Java泛型问题

Question

I have a class

public abstract class FakeClass<T extends MyClass> {
    protected HashMap<Character, T> myMap;

    private void myMethod(){
        myMap.put('c', /*???? I need to instatiate something of type T here.*/)
    }
}

As you can see, I can't figure out how to instantiate something of type T. Is this possible? If so can someone point me in the right direction to do it?

Thanks!

Answer 1

This can only be done by passing some information about T to myMethod() . One approach, described in this thread , is to pass a factory object for T . A special case of this, described in this thread , is to pass a Class<T> type argument to myMethod() , which can then use type.newInstance() to create a new T object using the default constructor. (This will fail if T does not have a default constructor.) The Class object serves as the factory object for T .

The reason we need all this is due to type erasure in Java.

Answer 2

Usually, the point of generics is to be able to accept an unknown-at-compile-time type as input.

You can't instantiate an unknown class (because it might not have a visible constructor). If you just need a placeholder to put into the map, you can put a null value.

Without knowing more context, there isn't much more that anyone can do.