Same source code, Eclipse build success but Maven (javac) fails

Question

Keep getting this error when compiling using Maven:

type parameters of <X>X cannot be determined; no unique maximal instance exists for type variable X with upper bounds int,java.lang.Object

Generics type interference cannot be applied to primitive types. But I thought since Java5, boxing/unboxing mechanism works seamlessly between primitive types and wrapper classes.

In any case, the strange thing is Eclipse doesn't report any errors and happily compiles. I'm using JDK1.6.0_12. What could possibly be the problem here?

Answer 1

This issue can occur when your code is generic and it calls another method that has a generic return type. Sometimes the compiler gets confused trying to figure out how to resolve the method call / return type.

It can be resolved by adding an explicit cast to your code.

// Old code:
public T getValue() {
    return otherMethod();  // otherMethod has the signature: <RT> RT otherMethod() { ... }
}

// New code:
@SuppressWarnings("unchecked")
public T getValue() {
    return (T) otherMethod();   // the cast tells the compiler what to do.
}