java泛型实例变量有界集合

Question

I have below example of Java Generics where I am trying to store a processors map which can store a Processor of its object. I have added a generic type for the Processor class. I want restrict the map processors to store only the it's key class' Processor object as it's value. Is there a way to use java wildcard or type bound on the processors collections? Please note that the the map itself can store multiple object of different classes in the key.

public class GenericsExample {

    private Map<?, Processor<?>> processors = new HashMap<>();

    public <O> void parameterisedMethod(O o) {
        processors.put(o, new Processor<O>());
    }

    static class Processor<T> {
        void process(T t) {
            System.out.println(t + " processed.");
        }
    }

    public static void main(String[] args) {
    }
}

Answer 1

As @LouisWasserman noted in his comment, you can't match the key and value types of a Map in java.

To implement this, you should use Object instead of wildcards for the key type in your map declaration:

private Map<Object, Processor<?>> processors = new HashMap<>();

The compiler will now let you insert any key with any associated processor in the map. It's your job to only allow matching types to be inserted:

<T> void addProcessor(T instance, Processor<? super T> processor) {
    processors.put(instance, processor);
}

To create the illusion of type-safety when reading from the map, you will need explicit casts; and perhaps some @SuppressWarning("unchecked") annotations. Eg:

@SuppressWarnings("unchecked")
<T> Processor<T> getProcessor(T instance) {
    return (Processor<T>) processors.get(instance);
}

Answer 2

Create a ParamAndProcessor<T> class to hold the parameter and its Processor together:

class ParamAndProcessor<T> {
  T param;
  Processor<T> processor;

  // Ctor.

  void process() {
    processor.process(param);
  }
}

Then store instances of this in the map:

Map<Object, ParamAndProcessor<?>> processors;

public void parameterisedMethod(Object o) {
  processors.put(o, new Processor<>());
}

If you want to put anything into processors , it can't have ? as the key type.

Note that your parameterizedMethod doesn't actually need to be parameterized. (Nor does the Processor class, at least for what you've shown).