Program/Source Code:

The source code to create the singleton class is given below. The given program is compiled and executed successfully.

// Java program to create Singleton class

class Singleton {
  private static Singleton singleRef = null;

  private Singleton() {
    System.out.println("Hello from Singleton class");
  }

  public void SayHello() {
    System.out.println("Hello world");
  }

  public static Singleton getSingletonInstance() {
    if (singleRef == null)
      singleRef = new Singleton();

    return singleRef;
  }
}

class Main {
  public static void main(String args[]) {
    Singleton obj = Singleton.getSingletonInstance();

    obj.SayHello();
  }
}

Output:

Hello from Singleton class
Hello world

Explanation:

In the above program, we created a singleton class Singleton and public class Main. The Singleton class contains constructor and two methods getSingletonInstance(), SayHello(). The getSingletonInstance() method returns instance of class. The SayHello() method prints "Hello world" message.

The Main class contains a static method main(). The main() is an entry point for the program. Here, we created the object of the Singleton class and called SayHello() method.