Singleton Class in Java

Singleton class provides a static method to get its static instance to outside world.

Singleton design pattern in java

• We can make constructor as private. So that We can not create an object outside of the class
• This property is useful to create singleton class in java.
• Singleton pattern helps us to keep only one instance of a class at any time
• The purpose of singleton is to control object creation by keeping private constructor.

Singleton Example

SingleObject.java

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(){
 System.out.println("Hello World!");
 }
}

SingletonPatternDemo.java

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();
//show the message
 object.showMessage();
 }
}

Output

Hello World!

The ClassicSingleton class maintains a static reference to the lone singleton instance and returns that reference from the static getInstance() method

ClassicSingleton class employs a technique known as lazy instantiation to create the singleton; as a result, the singleton instance is not created until the getInstance() method is called for the first time. This technique ensures that singleton instances are created only when needed.