Classes and Objects - Beginning Java 8 Fundamentals - page 252

public static T2 getInstance() {
return T2.instance;
}
/* Other code goes here */
}

You can use the T2.getInstance() method to get the reference of an object of the T2 class.
Internally, the T2 class does not create a new object every time you call the T2.getInstance()
method. Rather, it returns the same object reference for all calls to this method.

T2 t1 = T2.getInstance();
T2 t2 = T2.getInstance();

• Sometimes you want a class to have only static members. It may not make sense to create an

object of such a class. For example, the java.lang.Math class declares its constructor private .
The Math class contains static variables and static methods to perform numeric operations. It
does not make sense to create an object of the Math class.
You can also declare all constructors of a class •

private to prevent inheritance. Inheritance
lets you define a class by extending the definition of another class. If you do not want anyone
else to extend your class, one way to achieve this is to declare all constructors of your class
private . Another way to prevent your class from being extended is to declare it final . I will
discuss inheritance in detail in the chapter on inheritance.

Let's consider the class T3 whose constructor has protected access level as shown:

// T3.java
package com.jdojo.cls.p1;

public class T3 {
protected T3() {
}
}

A constructor with protected access level can be used anywhere in the same package or inside a descendant
class in any package. The class T3 is in the com.jdojo.cls.p1 package. You can write the following statement
anywhere in com.jdojo.cls.p1 package, which creates an object of the T3 class:

// Valid anywhere in the com.jdojo.cls.p1 package
new T3();

I will cover inheritance in detail later. However, to complete the discussion of a protected constructor, you will
use inheritance in the example below. Things about inheritance will be clearer when I discuss it in the chapter on
inheritance. You inherit (or extend) a class using the keyword extends . The following code creates a T3Child class by
inheriting it from the T3 class:

// T3Child.java
package com.jdojo.cls.p2;

import com.jdojo.cls.p1.T3;