13.4.4

Superclasses and Superinterfaces

BINARY COMPATIBILITY

cannot be assigned to a variable of type

Hyper

, because

Super

 is not a subclass of

Hyper

.

It is instructive to consider what might happen without the verification step:

the program might run and print:

s

This demonstrates that without the verifier the type system could be defeated by

linking inconsistent binary files, even though each was produced by a correct Java

compiler.

As a further example, here is an implementation of a cast from a reference

type to

int

, which could be made to run in certain implementations of Java if they

failed to perform the verification process. Assume an implementation that uses

method dispatch tables and whose linker assigns offsets into those tables in a

sequential and straightforward manner. Then suppose that the following Java code

is compiled:

class Hyper { int zero(Object o) { return 0; } }

class Super extends Hyper { int peek(int i) { return i; }  }

class Test extends Super {

public static void main(String[] args) throws Throwable {

Super as = new Super();

System.out.println(as);

System.out.println(Integer.toHexString(as.zero(as)));

}

}

The assumed implementation determines that the class

Super

 has two methods:

the first is method

zero

 inherited from class

Hyper

, and the second is the method

peek

. Any subclass of

Super

 would also have these same two methods in the first

two entries of its method table. (Actually, all these methods would be preceded in

the method tables by all the methods inherited from class

Object

 but, to simplify

the discussion, we ignore that here.) For the method invocation

as.zero(as)

, the

compiler specifies that the first method of the method table should be invoked; this

is always correct if type safety is preserved.

If the compiled code is then executed, it prints something like:

Super@ee300858

0

which is the correct output. But if a new version of

Super

 is compiled, which is

the same except for the

extends

 clause:

class Super { int peek(int i) { return i; }  }

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