Singleton class in java

• In object oriented programming, singleton is a class that can have only one object(an instance of class) at a time.
• After first time, if we try to instantiate the Singleton class multiple times, the new variable also points to the first instance created.
• It is used to provide global point of access to the object. 
• In terms of practical use Singleton patterns are used in logging, caches, thread pools, configuration settings, device driver objects.

How to design a singleton class:

1. Make construct as private
2. Need to write static method which will return object of singleton class.(lazy initialization concept)

Java program on singleton class:

public class singleton {

    private static singleton ston = null;

    public String s1 = "stringvariable";

    private singleton() {
        System.out.println("private constructor");
    }

    public static singleton getInstance() {
        if (ston == null)
            ston = new singleton();

        return ston;
    }

    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();
    // changing variable of instance x
    x.s1 = (x.s1).toUpperCase();
    System.out.println(x.s1);
    System.out.println(y.s1);
  }
}

Output:
Singleton class in java
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