Testing is completed to ensure that the software/application does what it is supposed to do

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True !! But it’s not that...The program should also not do what it is not supposed to do. Nice test cases are the only way to establish a reasonable level of confidence on the program product. The Tester should think of all possible scenarios based on the test requirements or test plan. Though there's various methods to prepare a gigantic
number of test cases like Equivalence Partioning, Boundary Value Analysis and so on,
a lot depends on one's own intuition to come up with some nice cases which reveals defects in the program.