There are various methods that can be used for testing the software application. These methods include:

Black Box testing: 

Black box testing is a method of testing the software application for any error without having the proper knowledge of the source code and the internal structure of the software. Black box testing is just like blind testing where we just test the application by providing the inputs and getting the outputs. Black box testing is very useful for testing the functionality of the software application. When we are providing input and checking the output, if the output is our desired output, then the function is working fine in the software, but if the function fails to provide the desired output, then there might be an error in the software. In this way, black box testing finds errors in software application.

Advantages of Black Box testing:

• Very useful for testing large applications containing large source code
• The tester don’t need to have the information about the source code or internal structure
• Testers having only limited knowledge about the software can test the application 
• Programming knowledge is not required for performing test cases
• Low complexity due to not having the knowledge of programming
• It takes very less time to produce test results

Disadvantages of Black Box testing:

• Only limited number of test cases can be performed due to limited knowledge of the software
• Test cases related to source code cannot be performed
• Difficulty occurs when creating test cases
• Errors related to source code cannot be reached because of not having the knowledge of source code
• It becomes difficult to find the cause of any error if found
• This type of testing is exhaustive

White Box Testing:

In white box testing, the tester has the complete knowledge about the internal structure of the software application, therefore can perform effective testing. White Box testing also called as glass testing, transparent testing, and open box testing. As the name suggests, that in this testing having all the information related to internal structure and the code of the software is must before performing the testing. In most cases, the developers do the white box testing, because the developers are the one who developed the software and they know very well about the structure and the code used in the software application. If any error is encountered in white box testing, the test removes the bugs and again do a single round of white box testing. After that the developer sends the software to the testing team for further rounds of testing.

Advantages of White Box testing:

• It is very useful as it gives better results of the tests because of having the knowledge of source code and internal workings
• The code optimization can be done in white-box testing approach
• The code which results in errors or faults can be easily removed as the tester has access to the source code of the application
• It can be performed by testers, end-users and developers too.

Disadvantages of White Box testing:

• To perform the white box testing, a skilled tester having the complete knowledge about the internal workings of a software is required
• Programming skills are also required for white box testing
• It is difficult to perform, as it becomes complex when it comes to test the code written for the application
• This testing cannot be performed for algorithm testing

Grey Box Testing:

This testing is the combined version of white-box testing and black-box testing. Such that having only sufficient knowledge about internal working and the source code of the application, and having the information about the inputs and results of the application, is required for performing grey box testing. Grey Box testing is very useful for web systems testing. The main use of Grey Box testing is in Integration testing and Penetration testing. Integration testing is the testing of application components in a group while Penetration testing is the testing of the software by producing those cases which can cause malicious attempts or which are vulnerable to the software. Penetration testing is used for protecting our software or system from vulnerable attacks that are done by hackers.

Advantages of Grey Box Testing:

• Grey Box testing is the combination of both white box and black box testing; therefore, it gives the benefits of both testing methods
• Partial knowledge of both white box and black box testing is required for performing grey box testing
• High-level programming skills are not required 
• Grey Box testing is system independent and language free

Disadvantages of Grey Box Testing:

• When the tester is doing testing using this strategy, tester do not have the complete access to source code as he has only partial access to source code, which makes it difficult to find errors in some cases
• Testing for distributed systems cannot be performed
• Running same tests in grey-box testing increases redundancy
• As the tester have partial access to the algorithms, he cannot perform algorithm testing 
• It is the most exhaustive and time-consuming testing

So, we can conclude that all the best testing method from all of them can be the one which fits best according to the requirements and the information provided. For instance, if no information is provided to the tester regarding internal workings and the source code of the application, then black box testing strategy suits best. White box testing is best when the tester have complete information about the functionality and the internal workings of source code. Grey code fits best when only partial knowledge about the working and limited input/output cases are provided for the testing. Hence, we can say that different testing methods have different usage.