Types, Values, and Variables - The Java Language Specification - page 79

class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {
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 ini-
tially assigned a reference to a new instance of class Point .
• The local variable cp similarly has as its type ColoredPoint , and is initially as-
signed 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 referen-
ce 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 . A ColoredPoint 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 .
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. However, the expression new Color-
able() { public void setColor... } is valid because it declares an anonymous class (§ 15.9.5 )
that implements the Colorable interface.