是否可以为程序中的每个类编写测试用例？

Im am just getting introduced to unit testing and test driven development. Thus far, I have only used Junit as testing framework. A question which emerged and for which I have not yet found a definite answer is: how much test cases do I need to write? Do I have to write a test case for every single class in my program? Or is this a stupid question because unit testing implies testing at the lowest (i.e. class) level?Im am just getting introduced to unit testing a