# Lesson 3: Method 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?

```java
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:

```java
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:

```java
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](https://practiceit.cs.washington.edu/problem/view/bjp4/chapter1/e7-Mantra)
2. [Complex method structure](https://practiceit.cs.washington.edu/problem/view/bjp4/chapter1/s23-Strange)
3. [Method parameters](https://practiceit.cs.washington.edu/problem/view/bjp4/chapter3/s2-MysteryNums)
4. [Tricky variables and method calls](https://practiceit.cs.washington.edu/problem/view/bjp4/chapter3/s15-mathExpressions2)

   *`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.*