Singletons

Enforce The Singleton property with a private constructor or an enum type

A Singleton is simply a class that is instantiated exactly once, can have only one object (an instance of the class) at a time.

To design a singleton class:

1. Make constructor as private.

2. Write a static method that has return type object of this singleton class. Here, the concept of Lazy initialization is used to write this static method.

Example:

//Singelton with public final field
public class Elvis{
  public static final Elvis INSTANCE = new Elvis();
  
  private Elvis(){....}
  
  public void leaveTheBuilding(){....}
 }

Could also be done as follows:

// Java program implementing Singleton class 
// with getInstance() method 
class Singleton 
{ 
    // static variable single_instance of type Singleton 
    private static Singleton single_instance = null; 
  
    // variable of type String 
    public String s; 
  
    // private constructor restricted to this class itself 
    private Singleton() 
    { 
        s = "Hello I am a string part of Singleton class"; 
    } 
  
    // static method to create instance of Singleton class 
    public static Singleton getInstance() 
    { 
        if (single_instance == null) 
            single_instance = new Singleton(); 
  
        return single_instance; 
    } 
} 

Here is what the driver class looks like:

// Driver Class 
class Main 
{ 
    public static void main(String args[]) 
    { 
        // instantiating Singleton class with variable x 
        Singleton x = Singleton.getInstance(); 
  
        // instantiating Singleton class with variable y 
        Singleton y = Singleton.getInstance(); 
  
        // instantiating Singleton class with variable z 
        Singleton z = Singleton.getInstance(); 
  
        // changing variable of instance x 
        x.s = (x.s).toUpperCase(); 
  
        System.out.println("String from x is " + x.s); 
        System.out.println("String from y is " + y.s); 
        System.out.println("String from z is " + z.s); 
        System.out.println("\n"); 
  
        // changing variable of instance z 
        z.s = (z.s).toLowerCase(); 
  
        System.out.println("String from x is " + x.s); 
        System.out.println("String from y is " + y.s); 
        System.out.println("String from z is " + z.s); 
    } 
} 

Output:

String from x is HELLO I AM A STRING PART OF SINGLETON CLASS
String from y is HELLO I AM A STRING PART OF SINGLETON CLASS
String from z is HELLO I AM A STRING PART OF SINGLETON CLASS
 
 
String from x is hello i am a string part of singleton class
String from y is hello i am a string part of singleton class
String from z is hello i am a string part of singleton class

Explanation:

1. In the Singleton class, when we first time call getInstance() method, it creates an object of the class with name single_instance and return it to the variable.

2. Since single_instance is static, it is changed from null to some object.

3. Next time, if we try to call getInstance() method, since single_instance is not null, it is returned to the variable, instead of instantiating the Singleton class again. This part is done by if condition.

Implementing Singleton class with method name as that of class name

   // static method to create instance of Singleton class 
    public static Singleton Singleton() 
    { 
        // To ensure only one instance is created 
        if (single_instance == null) 
        { 
            single_instance = new Singleton(); 
        } 
        return single_instance; 
    }