Question

I am working on a Java Springframework project and using JUnit4 to test it.

My class implements Comparable and overrides the compareTo method.

In my test, when I do

@Test
Class<myClass> m = myClass.class;
Method[] methods = m.getDeclaredMethods();
assertEquals(5, methods.length); 

This test fails even though there are exactly 5 methods in myClass. When I looked in the Eclipse debugger, I see that the compareTo method gets called twice.

I can’t find an explanation for this behavior. Why does this happen?

Answer 1

The compiler generates a bridge method for compareTo()

The mechanism is explained very well in Angelika Langer’s Generics FAQ: What is a bridge method?:

What is a bridge method?

A synthetic method that the compiler
generates in the course of type
erasure. It is sometimes needed when
a type extends or implements a
parameterized class or interface. The
compiler insert bridge methods in
subtypes of parameterized supertypes
to ensure that subtyping works as
expected.