Why are certain Object methods not callable from default methods?

Question

When creating a default method in Java 8, certain Object methods are not callable from within the default method. For example:

interface I {
    default void m() {
        this.toString(); // works.
        this.clone(); // compile-time error, "The method clone() is undefined for the type I"
        this.finalize(); // same error as above.
    }
}

It seems that clone() and finalize() are the only methods from Object that are not allowed. Coincidently, these are the only methods of Object that are protected, but this question is in particular regard to default methods, as they will be inherited by classes that do extend java.lang.Object. What is the reason for this?

Answer 1

It's not a coincidence that the protected methods from Object are not available in a default method in an interface.

Section 9.2 of the JLS states:

If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless an abstract method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.

An interface will not inherit anything from Object, but it will implicitly declare all public Object methods. This does not include any protected methods. This explains why clone and finalize can't be called; they are not declared in an interface.