Methods and varargs

Key points

• A method has a return type, a name and a parameter list. void means it returns nothing.
• Overloading means same name, different parameter list. The compiler picks the match from the argument types.
• Java is always pass-by-value. Reassigning a parameter inside a method never affects the caller.
• That value can be a reference, so mutating the object a parameter points at is visible to the caller.
• Varargs int... numbers accepts zero or more arguments and arrives inside the method as an array.
• Only one varargs parameter per method, and it must come last.

Example

import java.util.ArrayList;
import java.util.List;

public class Main {

    static int sum(int... numbers) {
        int total = 0;
        for (int n : numbers) total += n;
        return total;
    }

    static String greet(String name) {
        return greet(name, "Hello");
    }

    static String greet(String name, String word) {
        return word + ", " + name + "!";
    }

    static void tryToRename(String s) {
        s = "changed";
    }

    static void addItem(List<String> cart) {
        cart.add("book");
    }

    public static void main(String[] args) {
        System.out.println("sum()        = " + sum());
        System.out.println("sum(1,2,3)   = " + sum(1, 2, 3));
        System.out.println("sum(array)   = " + sum(new int[]{10, 20}));

        System.out.println(greet("Ada"));
        System.out.println(greet("Ada", "Welcome"));

        String original = "kept";
        tryToRename(original);
        System.out.println("String arg after call: " + original);

        List<String> cart = new ArrayList<>();
        addItem(cart);
        System.out.println("List arg after call:   " + cart);
    }
}

Pass-by-value always, but that value can be a reference to a mutable object.