What is Top - Down Integration Testing?

In Top down approach, testing takes place from top to down.Only the top module is unit tested.After lower modules are integrated one by one until all the modules are integrated and tested.But hence lower modules are still under development,that can not be integrated at this point of time.so we use stubs to test the modules. Stubs act as temporary replacement for a called module.

Top Down Integration - flow diagram
                                                                                                                                   

                                                                                

•  First, Integration between 1, 2, 3 is tested.
• Then,Integration between module 2 and Stub 1, Stub 2 tested.
•  And also Integration between module 3 and Stub 3,Stub 4 tested.