8.6.7

Default Constructor

CLASSES

8.6.7   Default Constructor

If a class contains no constructor declarations, then a

default constructor

 that

takes no parameters is automatically provided:

If the class being declared is the primordial class

Object

, then the default

constructor has an empty body.

Otherwise, the default constructor takes no parameters and simply invokes the

superclass constructor with no arguments.

A compile time error occurs if a default constructor is provided by the com 

piler but the superclass does not have a constructor that takes no arguments.

If the class is declared

public

, then the default constructor is implicitly given

the access modifier

public

 ( 6.6); otherwise, the default constructor has the

default access implied by no access modifier. Thus, the example:

public class Point {

int x, y;

}

is equivalent to the declaration:

public class Point {

int x, y;

public Point() { super(); }

}

where the default constructor is

public

 because the class

Point

 is

public

.

8.6.8   Preventing Instantiation of a Class

A class can be designed to prevent code outside the class declaration from creat 

ing instances of the class by declaring at least one constructor, to prevent the cre 

ation of an implicit constructor, and declaring all constructors to be

private

. A

public

 class can likewise prevent the creation of instances outside its package by

declaring at least one constructor, to prevent creation of a default constructor with

public

 access, and declaring no constructor that is

public

.

Thus, in the example:

class ClassOnly {

private ClassOnly() { }

static String just = "only the lonely";

}

the class

ClassOnly

 cannot be instantiated, while in the example:

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