Methods with Generics

[Alexander Bell]

There is a task which I don't unterstand

    
public class Test {
  public <T extends Comparable> T doSth(T x, T y) {
    return x;
  }
}
 
Test t = new Test();
// insert here possible code

Possible Code
A) Object x = t.find(123, "456");
B) int x = t.find(123, new Double(456));
C) int x = t.find(123, new Integer(456));
D) int x = (int)t.find(new Double(123), new Double(456));

In my opinion C and D are valid.
In fact A and C are valid.

So, why is A correct?

I thought you define with the first parameter the type for the metho.
So, in C the type is defined by the first parameter -> int therefore the second parameter must be an Integer as well.
For the same reason, I thougt D was correct as well.

Can anyone explain that?

mfg alex

[David Grant]

There seems to be some discrepancy in the method signature, but I'll assume it ought to be find(T x, T y)

Remember that the return type of the generic method is determined by the first common superclass of its parameters. If there is discrepancy, e.g. both could be Serializable too, the return type is determined by the assignment type.

A is correct because the first common supertype of Integer and String is Object, so the method signature becomes:

public Object find(Object x, Object y)

C is correct because both arguments are Integers (after autoboxing the first argument), and the return type of Integer can be autoboxed to an int

D is incorrect because whilst both arguments are Doubles, the return type of Double cannot be cast to an int.

Cheers,

Dave

[Alexander Bell]

Ok, I understand. It sounds plausible with your description

Thanks Dave