Step 1 and Step 2 To test API : Identify the initial condition and Input/Parameter Selection

STEP 1: Identify the initial condition
The testing of an application programming interface (API) would depend largely on the environment in which it is to be tested. Hence, initial condition plays a very vital role in understanding and verifying the behavior of the API under test. the initial conditions for testing APIs can be classified as:

- Mandatory Pre-setters
The execution of an API would require some minimal state, environment. These type of initial conditions are classified under the mandatory initialization for the API. For example, a non-static member function API requires an object to be created before it could be called. This is an essential activity required for invoking the API.

- Behavioral Pre-setters
To test the specific behavior of the API, some additional environmental state is required. These types of initial conditions are called the behavioral pre-setters category of initial condition. These are optional conditions required by the API and need to be set before invoking the API under test thus influencing its behavior. Since these influence the behavior of the API under test, they are considered as additional inputs other than the parameters.

Thus, to test any application programming interface, the environment required should also be clearly understood and set up. Without these criteria, API under test might not function as required and leave the tester's job undone.

STEP 2: Input/Parameter Selection
the list of valid input parameters need to be identified to verify that the interface actually performs the tasks that it was designed for. While there is no method that ensures this behavior will be tested completely, using inputs that return quantifiable and verifiable results is the next big thing. The techniques like boundary value analysis and equivalence partitioning need to be used while trying to consider the input parameter values. The boundary values or limits that would lead to errors or exceptions needs to be identified.

It would also be helpful if the data structures and other components that use these data structures apart from the API are analyzed. The data structure can be loaded by using the other components and the API can be tested while the other component is accessing these data structures.

The availability of the source code to the testers would help in analyzing the various input values that could be possible for testing the API. It would also help in understanding the various paths which could be tested. Therefore, not only are testers required to understand the calls, but also all the constants and data types used by the interface.