BINARY COMPATIBILITY

Class Body and Member Declarations

13.4.5

then the first method in the method table for

Super

 will now be

peek

, not

zero

.

Using the new binary code for

Super

 with the old binary code for

Hyper

 and

Test

 will cause the method invocation

as.zero(as)

 to dispatch to the method

peek

 in

Super

, rather than the method

zero

 in

Hyper

. This is a type violation, of

course; the argument is of type

Super

 but the parameter is of type

int

. With a few

plausible assumptions about internal data representations and the consequences of

the type violation, execution of this incorrect program might produce the output:

Super@ee300848

ee300848

A

poke

 method, capable of altering any location in memory, could be concocted

in a similar manner. This is left as an exercise for the reader.

The lesson is that a implementation of Java that lacks a verifier or fails to use

it will not maintain type safety and is, therefore, not a valid Java implementation.

13.4.5   Class Body and Member Declarations

No incompatibility with pre existing binaries is caused by adding a class member

that has the same name (for fields) or same name, signature, and return type (for

methods) as a member of a superclass or subclass. References to the original field

or method were resolved at compile time to a symbolic reference containing the

name of the class in which they were declared. This makes compiled Java code

more robust against changes than it might otherwise be. No error occurs even if

the set of classes being linked would encounter a compile time error. As an exam 

ple, if the program:

class Hyper { String h = "Hyper"; }

class Super extends Hyper { }

class Test extends Super {

public static void main(String[] args) {

String s = new Test().h;

System.out.println(s);

}

}

is compiled and executed, it produces the output:

Hyper

Suppose that a new version of class

Super

 is then compiled:

class Super extends Hyper { char h =  h ; }

If the resulting binary is used with the existing binaries for

Hyper

 and

Test

, then

the output is still:

245