A very simple explaned example of singleton class

Singleton.java

class Singleton {

  // This is an object of the same class, but give more attention towards
  // private :- This object cannot be accessed from outside, you have use getter (getInstance) method
  // static :-  This object is static as getInstance have to be static (Why?? You'll know) and 
  //            only static variables can be accessed in static methods
    private static Singleton INSTANCE = null;

  // Ofcourse a class is of no use if it does not have any data (So this is the data for which every thread will fight)
    private String criticalData;

  // Again look at private, (What the hell?? A private constructor), so basically it is done so that nobody should
  // be able to access this constructor from outside the class. We will use this in getInstance method. And that method
  // will expose it to the world
    private Singleton() {
        criticalData = "This should be unique and state should be universal";
    }

  // Here comes the hero of our story. THE getInstance() method.
  // So ofcourse it returns an object of the same class (how else it will return Instance :D )
  // but what about other two keywords :-
  // synchronized :- It ensures that only one thread will be calling this method at a time.
  // static       :- It ensures that an object of Singleton class is not required to invoke this method. it can be directly
  //                 access like this Singleton.getInstance()
    synchronized static Singleton getInstance() {
      
      
      // it says, create a instance of Singleton class only if it doesn't exists.
      // there is also another concept hidden in below lines it called Lazy init
      // which basiscally means that an instance of this class will be created untill
      // it is called.
        if (INSTANCE == null) {
          //create an instance if no previous instance is created
            INSTANCE = new Singleton();
        }
      
        // on this line, it will always have an instance of the class.
        return INSTANCE;
    }

    // These method need not be static (why? you tell me in comments )
    public String getString() {
        return this.criticalData;
    }

    public void setCriticalData(String value) {
        criticalData = value;
    }
}