What is "override-equivalence" and how is it related to @Override?

Question

Reading the Javadoc for the @Override annotation, I came across the following rule:

If a method is annotated with this annotation type compilers are required to generate an error message unless at least one of the following conditions hold:

• The method does override or implement a method declared in a supertype.
• The method has a signature that is override-equivalent to that of any public method declared in Object.

I'm clear on the first point, but I'm unsure about the second one.

What does it mean by "override-equivalent"? How are public methods of Object special in this respect? And why is this not covered under the first criterion?

Moreover, this is only true of the Java 7+ documentation. The Java 6 doc doesn't say anything about override-equivalence. Why the change?

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Update:

After further consulting the JLS (Section 8.4.2), I found the following explanation of override-equivalence:

The signature of a method m1 is a subsignature of the signature of a method m2 if either:

• m2 has the same signature as m1, or
• the signature of m1 is the same as the erasure (§4.6) of the signature of m2.

Two method signatures m1 and m2 are override-equivalent iff either m1 is a subsignature of m2 or m2 is a subsignature of m1.

As far as I can tell, this answers the first question ("What does it mean?") and the third question ("Why doesn't the first condition cover this?").

If I understand correctly (please inform me if I don't!), there is only one case where two methods are override-equivalent and which doesn't fall under the first condition of the original question. This is the case when the erasure of the signature of the subclass method is the same as the signature of the superclass method, but not the other way around.

The second condition of the original question, then, would only come into play when we attempt to add type parameters when attempting to "override" a public method of the Object class. I tried the following simple example to test this, with an unused type parameter:

public class Foo {
    @Override
    public <T> boolean equals(Object obj) {
        return true;
    }
}

Of course, this class doesn't compile, because the method doesn't actually override the equals method and thus clashes with it. But I also still receive a compiler error for using the @Override annotation. Am I wrong in assuming that this example meets the second condition for @Override usage? Or is the compiler generating this error despite not being required to?

Answer 1

The reason for this is to allow you to use the @Override annotation in interfaces, which do not inherit from Object but implicitly declare all public methods from Object (see JLS section 9.2 interface members). You are thus allowed to declare an interface like:

interface Bar { @Override int hashCode(); }

However, you would not be allowed to declare the following interface:

interface Quux { @Override Object clone(); }

since the clone() method is not implicitly declared in an interface (it is not public).

This is described in JLS section 9.6.3.4 @Override (the Javadoc for @Override still refers to an old section number)