Creating Java Object -- Part-III

This is the third approach of creating objects:

Singleton Property with private constructor

A singleton is a class which is instantiated only once. Making a class a singleton can make it difficult to test its client, as its impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.  There are few ways of implementing a singleton.

1. By keeping a private constructor and exposing a static final member by one method:

public class SingletonTest {

    public static final SingletonTest OBJECT= new SingletonTest();

   

    private SingletonTest() {

       

    }

   

    public void callMe() {

       

    }

}

The object is initialized only once and called as SingletonTest.OBJECT.  The private constructor will make sure that there no other instance will be created.  

2. One more similar way is to keep a public static factory method in addition to the above arrangement.

public class SingletonTest {

    public static final SingletonTest OBJECT = new SingletonTest();

   

    private SingletonTest() {

       

    }

   

    public static SingletonTest getInstance() {

        return OBJECT;

    }

   

    public void callMe() {

       

    } 

}

Here also the instance gets created only once. Thanks to the private constructor.  As you can see the only difference from the previous approach is the static method (which is in bold). We can call the instance by SingletonTest.getInstance();

The advantage of this approach is it gives a clear idea that the class is singleton.  

{Courtesy: Effective Java - Joshua Bloch}