Sanjay Prajapat's Blog

Singleton Class

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sanjay prajapat
·

1 min read

Singleton class is a class that can have only one instance. if you create new instance it will also points to first instance.

to design singleton class

  1. make constructor as private
  2. write static function that return object of singleton class
class Singleton {
    private static Singleton instance ;

    public int prop = 900;
    // private constructor 
    private Singleton(){
        System.out.println("it's private constructor ");
    }
    public static Singleton getInstance(){
        if(instance == null){
            instance = new Singleton();
        }
        return instance;
    }

}

public class Example1 {
    public static void main(String[] args) {
        // Singleton s1= new Singleton();  // will not work this 
        Singleton s1 = Singleton.getInstance();
        System.out.println(s1.prop);
        Singleton s2 = Singleton.getInstance(); // it also point to same instance as s1 
        System.out.println(s2.prop);

        // now change prop value it reflects in both, because both points to same object 
        s2.prop = 800 ;
        System.out.println(s1.prop); // 800
        System.out.println(s2.prop);


        // referenceid
        System.out.println(s1); // Singleton@5ca881b5
        System.out.println(s2);//Singleton@5ca881b5



    }   
}
Java