TYPES, VALUES, AND VARIABLES

Variables Have Types, Objects Have Classes

4.5.5

The example program:

class Point {

static int npoints;

int x, y;

Point root;

}

class Test {

public static void main(String[] args) {

System.out.println("npoints=" + Point.npoints);

Point p = new Point();

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

System.out.println("p.root=" + p.root);

}

}

prints:

npoints=0

p.x=0, p.y=0

p.root=null

illustrating the default initialization of

npoints

, which occurs when the class

Point

 is prepared ( 12.3.2), and the default initialization of

x

,

y

, and

root

, which

occurs when a new

Point

 is instantiated. See  12 for a full description of all

aspects of loading, linking, and initialization of classes and interfaces, plus a

description of the instantiation of classes to make new class instances.

4.5.5   Variables Have Types, Objects Have Classes

Every object belongs to some particular class: the class that was mentioned in the

creation expression that produced the object, the class whose class object was

used to invoke the

newInstance

 method ( 20.3.6) to produce the object, or the

String

 class for objects implicitly created by the string concatenation operator

+

( 15.17.1). This class is called the

class of the object

. (Arrays also have a class, as

described at the end of this section.) An object is said to be an instance of its class

and of all superclasses of its class.

(Sometimes a variable or expression is said to have a  run time type  but that

is an abuse of terminology; it refers to the class of the object referred to by the

value of the variable or expression at run time, assuming that the value is not

null

. Properly speaking, type is a compile time notion. A variable or expression

has a type; an object or array has no type, but belongs to a class.)

The type of a variable is always declared, and the type of an expression can be

deduced at compile time. The type limits the possible values that the variable can

hold or the expression can produce at run time. If a run time value is a reference

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