13.4.8

final

Fields and Constants

BINARY COMPATIBILITY

recompiling

Hyper

 and

Super

, and executing the resulting new binaries with the

old binary of

Test

 produces the output:

hyper

The field

h

 of

Hyper

 is output by the original binary of

main

 no matter what type

field

h

 is declared in

Super

. While this may seem surprising at first, it serves to

reduce the number of incompatibilities that occur at run time. (In an ideal world,

all source files that needed recompilation would be recompiled whenever any one

of them changed, eliminating such surprises. But such a mass recompilation is

often impractical or impossible, especially in the Internet. And, as was previously

noted, such recompilation would sometimes require further changes to the source

code.)

Deleting a field from a class will break compatibility with any pre existing

binaries that reference this field, and a

NoSuchFieldError

 will be thrown when

such a reference from a pre existing binary is linked. Only

private

 fields may be

safely deleted from a widely distributed class.

13.4.8

final

Fields and Constants

If a field that was not

final

 is changed to be

final

, then it can break compatibil 

ity with pre existing binaries that attempt to assign new values to the field. For

example, if the program:

class Super { static char s; }

class Test extends Super {

public static void main(String[] args) {

s =  a ;

System.out.println(s);

}

}

is compiled and executed, it produces the output:

a

Suppose that a new version of class

Super

 is produced:

class Super { static char s; }

If

Super

 is recompiled but not

Test

, then running the new binary with the exist 

ing binary of

Test

 results in a

IncompatibleClassChangeError

. (In certain

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