how to create only one instance of class

[purijatin]

can you pl help me out on this problem ..i have spent a lot of time on this question but iam unable to find the answer
What if i want to write a class in such a way that only one instance of it can be created , and anyone who wants to use an instance of the class will always use that one single instance

i know i will have to use static variables and constructors some where and later once the object is created the constructor must turn private..can anyone pl help how should i overcome this problem given above

[JamesCherrill]

private static variable to hold the instance.
public static method getInstance() that creates an instance if necessary, and returns the instance. Should be synchronized if it's a multi-threaded app.
private constructor, so you can't call the constructor from outside.

This is called the "singleton" pattern, you'll find lots of info on the web.

private static MyClass currentInstance;

	private MyClass () {
		currentInstance = this;
	}

	public static synchronized MyClass  getInstance() {
                if (currentInstance == null) currentInstance = new MyClass();
		return currentInstance;
	}

[~s.o.s~]

If you can afford eager loading, a better way would be to declare your singleton instance as:

public class MySingleton {

  private static final MySingleton instance = 
    new MySingleton();

  private MySingleton() {
    // initialization code goes here
  }

  public static MySingleton getInstance() {
    return instance;
  }

  public void doSomething() {
    // do something useful
  }

}

This approach does away with all the crazy synchronization stuff and is guaranteed to work since we can be rest assured that by the time `getInstance' is called, the static field `instance' would have been initialized to a new instance of `MySingleton'.

As of Java 5, the best way to implement the Singleton Pattern is to use an `enum' [as mentioned in Effective Java 2nd Edition].

public enum MySingleton {

  SINGLETON;
  
  private MySingleton() {
    // initialization code goes here
  }

  public void doSomething() {
    // do something useful
  }

}

This provides all the serialization and synchronization for free [serialization gotchas exist when using Singletons].

[JamesCherrill]

Interesting solutions sos. In practice I suspect the majority of uses of singleton (in fact 100% of all the cases I can remember using) needed to be initialised the first time they were needed, not when the class was loaded. However, if that's not a problem, I really like the enum.

[Devendra1]

Hi code like this:

public class OnlyOne{
 private static OnlyOne one = new OnlyOne();
 private OnlyOne() {} // private constructor this can't be instantiated from outside.

public static OnlyOne getInstance(){
  return one;
}
}

// look at the code above,.. static variable will get initialised when the class get loaded, and then it will have reference value of object.
and when u call getInstance() you will get the same instance..

Try it.. this is practically used by me.. this is the solution.

// To use it OnlyOne myOne = OnlyOne.getInstance(); if i am helpful add to my reputation.

[JamesCherrill]

if i am helpful add to my reputation.

All you did is repeat part of sos's post from 2 days ago. If you want to build a reputation you need to read the earlier posts, then make a contribution by adding something new.