A computer is a machine that follows a list of instructions called a program. An Android device is a computer and an app is a program. The instructions of an app are divided into sections called methods. When we tell the device to execute a method, we say we are calling the method.

If we know how to call a method, feed information into it, and receive its results, we can treat the method as a black box: something that we can safely use even though we don’t know how it works.