Java - Interfaces and methods

Question

I'm looking through some interfaces at the moment and I'm wondering why this does not work:

interface I {
    public void doSomething(String x);
}
class MyType implements I {
    public int doSomething(String x) {
        System.out.println(x);
        return(0); 
    }
}

Basically, why can't I implement the method in the interface? THey have different signatures as one has a return type? Isn't the name, parameters and return type what make a method unique?

Answer 1

You can't have different return types. Imagine the following

class Foo implements I {
  public int doSomething(String x) {
    System.out.println(x);
    return(0);
  }
}
class Bar implements I {
  public void doSomething(String x) {
    System.out.println(x);
    return;
  }
}

List<I> l = new ArrayList();
l.add(new Foo());
l.add(new Bar());

for (I i : l) {
  int x = i.doSomething();  // this makes no sense for Bar!
}

Therefore, the return types must also be the same!

Answer 2

Yeah, you're basically correct. Java doesn't allow overloading methods by return type, which would be neat. However, the interface return type must still match.

Answer 3

方法签名由方法的名称和参数类型组成，因此您不能声明多个具有相同名称，相同数量和参数类型的方法，因为编译器无法区分它们。

Answer 4

Think of a typical use for interfaces: eg anything implementing the java List interface must implement boolean add(Object o)

The caller is probably going to do something like:

if (!impl.add(o)) { /* report error */ }

If you were allowed to change the return type, you'd hit all types of problems.

void add(Object o)
if (!impl.add(o)) { // ... your method returns void, so this makes no sense

float add(Object o)
if (!impl.add(o)) { // float to boolean ? are you sure that is what you meant?