BINARY COMPATIBILITY

Field Declarations

13.4.7

}

}

then these classes compile and

Test

 executes to produce the output:

Test

If the method

print

 in class

Point

 is changed to be

public

, and then only the

Point

 class is recompiled, and then executed with the previously existing binary

for

Test

 then no linkage error occurs, even though it is improper, at compile time,

for a

public

 method to be overridden by a

protected

 method (as shown by the

fact that the class

Test

 could not be recompiled using this new

Point

 class unless

print were changed to be

public

.)

Allowing superclasses to change

protected

 methods to be

public

 without

breaking binaries of preexisting subclasses helps make Java binaries less fragile.

The alternative, where such a change would cause a linkage error, would create

additional binary incompatibilities with no apparent benefit.

13.4.7   Field Declarations

Adding a field to a class will not break compatibility with any pre existing bina 

ries that are not recompiled, even in the case where a class could no longer be

recompiled because a field access previously referenced a field of a superclass

with an incompatible type. The previously compiled class with such a reference

will continue to reference the field declared in a superclass. Thus compiling and

executing the code:

class Hyper { String h = "hyper"; }

class Super extends Hyper { String s = "super"; }

class Test {

public static void main(String[] args) {

System.out.println(new Super().h);

}

}

produces the output:

hyper

Changing

Super

 to be defined as:

class Super extends Hyper {

String s = "super";

int h = 0;

}

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