Problem With Java Generic Interface passed to Generic Constructor

Question

I have an interface and a class and both compile just fine. But when I try to instantiate a Separator with an anonymous implementation of Namer It fails to compile.

private interface Namer<N>
{
    String getName(N o);
};

private static class Separator<H>
{
    HashSet<H> oldDiff;
    HashMap<String, H> newFaMap;

    public Separator(HashSet<H> older, HashSet<H> newer, Namer<H> namer)
    {
        oldDiff = new HashSet<H> (older);
        HashSet<H> newDiff = new HashSet<H> (newer);
        oldDiff.removeAll(newer);
        newDiff.removeAll(older);
        newFaMap = makeMap(newDiff, namer);
    }

    private HashMap<String, H> makeMap(HashSet<H> set, Namer<H> namer)
    {
        HashMap<String, H> map = new HashMap<String, H>();
        for (H holder : set)
        {
            map.put(namer.getName(holder), holder);
        }

        return map;
    }
}

in a method

        Namer<FAHolder> namer = new Namer<FAHolder>() {

            public String getName(FAHolder o)
            {
                return o.getName();
            }
        };

        new Separator<FAHolder>(older, newer, namer);

The compile error is:

The constructor MyClass.Separator<FAHolder>(Set<FAHolder>, Set<FAHolder>, MyClass.Namer<FAHolder>) is undefined

What have I overlooked?

score 3 · Accepted Answer · answered Feb 17 '11 at 22:56

3

It looks like the static type of older and newer is Set<FAHolder> and not HashSet<FAHolder>. Change your constructor's signature from:

public Separator(HashSet<H> older, HashSet<H> newer, Namer<H> namer)

to

public Separator(Set<H> older, Set<H> newer, Namer<H> namer)

answered Feb 17 '11 at 22:56

ide

19,942
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I was so focused on the anonymous interface I overlooked the obvious mistake. – Aaron Feb 17 '11 at 23:00
[Program to interfaces](http://stackoverflow.com/questions/383947/what-does-it-mean-to-program-to-an-interface) unless you have a need not to. – ide Feb 17 '11 at 23:02

score 2 · Answer 2 · answered Feb 17 '11 at 22:58

2

I do not know the declaration of older and newer in the constructor calling code, but why you use HashSet instead of Set in the Separator class? Maybe older and newer are declared as Set?

answered Feb 17 '11 at 22:58

Arne Burmeister

20,046
8
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score 0 · Answer 3 · answered Feb 17 '11 at 22:58

0

Your older and newer objects seem to be of type "Set" while your constructor parameters are "HashSet".

answered Feb 17 '11 at 22:58

Carles Barrobés

11,608
5
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Problem With Java Generic Interface passed to Generic Constructor

3 Answers3