4.3.2

The Class

Object

TYPES, VALUES, AND VARIABLES

The example program:

class Value { int val; }

class Test {

public static void main(String[] args) {

int i1 = 3;

int i2 = i1;

i2 = 4;

System.out.print("i1==" + i1);

System.out.println(" but i2==" + i2);

Value v1 = new Value();

v1.val = 5;

Value v2 = v1;

v2.val = 6;

System.out.print("v1.val==" + v1.val);

System.out.println(" and v2.val==" + v2.val);

}

}

produces the output:

i1==3 but i2==4

v1.val==6 and v2.val==6

because

v1.val

 and

v2.val

 reference the same instance variable ( 4.5.3) in the

one

Value

 object created by the only

new

 expression, while

i1

 and

i2

 are differ 

ent variables.

See  10 and  15.9 for examples of the creation and use of arrays.

Each object has an associated lock ( 17.13), which is used by

synchronized

methods ( 8.4.3) and the

synchronized

 statement ( 14.17) to provide control

over concurrent access to state by multiple threads ( 17.12,  20.20).

4.3.2   The Class

Object

The standard class

Object

 is a superclass ( 8.1) of all other classes. A variable of

type

Object

 can hold a reference to any object, whether it is an instance of a class

or an array ( 10). All class and array types inherit the methods of class

Object

,

which are summarized here and completely specified in  20.1:

package java.lang;

public class Object {

public final Class getClass() {

. . .

 }

public String toString() {

. . .

 }

public boolean equals(Object obj) {

. . .

 }

public int hashCode() {

. . .

 }

protected Object clone()

throws CloneNotSupportedException {

. . .

 }

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