4.5.5

Variables Have Types, Objects Have Classes

TYPES, VALUES, AND VARIABLES

that is not

null

, it refers to an object or array that has a class (not a type), and that

class will necessarily be compatible with the compile time type.

Even though a variable or expression may have a compile time type that is an

interface type, there are no instances of interfaces. A variable or expression whose

type is an interface type can reference any object whose class implements ( 8.1.4)

that interface.

Here is an example of creating new objects and of the distinction between the

type of a variable and the class of an object:

public interface Colorable {

void setColor(byte r, byte g, byte b);

}

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

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 refer 

ence 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

.

48