Differences between two singleton examples

Question

Class A 1

public class A {

    private static final A instance = new A();

    public static A getInstance() {
        return new A();
    }

}

Class A 2

public class A {

    private static final A instance = new A();
    private A(){}
    public static A getInstance() {
        return instance;
    }

}

I just start to learn singleton and I saw two java examples that using class A 1 example and class A 2 example. Is the class A 1 getInstance() is singleton? I also like to know what is the differences between these two class A getInstance() method? Thank you

First returns unnamed object that is different then `instance` object, whereas second code returns same object `instance` that is class level object (static). — Grijesh Chauhan, Jul 08 '13 at 07:58
"I just start to learn singleton" please stop where you are. Singletons are an almost-anti pattern and most of the times not the solution you want, especially if it has an internal state, like `String status`. — Cephalopod, Jul 08 '13 at 08:00
I suggest you use a simple enum Singleton `class A { INSTANCE; private String status; }` You only need to add methods for the status. — Peter Lawrey, Jul 08 '13 at 08:04
@gjman2 The class should be final and the instance field should be final. — Peter Lawrey, Jul 08 '13 at 08:07
Personally I only use stateless singletons if I can help it. Singletons with state are very difficult to test. — Peter Lawrey, Jul 08 '13 at 08:08

score 6 · Accepted Answer · edited May 23 '17 at 12:11

6

In A1, A is not singleton.. getInstance() is returning a new instance of A everytime

In A2, A is not singleton again, cause the default constructor is still public (implicitly) . One can easily create more instances from outside

EDIT:

Since you have edited the class A in A2, now it becomes singleton.

Here A is created eagerly and will be thread-safe by default. Check lazy vs eager intialization

edited May 23 '17 at 12:11

Community

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answered Jul 08 '13 at 07:57

sanbhat

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@gjman2 The class is not final and the instance field is not final. – Peter Lawrey Jul 08 '13 at 08:05
Thank you so much guys for helping me out. Guys, One of the benefit of singleton is to prevent from creating unnecessary objects right? – gjman2 Jul 08 '13 at 08:20

AllTooSir · Answer 2 · 2013-07-08T08:22:25.890

I also like to know what is the differences between these two class A getInstance() method

Class A 1:

If you look at the code :

 public static A getInstance() {
    return new A();
}

You are returning a new instance of A on each call of getInstance() method and hence it is not a Singleton. Also you have not made the default constructor private in this case and any code outside your class can easily create instances of the class breaking the Singleton paradigm.

Class A 2:

Looking at this code :

public class A {

  private static final A instance = new A();
  private A(){}
  public static A getInstance() {
     return instance;
  }
}

You are returning the same instance for each call of getInstance().Now your class behaves like Singleton , You are actually doing an eager instantiation of the Singleton instance here and this Singleton instance should be thread-safe. Also make the class final so that no one can sub class it and break the Singleton.

score 0 · Answer 3 · answered Jul 08 '13 at 07:58

In first case

each time a new instance of A is creating .

*in second case *

As per single ton pattern ,it should be

public class A {
   private static A instance = null;
   private  A() {

   }
   public static A getInstance() {
      if(instance == null) {
         instance = new A();
      }
      return instance;
   }
}

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

score 0 · Answer 4 · answered Jul 08 '13 at 08:00

Is the class A 1 getInstance() is singleton?

No because everytime you are calling this method, a new instance of A is created.

I also like to know what is the differences between these two class A getInstance() method?

The first getInstance() will always create a new instance of class A, the second getInstance() will always return the same instance created of class A.

score 0 · Answer 5 · answered Jul 08 '13 at 10:59

        There are two ways of creating a singleton class
        1, Dynamic Singleton
        2, Static Singleton

        Static Singleton :

        public class A {
          //Create a object of class A and make it final so that nobody can modify it
          private static final A instance = new A();

          //make the constructor private so that new objects can not be created
          private A(){}

          //provide a method to access the object
          public static A getInstance() {
             return instance;
          }
        }

        Dynamic Singleton :

a, Single Check
        public class A {
    //Create a object reference of class A
    private static A instance = null;

    //make the constructor private so that new objects can not be created
    private  A() {}


    public static A getInstance() {
        //check if instance is null or not
        if(instance == null) {
            if(instance == null) {
                //if null then create an instance of class A and assign it to the final reference
                instance = new A();
            }

        }
        return instance;
    }
}

b, double check
public class A {
    //Create a object reference of class A
    private static A instance = null;

    //make the constructor private so that new objects can not be created
    private  A() {}


    public static A getInstance() {
        //check if instance is null or not
        if(instance == null) {
            synchronized(A.class){
                //Double Check
                if(instance == null) {
                    //if null then create an instance of class A and assign it to the final reference
                    instance = new A();
                }
            }
        }
        return instance;
    }
}

Differences between two singleton examples

5 Answers5