How to create Singleton class in java?

Step 1: Create Class “SingleObject.java”
Step 2: Create a private constructor so no one can instantiate  this class.
Step 3: Create the static method getInstance and this will return the object of SingleObject class.
Step 4: Create client program and call this static method of SingleObject class this method will return the same object each time.
Step 5: We can verify the the singleton behavior by hashCode. The hasCode is same for both object.



SingleObject.java

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package com.ashish.singletontest; public class SingleObject { //create an object of SingleObject private static SingleObject instance = new SingleObject(); //make the constructor private so that this class cannot be //instantiated private SingleObject(){} //Get the only object available public static SingleObject getInstance(){ return instance; } public void showMessage(int a){ System.out.println("Hello World! : "+a); } }

SingletonPatternDemo.java

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package com.ashish.singletontest; public class SingletonPatternDemo { public static void main(String[] args) { //illegal construct //Compile Time Error: The constructor SingleObject() is not visible //SingleObject object = new SingleObject(); //Get the only object available SingleObject object = SingleObject.getInstance(); SingleObject object1 = SingleObject.getInstance(); //show the message object1.showMessage(20); object.showMessage(30); object1.showMessage(30); System.out.println(("hashCode object : "+object.hashCode()); System.out.println(("hashCode object1 : "+object1.hashCode()); System.out.println(object1==object); System.out.println(object1.equals(object)); } }

Output:

Hello World! : 20
Hello World! : 30
Hello World! : 30
hashCode object : 17008065
hashCode object1 : 17008065
true
true

Note : Observe both the hasCode are same  17008065.