How can I fix this Java generic pattern error?

Question

In this question , TofuBeer was having trouble creating a generic one IterableEnumeration.

A response from jcrossley3 pointed to this link http://www.javaspecialists.eu/archive/Issue107.html , which pretty much solved the problem.

There is one more thing that I do not understand. The real problem, as Erickson effectively noted, was this:

You cannot specify a wildcard when constructing a parameterized type

But deleting the template in the declaration didn't work either:

final IterableEnumeration<ZipEntry> iteratable 
                  = new IterableEnumeration<ZipEntry>(zipFile.entries());

Results with the following error:

Main.java:19: cannot find symbol
symbol  : constructor IterableEnumeration(java.util.Enumeration<capture#469 of ? extends java.util.zip.ZipEntry>)
location: class IterableEnumeration<java.util.zip.ZipEntry>
        final IterableEnumeration<ZipEntry> iteratable = new IterableEnumeration<ZipEntry>(  zipFile.entries());
                                                         ^
1 error

But the samples in the JavaSpecialist work:

  IterableEnumeration<String> ie =
              new IterableEnumeration<String>(sv.elements());

The only difference I can notice is that the JavaSpecialists blog Enumerationcomes from Vector, whose signature is:

public Enumeration<E> elements()

, , ZipFile, :

public Enumeration<? extends ZipEntry> entries()

, for-each ,

for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() ))  {
    if(!(entry.isDirectory())) {
        names.add(entry.getName());
    }
}

!! , , , !!

, :

?

IterableEnumeration, Enumeration, <? extends SomeClass>? make ?!!!

:

for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() ))  {

?

final IterableEnumeration<ZipEntry> iteratable
                     = IterableEnumeration.make( zipFile.entries() );

() TofuBeer:

import java.util.Enumeration;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;
import java.util.Vector;

public class Main {
    private ZipFile zipFile;

    public Set<String> entries() {

        final Vector<ZipEntry>    vector = new Vector<ZipEntry>();
        // why this works.
        //final IterableEnumeration<ZipEntry> iteratable = new IterableEnumeration<ZipEntry>( vector.elements() );

        // but this do not.
        //final IterableEnumeration<ZipEntry> iteratable = new IterableEnumeration<ZipEntry>( zipFile.entries() );

        // nor this 
        final IterableEnumeration<ZipEntry> iteratable = IterableEnumeration.make( zipFile.entries() );

        // And what with the for-each that doesn't care about the type?    
        final Set<String>   names = new HashSet<String>();

        for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() ))  {
            if(!(entry.isDirectory())) {
                names.add(entry.getName());
            }
        }

        return (names);
    }
}

class IterableEnumeration<T> implements Iterable<T> {
    private final Enumeration<T> enumeration;

    public IterableEnumeration(final Enumeration<T> e) {
        enumeration = e;
    }

    public Iterator<T> iterator() {
        return new Iterator<T>() { 
             public boolean hasNext() {
                return (enumeration.hasMoreElements());
            }

            public T next() {
                return (enumeration.nextElement());
            }

            public void remove() {
                throw new UnsupportedOperationException("Cannot remove via an Enumeration");
            }
        };
    }
    // As suggested by http://www.javaspecialists.eu/archive/Issue107.html
    // but doesn't help with: final IterableEnumeration<ZipEntry> iteratable = IterableEnumeration.make( zipFile.entries() );
    public static <T> Iterable<T> make(Enumeration<T> en) {
        return new IterableEnumeration<T>(en);
    }
}

!

Answer 1

, foreach, IterableEnumeration, , ZipFile.entries().

    private final Enumeration<T> enumeration;

    public IterableEnumeration(final Enumeration<T> e) {
        enumeration = e;
    }

    public static <T> Iterable<T> make(Enumeration<T> en) {
        return new IterableEnumeration<T>(en);
    }

    private final Enumeration<? extends T> enumeration;

    public IterableEnumeration(final Enumeration<? extends T> e) {
        enumeration = e;
    }

    public static <T> Iterable<T> make(Enumeration<? extends T> en) {
        return new IterableEnumeration<T>(en);
    }

Answer 2

, ZipFile , entries() methods Enumeration<? extends ZipEntry> (, JarFile Enumeration<JarEntry>). , .

Enumeration<T> ( - - ), Enumeration<? extends T>.