Example to learn about Singleton Pattern in JAVA

SingletonPatternDemo_SingletonOptionOne.java

package SingletonPatternDemo;
/*
*Example of Eager Initialization
* */
public class SingletonOptionOne {
    private static final SingletonOptionOne INSTANCE = new SingletonOptionOne();
    private SingletonOptionOne(){}
    private String messageData;
    /**
     * To get singleton instance of SingletonOptionOne class
     * */
    public  static SingletonOptionOne getInstance(){
        return  INSTANCE;
    }

    public String getMessageData() {
        return messageData;
    }

    public void setMessageData(String messageData) {
        this.messageData = messageData;
    }
}

SingletonPatternDemo_SingletonOptionOneDemo.java

package SingletonPatternDemo;
/**
 * To use object of SingletonOptionOne class - which is singleton
 * Example to consume single class instance
 * */
public class SingletonOptionOneDemo {
    public static void main(String[] args) {
        SingletonOptionOne obj1 = SingletonOptionOne.getInstance();
        obj1.setMessageData("Hello From obj1");
        System.out.println(obj1.getMessageData());

        SingletonOptionOne obj2 = SingletonOptionOne.getInstance();
        obj2.setMessageData("Hello From obj2");
        System.out.println(obj2.getMessageData());
        System.out.println(obj1.getMessageData());
    }
}

SingletonPatternDemo_SingletonOptionTwo.java

package SingletonPatternDemo;
/**
 * Learning Example: Singleton pattern - Lazy Initialization Method
 * */
public class SingletonOptionTwo {
    private static  SingletonOptionTwo INSTANCE =null;
    private SingletonOptionTwo(){}
    private String messageData;
    /**
     * To get singleton instance of SingletonOptionTwo class
     * */
    public synchronized static SingletonOptionTwo getInstance(){
        if(INSTANCE==null){
            INSTANCE = new SingletonOptionTwo();
        }
        return  INSTANCE;
    }

    public String getMessageData() {
        return messageData;
    }

    public void setMessageData(String messageData) {
        this.messageData = messageData;
    }
}

SingletonPatternDemo_SingletonOptionTwoDemo.java

package SingletonPatternDemo;

public class SingletonOptionTwoDemo {
    public static void main(String[] args) {
        SingletonOptionTwo obj1;
        obj1 = SingletonOptionTwo.getInstance();
        obj1.setMessageData("Hello from obj1 ");
        System.out.println(obj1.getMessageData());

        SingletonOptionTwo obj2;
        obj2 = SingletonOptionTwo.getInstance();
        System.out.println(obj2.getMessageData());

        obj2.setMessageData("Hello from obj2");
        System.out.println(obj1.getMessageData());
        System.out.println(obj2.getMessageData());
    }
}