4.3.1

Objects

TYPES, VALUES, AND VARIABLES

The sample code:

class Point { int[] metrics; }

interface Move { void move(int deltax, int deltay); }

declares a class type

Point

, an interface type

Move

, and uses an array type

int[]

(an array of

int

) to declare the field

metrics

 of the class

Point

.

4.3.1   Objects

An

object

 is a

class instance

 or an array.

The reference values (often just

references

) are

pointers

to these objects, and a

special null reference, which refers to no object.

A class instance is explicitly created by a class instance creation expression

( 15.8), or by invoking the

newInstance

 method of class

Class

 ( 20.3.8). An

array is explicitly created by an array creation expression ( 15.8).

A new class instance is implicitly created when the string concatenation oper 

ator + ( 15.17.1) is used in an expression, resulting in a new object of type

String

 ( 4.3.3,  20.12). A new array object is implicitly created when an array

initializer expression ( 10.6) is evaluated; this can occur when a class or interface

is initialized ( 12.4), when a new instance of a class is created ( 15.8), or when a

local variable declaration statement is executed ( 14.3).

Many of these cases are illustrated in the following example:

class Point {

int x, y;

Point() { System.out.println("default"); }

Point(int x, int y) { this.x = x; this.y = y; }

//

A

Point

 instance is explicitly created at class initialization time:

static Point origin = new Point(0,0);

//

A

String

 can be implicitly created by a

+

 operator:

public String toString() {

return "(" + x + "," + y + ")";

}

}

class Test {

public static void main(String[] args) {

//

A

Point

 is explicitly created using

newInstance:

Point p = null;

try {

p = (Point)Class.forName("Point").newInstance();

} catch (Exception e) {

System.out.println(e);

}

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