Why does autoboxing make some calls ambiguous in Java?

Question

I noticed today that auto-boxing can sometimes cause ambiguity in method overload resolution. The simplest example appears to be this:

public class Test {
           


        

Answer 1

The Java compiler resolves overloaded methods and constructors in phases. In the first phase [§15.12.2.2], it identifies applicable methods by subtyping [§4.10]. In this example, neither method is applicable, because int is not a subtype of Object.

In the second phase [§15.12.2.3], the compiler identifies applicable methods by method invocation conversion [§5.3], which is a combination of autoboxing and subtyping. The int argument can be converted to an Integer, which is a subtype of Object, for both overloads. The boolean argument needs no conversion for the first overload, and can be converted to Boolean, a subtype of Object, for the second. Therefore, both methods are applicable in the second phase.

Since more than one method is applicable, the compiler must determine which is most specific [§15.12.2.5]. It compares the parameter types, not the argument types, and it doesn't autobox them. Object and boolean are unrelated types, so they are considered equally specific. Neither method is more specific than the other, so the method call is ambiguous.

One way to resolve the ambiguity would be to change the boolean parameter to type Boolean, which is a subtype of Object. The first overload would always be more specific (when applicable) than the second.