Functional Testing is the process through which QAs determine if a piece of software is acting as per pre-determined requirements. It uses black-box testing techniques, in which the tester does not know the internal system logic. Functional testing is only concerned with validating if a system works as intended.
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Black box testing is a technique of software testing which examines the functionality of the software without peering into its internal structure or coding. The primary source of black box testing is a specification of requirements that are stated by the customer.
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White box testing is a testing technique, that examines the program structure and derives test data from the program logic/code.
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Positive testing is a method of checking if the application does what it is expected to do. When a software tester writes the test cases for a set of specified outputs, it is called as a positive test.
Since positive testing is carried out against the valid inputs, negative testing is just the opposite.The application is validated against the invalid inputs. The objective behind this is to make sure the system shows an error when it’s supposed to and does not show an error when it’s not supposed to.