CODEWITHSUNDEEP
javacollections
CPerkins
CPerkins

Reputation: 9018

Generics and collections - method undefined for type

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:

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.

Upvotes: 3

Views: 970

Answers (2)

sfussenegger
sfussenegger

Reputation: 36095

  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

Upvotes: 4

Konrad Rudolph
Konrad Rudolph

Reputation: 545588

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

Upvotes: 5

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