Why Unit Testing

• Software development is a Team work nowadays. So once code written by one team member should not get break by other, Junit can prevent it.
• Agile promotes TDD Test Driven Development. It means write test code first and then write production ready code.
• A fix done for defect should not introduces new error in code.
• Mistake is part of human nature. Even a good programmer can write wrong code. But Junit test alerts to fix it timely.
• TDD (Test Driven Design) helps to make code simple and reusable , it helps in designing of code.
• Making test cases automates will reduce lot of effort to test code.
• It helps to understand existing code.
• Unit testing must be written before writing code and should be maintained through development life cycle.
• Also it reduces development cost by providing test failure reports.

Unit testing provides below benefits

• Specification of system : like what should be input and expected output and business conditions.
• Error Detection : Timely test reports helps to catch defect .
• Good Design : Individual modules makes easy to test.
• Save cost and time of project.
• Test cases itself are documents to describe expectation of code.

Good Unit test cases are 

• Readable : Follow Java Naming conventions/comments and provides Java Doc as well.
• Fast : It should uses mock to object to get it finish soon.
• Independent and Isolated : less complex and easy to test.
• Correct : Should test exact business logic of code.
• Repeatable :For random Test data it should produce expected result even it get executed many times.