3_method_calls.md

Method (Function) Calls

You're probably familiar with functions from your math class: you pass the function an input (in f(x) this is the x), and you get an output (this is f(x), or y; in programming the output you get is often called the return value). Java has something similar, with a few twists.

Here are a couple of major differences:

1. In Java, we call functions methods. Think of them as the same thing.
2. In math, a function has one input and one output. In Java, a function may have 0 or more inputs and 0 or 1 outputs.
3. In math, functions are deterministic (for input x, f(x) will always output y) and have no side effects (pure functions). In Java, functions are not deterministic (for input x, f(x) can output anything), and have side effects (they may modify the state of other parts of the program).

You've actually already seen method calls. Remember this?

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

System.out.println is a method call in this example (we're calling a method called println). Here, it's taking a String, returning nothing (a return of nothing is also called void)

As you can see, the syntax for methods is relatively simple: <function name>(<function parameters>).

Another useful set of functions are defined in the Math class. Exponentiation can be performed with the Math.pow method (pow as in power). The method's signature is public static double pow(double base, double exponent). This means that it takes two doubles, and returns (outputs) a double. Don't worry about fully understanding every part of the method signature yet.

Here's how you would call that:

Math.pow(3.0, 2.0); // Returns 9.0

A key part of this method is that it returns a value. System.out.println returns void, which means it returns nothing. This method returns a double. This means you could do the following:

double num = Math.pow(3.0, 2.0);
System.out.println("3^2 is "+num);

And you would end up printing out 3^2 is 9.

Conclusion

Here are the PracticeIt problems for this section:

1. Method calls without parameters or returns
2. Complex method structure
3. Method parameters
4. Tricky variables and method calls Math.sqrt is square root, Math.min gets the smallest of the two numbers you pass in, and Math.round rounds the number you pass in to the closest int. When this asks for "grade =", it is asking for the value of the grade variable at that line.