13.4.6

Access to Members and Constructors

BINARY COMPATIBILITY

class Super extends Hyper {

void hello() { System.out.println("hello from Super"); }

}

If

Super

 and

Hyper

 are recompiled but not

Test

, then running the new binaries

with the existing binary of

Test

 produces the output:

hello from Super

as you might expect. (A flaw in some early versions of Java caused them to print:

hello from Hyper

incorrectly.)

13.4.6   Access to Members and Constructors

Changing the declared access of a member or constructor to permit less access

may break compatibility with pre existing binaries, causing a linkage error to be

thrown when these binaries are resolved. Less access is permitted if the access

modifier is changed from default access to

private

 access; from

protected

access to default or

private

 access; or from

public

 access to

protected

,

default, or

private

 access. Changing a member or constructor to permit less

access is therefore not recommended for widely distributed classes.

Perhaps surprisingly, Java is defined so that changing a member or constructor

to be more accessible does not cause a linkage error when a subclass (already)

defines a method to have less access. So, for example, if the package

points

defines the class

Point

:

package points;

public class Point {

public int x, y;

protected void print() {

System.out.println("(" + x + "," + y + ")");

}

}

used by the

Test

 program:

class Test extends points.Point {

protected void print() { System.out.println("Test"); }

public static void main(String[] args) {

Test t = new Test();

t.print();

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