.html

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 {

This is your problem, Iterable needs to be
Iterable<Hashtable<String,Object>>

    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;

When you change Iterable above, you'll need to change this Iterator to
match.

    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;

new MyIter does not look plausible. The way you get an Iterator is
nearly always SomeCollection.iterator()

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).

System.out.println( row ) won't print anything interesting. You need
an inner loop to iterate over the objects in the row.

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

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>