Return Values

In the methods lesson, we saw that a method can return a value instead of just printing. Let's dive deeper into return values: they're one of the most powerful tools in your Java toolbox

Think of a method with a return value like a vending machine. You put something in (parameters), the machine does its thing, and it gives you something back (the return value). A void method is like a loudspeaker: it does something (plays sound) but doesn't hand you anything

public class Main {
    public static int square(int n) {
        return n * n;
    }

    public static void main(String[] args) {
        int result = square(5);
        System.out.println(result);   // 25
    }
}

Output 25

The return type int before the method name tells Java: "this method will give back an int". Inside the method, return sends that value back to whoever called it

We can return any type, not just int. Here are a few examples

public class Main {
    public static String greet(String name) {
        return "Welcome to Vice City, " + name;
    }

    public static double half(double n) {
        return n / 2.0;
    }

    public static boolean isVIP(String name) {
        return name.equals("Tommy Vercetti");
    }

    public static void main(String[] args) {
        System.out.println(greet("Lance Vance"));
        System.out.println(half(100));
        System.out.println(isVIP("Tommy Vercetti"));
        System.out.println(isVIP("Phil Cassidy"));
    }
}

Output

Welcome to Vice City, Lance Vance
50.0
true
false

Each method declares its return type right before its name: String, double, boolean. The type must match what you actually return. Java won't let you return a String from a method that promises an int

The cool thing about return values is that you can use them in expressions, just like any other value

public class Main {
    public static int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        // use the return value directly in math
        int total = add(10, 20) + add(5, 5);
        System.out.println(total);   // 40

        // use it directly in println
        System.out.println(add(100, 200));   // 300

        // use it in a condition
        if (add(2, 3) > 4) {
            System.out.println("Yep, 5 > 4");
        }
    }
}

Output

40
300
Yep, 5 > 4

You can think of a method call like add(10, 20) as being replaced by its return value. So add(10, 20) + add(5, 5) becomes 30 + 10 which is 40

A common mistake: trying to return inside a void method, or forgetting to return in a non-void method

public class Main {
    // error: void method can't return a value
    public static void doStuff() {
        return 42;   // won't compile
    }

    // error: method promises int but has no return
    public static int getNumber() {
        int x = 42;
        // forgot to return x
    }
}

Java catches both of these at compile time, before the program even runs

void vs return: when do you use which?

Use void when the method just does something (prints, modifies data, etc.)
Use a return type when the method computes something and you need the result

Tommy doesn't just run missions, he brings back the money. That's a return value. If he just causes chaos with no reward, that's void

Mission: Growing the Empire

Tommy's empire grows day by day. The stash starts at 1, and each day it multiplies by a fixed factor. You want a method that tells you how much you have after a given number of days.

Write a method that takes the factor and the number of days and returns the final stash. Compute it with a loop: start at 1, then multiply by the factor a number of times equal to the days. If 0 days have passed, the loop doesn't run at all and you're left with 1. Then, in main, store the factor in a variable named factor and the number of days in a variable named days, call the method, and print the result.

Example

For factor 2 over 3 days (2 × 2 × 2), your program should print