Difference between static class and singleton patt

Question

What real (i.e. practical) difference exists between a static class and a singleton pattern?

Both can be invoked without instantiation, both provide only one "Instance" and neither of them is thread-safe. Is there any other difference?

Answer 1

I'm agree with this definition:

The word "single" means single object across the application life cycle, so the scope is at application level.

The static does not have any Object pointer, so the scope is at App Domain level.

Moreover both should be implemented to be thread-safe.

You can find interesting other differences about: Singleton Pattern Versus Static Class

Answer 2

Another advantage of a singleton is that it can easily be serialized, which may be necessary if you need to save its state to disc, or send it somewhere remotely.

Answer 3

As I understand the difference between a Static class and non-Static Singleton class, the static is simply a non-instantiated "type" in C#, where the Singleton is a true "object". In other words, all the static members in a static class are assigned to the type but in the Singleton are housed under the object. But keep in mind, a static class still behaves like a reference type as its not a value type like a Struct.

That means when you create a Singleton, because the class itself isnt static but its member is, the advantage is the static member inside the Singleton that refers to itself is connected to an actual "object" rather than a hollow "type" of itself. That sort of clarifies now the difference between a Static and a Non-Static Singleton beyond its other features and memory usage, which is confusing for me.

Both use static members which are single copies of a member, but the Singleton wraps the referenced member around a true instantiated "object" who's address exists in addition to its static member. That object itself has properties wherein in can be passed around and referenced, adding value. The Static class is just a type so it doesn't exist except to point to its static members. That concept sort of cemented the purpose of the Singleton vs Static Class beyond the inheritance and other issues.

Answer 4

To expand on Jon Skeet's Answer

The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that's less common IME), so you can pass around the singleton as if it were "just another" implementation.

Singletons are easier to work with when unit testing a class. Wherever you pass singletons as a parameter (constructors, setters or methods) you can instead substitute a mocked or stubbed version of the singleton.

Answer 5

Here's a good article: http://javarevisited.blogspot.com.au/2013/03/difference-between-singleton-pattern-vs-static-class-java.html

Static classes

• a class having all static methods.
• better performance (static methods are bonded on compile time)

• can't override methods, but can use method hiding. (What is method hiding in Java? Even the JavaDoc explanation is confusing)

  public class Animal {
      public static void foo() {
          System.out.println("Animal");
      }
  }
  
  public class Cat extends Animal {
      public static void foo() {  // hides Animal.foo()
          System.out.println("Cat");
      }
  }
  

Singleton

• an object that can only be instantiated once.
• methods can be overridden (Why doesn't Java allow overriding of static methods?)
• easier to mock then static methods
• better at maintaining state

In summary, I would only use static classes for holding util methods, and using Singleton for everything else.

---

Edits

• static classes are lazy loaded as well. Thanks @jmoreno (When does static class initialization happen?)

• method hiding for static classes. Thanks @MaxPeng.

Answer 6

Singleton is better approach from testing perspective. Unlike static classes , singleton could implement interfaces and you can use mock instance and inject them.

In the example below I will illustrate this. Suppose you have a method isGoodPrice() which uses a method getPrice() and you implement getPrice() as a method in a singleton.

singleton that’s provide getPrice functionality:

public class SupportedVersionSingelton {

    private static ICalculator instance = null;

    private SupportedVersionSingelton(){

    }

    public static ICalculator getInstance(){
        if(instance == null){
            instance = new SupportedVersionSingelton();
        }

        return instance;
    }

    @Override
    public int getPrice() {
        // calculate price logic here
        return 0;
    }
}

Use of getPrice:

public class Advisor {

    public boolean isGoodDeal(){

        boolean isGoodDeal = false;
        ICalculator supportedVersion = SupportedVersionSingelton.getInstance();
        int price = supportedVersion.getPrice();

        // logic to determine if price is a good deal.
        if(price < 5){
            isGoodDeal = true;
        }

        return isGoodDeal;
    }
}


In case you would like to test the method isGoodPrice , with mocking the getPrice() method you could do it by:
Make your singleton implement an interface and inject it. 



  public interface ICalculator {
        int getPrice();
    }

Final Singleton implementation:

public class SupportedVersionSingelton implements ICalculator {

    private static ICalculator instance = null;

    private SupportedVersionSingelton(){

    }

    public static ICalculator getInstance(){
        if(instance == null){
            instance = new SupportedVersionSingelton();
        }

        return instance;
    }

    @Override
    public int getPrice() {
        return 0;
    }

    // for testing purpose
    public static void setInstance(ICalculator mockObject){
        if(instance != null ){
instance = mockObject;
    }

test class:

public class TestCalculation {

    class SupportedVersionDouble implements ICalculator{
        @Override
        public int getPrice() { 
            return 1;
        }   
    }
    @Before
    public void setUp() throws Exception {
        ICalculator supportedVersionDouble = new SupportedVersionDouble();
        SupportedVersionSingelton.setInstance(supportedVersionDouble);

    }

    @Test
    public void test() {
          Advisor advidor = new Advisor();
          boolean isGoodDeal = advidor.isGoodDeal();
          Assert.assertEquals(isGoodDeal, true);

    }

}

In case we take the alternative of using static method for implementing getPrice() , it was difficult to the mock getPrice(). You could mock static with power mock, yet not all product could use it.