Methods · Java · CodeMIRAI

A method is a named block of code you can run whenever you need it. main is a method; here's how to write your own.

Java

public class Main {
    static void greet(String name) {
        System.out.println("Hello, " + name + "!");
    }

    public static void main(String[] args) {
        greet("Asha");
        greet("Leo");
    }
}

static means the method belongs to the class itself, so main can call it directly. (Methods that belong to objects come in the classes lesson.)
void means the method doesn't return anything.
String name is a parameter: a variable that receives the value passed in, called the argument.

Returning a value

Java

public class Main {
    static double area(double width, double height) {
        return width * height;
    }

    static boolean isEven(int n) {
        return n % 2 == 0;
    }

    public static void main(String[] args) {
        double room = area(5.0, 3.5);
        System.out.println("Room: " + room + " square metres");
        System.out.println(isEven(10) + " " + isEven(7));
    }
}

The return type, here double or boolean, comes before the method's name, and return sends a value back to the caller. A method with a return type must return a value on every path through it, or it won't compile.

Methods calling methods

Java

public class Main {
    static double toFahrenheit(double celsius) {
        return celsius * 9 / 5 + 32;
    }

    static String feel(double celsius) {
        if (celsius >= 30) return "hot";
        if (celsius >= 15) return "mild";
        return "cold";
    }

    static String describe(double celsius) {
        return celsius + " C is " + toFahrenheit(celsius) + " F, which feels " + feel(celsius);
    }

    public static void main(String[] args) {
        System.out.println(describe(35));
        System.out.println(describe(20));
        System.out.println(describe(-5));
    }
}

Small methods that each do one job, combined by others, are far easier to read, test and fix than one long main. A return also ends the method immediately, which is what lets feel skip the ifs it doesn't need.

Scope

A variable declared inside a method, or inside any { } block, exists only there:

Java

static void demo() {
    int x = 5;
    if (x > 0) {
        int y = x * 2;    // y exists only inside this if
    }
    // y doesn't exist here, and x doesn't exist outside demo()
}

Pass by value

Java

public class Main {
    static void tryToChange(int number, int[] numbers) {
        number = 100;        // changes this method's copy only
        numbers[0] = 100;    // changes the array both variables refer to
    }

    public static void main(String[] args) {
        int n = 1;
        int[] arr = {1, 2, 3};
        tryToChange(n, arr);
        System.out.println(n);         // 1
        System.out.println(arr[0]);    // 100
    }
}

Java always passes a copy of the value. For a primitive like int, that's a copy of the number, so the caller's variable can't change. For an array or an object, it's a copy of the reference, so the method can change the object both refer to, but not make the caller's variable point somewhere else.