Simple Functions

This lesson will be covering the basics of functions.

What you (hopefully) know

You hopefully know what a function is from various math classes over the years. As a refresher, it may look something like this:

This function’s y value is always equal to 3 times its x value.

Java Function

Let’s see what that function would look like in Java.

double f(double x) {
    return x * 3.0d;
}

The first double represents the return type. This is to signify that the function returns a decimal value. f is our name. You would usually use something more descriptive such as triple. The double x is the argument, we specify type and name. It’s the x in f(x). The { ... } shows the inside of the function, anything in here happens when the function is ‘called’. return is often the final line of functions. That’s where you put whatever you want to come out of the function, a double in our case. return additionally ends a functions processing and ‘return’s to wherever you call it from. Let’s see why we want a return.

double startNumber = 6.0d;

double endNumber = f(startNumber); // This will evaluate to 18.0

This hopefully explains functions. Some more examples are below:

// Return the absolute value of a number
double absoluteValue(double number) {
    double finalNumber = number
    if (number < 0.0d) {
        finalNumber *= -1.0d;
    }
    return finalNumber;
}

// Add 1, then divide by 2
double approachOneHalf(double number) {
    double finalNumber = number
    finalNumber += 1;
    finalNumber /= 2;
    return finalNumber;
}

boolean isEven(int number) {
    if (number % 2 == 0) {
        return true; // Notice this return will end the function early, so the return false doesn't happen if the number is even.
    }
    return false;
}

int conjecture(int number) {
    int finalNumber = number;
    if (isEven(number)) { // We can call other functions from within functions
        finalNumber /= 2;
    } else {
        finalNumber *= 3;
        finalNumber -= 1;
    }
    return finalNumber;
}