Why do we need Unit test

The test is the executable specification
Focus on the purpose first, then about the possible implementations
Understand the program‘s behavior by looking at the tests – the tests tell you more than an API description, they show the dynamics, how to use the API.
You develop just enough
You get to the goal as quick as possible
You don‘t develop unnecessary code
There is no code without a test
There is no test without a user requirement
Once you get one test working, you know it is working now and forever
You use the tests as regression tests
The tests give us the courage to refactor
You can prove that everything still works after the refactoring by simply executing the tests
Code quality – only code that is tested is checked into versioning system!
Well-written tests provide documentation on expected and unexpected use
Tests are fined grained – can be run independently
Tests can be aggregated easily
Which tests fail give you a hint of where a bug was introduced
Helps ensure you made a shippable product