Interface (two methods, different return types depending on class) Java

Question

Any class that extends an interface must implement the methods declared in the interface. Not sure if this is possible but what I want to do is the following :

Answer 1

This compiles, although I'm not sure I recommend doing things this way:

interface test {
   Object get();
}

class A implements test {
  int val;

  public A(int x) {
    val = x;
  }

  public Integer get() {
    return val;
  } 
}

class B implements test {
  String val;

  public B(String x) {
    val = x;
  }

  public String get() {
    return val;
  } 
}

It's legal in Java to override a method using a return type that is a subclass of the original method's return type. This is called "covariance". It only works on return types, not on parameters. Note that to get this to compile, I had to make get in class A return Integer and not int. Also: use implements, not extends, when implementing an interface; and make sure that you use the correct letter case when referring to variables (Val in your posted code does not work).

As I said, I'm not sure I recommend this, because in any code that declares something of type test, the result of get() is an Object, which you will have to cast to something else to make it useful. If possible, generics should be used instead. However, it's not always possible. I would think carefully about your design before writing an interface like this. It may be that an interface isn't really what you want. However, I can't tell without more details about what you're trying to accomplish.