The Java Language Specification, Third Edition

4.12.6 Types, Classes, and Interfaces TYPES, VALUES, AND VARIABLES
74
byte r, g, b;
public void setColor(byte rv, byte gv, byte bv) {
r = rv; g = gv; b = bv;
}
}
class Test {
public static void main(String[] args) {
Point p = new Point();
ColoredPoint cp = new ColoredPoint();
p = cp;
Colorable c = cp;
}
}
In this example:
• The local variable p of the method main of class Test has type Point and is
initially assigned a reference to a new instance of class Point.
• The local variable cp similarly has as its type ColoredPoint, and is initially
assigned a reference to a new instance of class ColoredPoint.
• The assignment of the value of cp to the variable p causes p to hold a reference
to a ColoredPoint object. This is permitted because ColoredPoint is a
subclass of Point, so the class ColoredPoint is assignment compatible
(§5.2) with the type Point. AColoredPoint object includes support for all
the methods of a Point. In addition to its particular fields r, g, and b, it has
the fields of class Point, namely x and y.
• The local variable c has as its type the interface type Colorable, so it can
hold a reference to any object whose class implements Colorable; specifically,
it can hold a reference to a ColoredPoint.
DISCUSSION
DRAFT
Note that an expression such as new Colorable() is not valid because it is not possible to
create an instance of an interface, only of a class.
Every array also has a class; the method getClass, when invoked for an array
object, will return a class object (of class Class) that represents the class of the
array.