Bdtjtk

This code will not compile:

class App {

    Short foo() {

        return 3;

    }



    void bar(Short s){



    }



    public static void main(String args) {

        new App().foo();

        new App().bar(3);

    }

}

with the message:

App.java:12: error: incompatible types: int cannot be converted to Short

new App().bar(3);

How come the compiler has no issues returning 3 in foo which has return type Short but will not accept 3 for the Short in the parameter list?

I found myself chasing my own tail on this question, until I read the JLS and basically just concluded that the answer is, because the creators of Java made it this way.

Regarding why new App().bar(3) fails, the JLS has one rule for method invocations using boxing:

5.3. Method Invocation Conversion

a boxing conversion (§5.1.7) optionally followed by widening reference conversion

A reference conversion in this context means:

A widening reference conversion exists from any reference type S to any reference type T, provided S is a subtype (§4.10) of T.

So, the compiler will go so far as to try:

new App().bar(new Integer(3))

But since Short and Integer are not subclass of each other (they are in fact both subclasses of Number), a widening reference conversion is not possible. So this method call fails with the compiler error you were seeing.

As for why the following is allowed:

Short foo() {

    return 3;

}

we can also look to the JLS' discussion of assignment conversion. Buried in there we find the following:

5.2. Assignment Conversion
A narrowing primitive conversion followed by a boxing conversion may be used if the type of the variable is:

• Byte and the value of the constant expression is representable in the
  type byte.


• Short and the value of the constant expression is representable in
  the type short.


• Character and the value of the constant expression is representable
  in the type char.

So the compiler can handle the foo() method as:

Short foo() {

    return Short.valueOf(3);

}