How be implement a singleton class in java ?

Singleton pattern used when we want to allow only a single instance of a class can be created inside our application. Using this pattern ensures that a class only have a single instance by protecting the class creation process, by setting the class constructor into private access modifier.

To get the class instance, the singleton class can provide a method for example a getInstance() method, this will be the only method that can be accessed to get the instance.

package org.kodejava.example.pattern.factory;

public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {
    }

    public static synchronized Singleton getInstance() {
        return instance;
    }

    public void doSomething() {
        for (int i = 0; i < 10; i++) {
            System.out.println("i = " + i);
        }
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException("Clone is not allowed.");
    }
}

There are some rules that need to be followed when we want to implement a singleton.

1. From the example code above you can see that a singleton has a static variable to keep it sole instance.
2. You need to set the class constructor into private access modifier. By this you will not allowed any other class to create an instance of this singleton because they have no access to the constructor.
3. Because no other class can instantiate this singleton how can we use it? the answer is the singleton should provide a service to it users by providing some method that returns the instance, for example getInstance().
4. When we use our singleton in a multi threaded application we need to make sure that instance creation process not resulting more that one instance, so we add a synchronized keywords to protect more than one thread access this method at the same time.
5. It is also advisable to override the clone() method of the java.lang.Object class and throw CloneNotSupportedException so that another instance cannot be created by cloning the singleton object.

And this is how we use the Service singleton class.

package org.kodejava.example.pattern.factory;

public class SingletonDemo {
    public static void main(String[] args) throws Exception {
        //
        // Gets an instance of Service object and calls the
        // doSomething method.
        //
        Singleton service = Singleton.getInstance();
        service.doSomething();
    }


}