BINARY COMPATIBILITY

Superclasses and Superinterfaces

13.4.4

13.4.4   Superclasses and Superinterfaces

A

ClassCircularityError

 is thrown at load time if a class would be a super 

class of itself. Changes to the class hierarchy that could result in such a circularity

when newly compiled binaries are loaded with pre existing binaries are not rec 

ommended for widely distributed classes.

Changing the direct superclass or the set of direct superinterfaces of a class

type will not break compatibility with pre existing binaries, provided that the total

set of superclasses or superinterfaces, respectively, of the class type loses no

members.

Changes to the set of superclasses of a class will not break compatibility with

pre existing binaries simply because of uses of class variables and class methods.

This is because uses of class variables and class methods are resolved at compile

time to symbolic references to the name of the class that declares them. Such uses

therefore depend only on the continuing existence of the class declaring the vari 

able or method, not on the shape of the class hierarchy.

If a change to the direct superclass or the set of direct superinterfaces results

in any class or interface no longer being a superclass or superinterface, respec 

tively, then link time errors may result if pre existing binaries are loaded with the

binary of the modified class. Such changes are not recommended for widely dis 

tributed classes. The resulting errors are detected by the verifier of the Java Virtual

Machine when an operation that previously compiled would violate the type sys 

tem. For example, suppose that the following test program:

class Hyper { char h =  h ; }

class Super extends Hyper { char s =  s ; }

class Test extends Super {

    public static void main(String[] args) {

        Hyper h = new Super();

        System.out.println(h.h);

    }

}

is compiled and executed, producing the output:

h

Suppose that a new version of class

Super

 is then compiled:

class Super { char s =  s ; }

This version of class

Super

 is not a subclass of

Hyper

. If we then run the existing

binaries of

Hyper

 and

Test

 with the new version of

Super

, then a

VerifyError

is thrown at link time. The verifier objects because the result of

new Super()

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