Static Variables and Methods

A beginner often assumes that once a function is called, its result is automatically useful. Calling a function is only the first half of the job. The function computes or retrieves a value and hands that value back to the code that called it, and the calling code has to decide what happens next. If nothing receives the value, the work the function did is simply lost.

A function returns a value. Think of a function call as producing a value that must be connected to the next job in the program. The call is not finished when the function's body finishes; the value still has to travel back to the line that made the call. From there it can go to one of two direct destinations: it can be displayed, or it can be assigned to a variable. The call can also take part in a larger numerical expression, because wherever a suitable numeric literal could be written, a function call that returns that kind of value can be written instead.

The name before the dot identifies the class that supplies the member. Math.sqrt is a static method that performs a calculation and returns a value. Math.PI and Math.E are static variables that contain mathematical constants. The shared Math prefix shows their relationship to the class, while the member after the dot identifies the particular variable or method. Because these members belong to the class itself, you call them through the class name and do not need to create a Math object first.

The key fact is where these variables live. Instance variables are part of the object, not part of the class. The class describes which variables an object will have, but the values belong to each individual object. This is why two objects created from the same class can hold different data at the same time.

The same idea extends to behavior. The methods, meaning the subroutines, that an object contains are called instance methods. These are the non-static methods, and they become part of objects created from the class. Because an instance method is logically part of the object, you can think of the object as carrying its behavior along with its data: it contains both its instance variables and its instance methods in the logical sense. The source notes that the examples in its discussion do not include methods, but that methods work similarly to variables.

A common picture treats the class as a cookie cutter and every object as a full, independent copy of everything the class describes, including its behavior. Under that picture, three objects created from a class with an instance method would seem to have three separate copies of the method's code in memory. This picture breaks down for methods.

Not every method belongs to objects. The source contrasts two kinds. Static methods are part of the class and are associated with the class, not with objects. Non-static methods, which are the instance methods, become part of objects created from the class. Keeping this contrast in mind prevents a common confusion about where a method lives. Math.sqrt is the static case: you call it on the class name. An instance method is called on an object that was created from its class.