接口方法Java中的泛型类型

Question

I have this method in my interface:

  public String getValue();

String is just for the example here, I don't want to return a String.

And I want to put the method in classes that will return ints, chars or bools (depending on the class). For example, I have a class Animal that implements my interface and has this method. I want it to return an int when this method is invoked in an animal. I have another class person that has this method, and when this method is invoked on a Person I want it to return a char.

public class Animal implements myInterface {
  @Override
  public int getValue() {
    return 5;
  }
}

public class Person implements myInterface {
  @Override
  public char getValue() {
    return 'c';
  }
}

I understand I have to do this with generics. But how exactly do I do it?

If I replace String in the interface with a generic type, like:

public <T> T getValue();

then it says I cannot cast from int (or char) to T.

What should I do? Thanks.

Answer 1

The proper way to do that is by having a generic interface:

public interface MyInterface<T> {
  public T getValue();
}

public class Animal implements MyInterface<Integer> {
  @Override
  public Integer getValue() {
    return 5;
  }
}

However, this really reeks like an X/Y problem; not to mention that it's highly inefficient, boxing and unboxing all those primitives.

What are you trying to achieve? What is your goal?

Answer 2

Actually

    public <T> T getValue();

can be overridden by

    public Character getValue()

This is weird ... it was for some backward-comparability mumbo jumbo. Don't do it.

---

Usually, we say return types can be covariant, ie we can return a subtype in the overriding method. Since char is a subtype of int , we may expect that this should work

    public int getValue();

    // overridden by
    public char getValue()

however, java requires that if the return type is primitive, it must be identical, no covariance there.