I would like to test a method from class1 which calls the singleton class getInstance:
Class ivDomain {
public String method1() {
id=Singleton.getInstance().generateId()
... code
}
}
When I do the test using Junit I am getting NullPointerException on the singleton class. How can I fix this?
Classes using statically accessed singletons are a pain to test. Change you ivDomain class to rather take the singleton instance as a parameter to it's constructor and then mock it normally. You should use a Dependency Injection framework (such as Guice) going forward to make this style of development easier.
You shouldn't have to mock Singleton; just change the code so getInstance() doesn't return a null.
The most likely reason - without examining your Singleton's code - is that it's initialization fails to load some external configuration and therefore fails.
Singletons in Automatic Unit Tests can be a problem since sometimes you like them to behave differently for the specific scenario you test (e.g. you want one scenario where generateId returns -1, another when it returns 4354353 and another when it throws a RuntimeException - just so you can see how the code that uses the Singleton works. In such cases a change of design is recommended as Singletons are not amongst the favored design patterns and are somewhat regarded as anti-patterns.
