assertEquals doesn't work without second parameter casting

Question

Folks, why I'm getting the "The method assertEquals(String, Object, Object) is ambiguous for the type DictionaryTest" error for this JUnit test?

@Test
 public void testEditCard() {
  Integer a = 10;
  Integer b = 12;
  Integer c = 2;
  assertEquals("test", a-b, c);
 }

Adding casting assertEquals("test", (Integer)(a-b), c); resolves the problem.

Answer 1

Because of the wonders of autoboxing and -unboxing:

assertEquals("test", /* this is an int */ a-b, /* this is an Integer */ c);

Can be evaluated as

assertEquals(String, long, long);
// in this case the second parameter is unboxed
// (and the first one silently casted)

or as

assertEquals(String, Object, Object);
// in this case the first parameter is boxed

If you declare all variables as int (not Integer), there should be no ambiguity.

Answer 2

It's because the compiler can't tell if you want to call assertEquals(String, Object, Object) or assertEquals(String, long, long). Since a-b and c can be automatically coerced to long the compiler sees an ambiguity.

Your explicit cast tells the compiler that you want the Object version.

Note that in this case you could use int rather than Integer variables which would also fix the ambiguity.