Unit-testing – How to write unit test cases

It's perfectly legit. Red, Green, Refactor.

The first test passes.

Add the second test, with a new input.

Now quickly get to green, you could add an if-else, which works fine. It passes, but you are not done yet.

The third part of Red, Green, Refactor is the most important. Refactor to remove duplication. You WILL have duplication in your code now. Two statements returning integers. And the only way to remove that duplication is to code the function correctly.

I'm not saying don't write it correctly the first time. I'm just saying it's not cheating if you don't.

"what exactly is the purpose of setUp() method in Junit that runs before each test case ?"

Isolation.

If a shared resource e.g. a fake persistent store, is used by each test case, you'll need to ensure it's in the same state at the start of each test.

Otherwise, if you don't initialise a store in that way and your tests (with the naming convention: MethodUnderTest_Condition_ExpectedBehaviour) execute in an order, say:

1: AddObject_GivenObject_AddsObjectToStore
2: GetObjectById_IdThatExistsInStore_ReturnsObject

then test 2 depends on test 1 adding the object to the store and is no longer a proper unit test. If they run in a different order, the working code of GetObjectById() will fail the unit test, giving a false negative.

Good Unit tests do not have a required order to be run.

"this dependency may cause large number of test cases to fail if something changes ."

That's a good thing, those unit tests each test something different, and will fail appropriately. If it's a small change that caused them to fail, it will be a small fix that will make the tests pass again.