8.2.1

Examples of Inheritance

CLASSES

8.2.1.4

Accessing Members of Inaccessible Classes

Even though a class might not be declared

public

, instances of the class might be

available at run time to code outside the package in which it is declared if it has a

public

 superclass or superinterface. An instance of the class can be assigned to a

variable of such a

public

 type. An invocation of a

public

 method of the object

referred to by such a variable may invoke a method of the class if it implements or

overrides a method of the

public

 superclass or superinterface. (In this situation,

the method is necessarily declared

public

, even though it is declared in a class

that is not

public

.)

Consider the compilation unit:

package points;

public class Point {

public int x, y;

public void move(int dx, int dy) {

x += dx; y += dy;

}

}

and another compilation unit of another package:

package morePoints;

class Point3d extends points.Point {

public int z;

public void move(int dx, int dy, int dz) {

super.move(dx, dy); z += dz;

}

}

public class OnePoint {

static points.Point getOne() { return new Point3d(); }

}

An invocation

morePoints.OnePoint.getOne()

 in yet a third package would

return a

Point3d

 that can be used as a

Point

, even though the type

Point3d

 is

not available outside the package

morePoints

. The method

move

 could then be

invoked for that object, which is permissible because method

move

 of

Point3d

 is

public

 (as it must be, for any method that overrides a

public

 method must itself

be

public

, precisely so that situations such as this will work out correctly). The

fields

x

 and

y

 of that object could also be accessed from such a third package.

While the field

z

 of class

Point3d

 is

public

, it is not possible to access this

field from code outside the package

morePoints

, given only a reference to an

instance of class

Point3d

 in a variable

p

 of type

Point

. This is because the

expression

p.z

 is not correct, as

p

 has type

Point

 and class

Point

 has no field

named

z

; also, the expression

((Point3d)p).z

 is not correct, because the class

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