Generics and collections - method undefined for type

Question

I'm working on a project, and I'm slightly stuck on one aspect of it. I need to create a class which can manipulate collections of instances of other classes. As part of this, it creates a wrapper object for each instance which has to be able to not only hold the instance, but perform certain operations on it (including equals/hashcode).

So the constructor takes not only the wrapped object, but also a function object which caller tells us can perform these operations in the way desired (which may differ from the native behavior for the contained objects).

Incidentally, I know that what I'm describing here sounds like I'm reinventing part of the Collections framework, but I've simplified here.

public class MapWrapper<K,V> {
    private class KeyWrapper<K> {
        K key;
        public KeyWrapper(K key) {
            // ...
        }
    }
    private class ValueWrapper<V> {
        V value;
        public ValueWrapper(V value) {
            // ...
        }
    }

    // ...
    HashMap<KeyWrapper<K>, ValueWrapper<V>> map 
            = new HashMap<KeyWrapper<K>, ValueWrapper<V>> ();
    // ...

Everything seems to be okay so far, but I seem to be unable to add entries to my contained map:

    public MapWrapper (HashMap<K, V> map) {
        // ...
        map.put(new KeyWrapper<K>(key), new ValueWrapper<V>(val));
        // ...
    }

This fails compilation, with this message:

"The method put(K, V) in the type HashMap<K,V> is not applicable for the arguments (HashPlus.KeyWrapper, HashPlus.ValueWrapper)

And I'm at a loss. Is this type erasure at work? I wouldn't think so - it's all in the same compilation unit, and it's not dropping to HashMap<Object,Object> - it's dropping to HashMap<K,V>, which seems odd.

Thoughts? Pointers to glaring omissions?

Update and solution: Based on recommendations from several responders, I replaced the original "add" line with:

        this.map.put(new KeyWrapper<K>(key), new ValueWrapper<V>(val));

And this sorted my problem. Note: one responder suggested that I don't need to parameterize KeyWrapper and ValueWrapper on the put line, but removing that yields "raw type" warnings, so I have left them in.

Thanks to all.

Answer 1

The error says it all. The method you’re looking for is called put, not add.

Answer 2

1. it's put(..) not add(..)
2. you're trying to add KeyWrapper and ValueWrapper to the map parameter, not the map field. Use this.map.put(..)
3. you don't need to parameterize KeyWrapper and ValueWrapper. K and V are already available through MapWrapper