Difference and advantage between singleton and static

Difference between Singleton vs Static in Java

These are  few  differences between static and singleton in Java.

1) Static class provides better performance than Singleton pattern, because static methods are bonded on compile time.

2) One more difference between Singleton and static is, ability to override. Since static methods in Java cannot be overridden, they leads to inflexibility. On the other hand, you can override methods defined in Singleton class by extending it.

3) Static classes are hard to mock and consequently hard to test than Singletons, which are pretty easy to mock and thus easy to test. It’s easier to write JUnit test for Singleton than static classes, because you can pass mock object whenever Singleton is expected, e.g. into constructor or as method arguments.

4) If your requirements needs to maintain state than Singleton pattern is better choice than static class, because
maintaining  state in later case is nightmare and leads to subtle bugs.

5) Singleton classes can be lazy loaded if its an heavy object, but static class doesn't have such advantages and always eagerly loaded.

6) Many Dependency Injection framework manages Singleton quite well e.g. Spring, which makes using them very easy.

Advantage of Singleton Pattern over Static Class in Java

Main advantage of Singleton over static is that former is more object oriented than later. With Singleton, you can use Inheritance and Polymorphism to extend a base class, implement an interface and capable of providing different implementations. If we talk about java.lang.Runtime, which is a Singleton in Java, call to getRuntime() method return different implementations based on different JVM, but guarantees only one instance per JVM, had java.lang.Runtime an static class, it’s not possible to return different implementation for different JVM.