A method and system for automatically generating unit test cases for a computer program that can reproduce runtime problems. The method comprises: modifying the computer program according to one or more interested target program units in the program and possibly occurring run time problems; test executing the modified program; and automatically generating unit test cases according to the interested runtime problems occurring during the execution of the interested target program units. Wherein the modifying step adds captor code and problem detective code into the program, the captor code being configured to record the execution paths and execution contexts of the interested target program units in the program; and the problem detective code being configured to detect the interested unexpected exceptions possibly raised and the interested violations of predefined behavior rules possibly produced by the execution of the program units. The present invention further provides methods and systems for debugging and for regression testing using the above method, and a computer program testing method and system.