Java Singleton Pattern

Question

Edit: Answered - error was method wasn't static

I'm used the Singleton Design Pattern

 public class Singleton {
   private static final Singleton INSTANCE = new Singleton();

   // Private constructor prevents instantiation from other classes
   private Singleton() {}

   public static Singleton getInstance() {
      return INSTANCE;
   }
 }

My question is how do I create an object of class Singleton in another class?

I've tried:

Singleton singleton = new Singleton(); 
// error - constructor is private
Singleton singleton = Singleton.getInstance();
// error - non-static method cannot be referenced from a static context

What is the correct code?

Thanks, Spencer

Answer 1

Singleton singleton = Singleton.getInstance();

is the correct way. Make sure your getInstance() method is indeed static.

Since your Singleton implementation is far from being safe - your object can be instantiated via reflection, you may want to create a singleton based on enum

Answer 2

Singleton singleton = Singleton.getInstance(); should work -- that error doesn't make sense, given your code; are you sure you're reporting it correctly? (It would make sense if you had forgotten to make the getInstance method static, which you've done in your code above.)

The code you've given us for the class is correct.

Lastly, one conceptual note: First, you aren't "creating an object of class Singleton" -- that's the whole point of a Singleton. :) You're just getting a reference to the existing object.

Answer 3

This one:

 Singleton singleton = Singleton.getInstance();

should work. This is how you call static methods in Java. And the getInstance() method is declared as static. Are you sure you are using the very same Singleton class? Or maybe you have imported a class called the same in some other package.

Answer 4

1. since the constructor is private, it does not make sense to create an object using the constructor.

2. you should be using public static Singleton getInstance(), but the implementation is not very correct.

   if (instance == null) {
instance = new Singleton();
}
return instance;

This is how you should be doing it. This ensure that it creates the instance if it does not exist, or simply returns the existing instance. Your code would also do the same thing, but this add to the readability.