Lesson 13 of 20

Methods and Parameters

Organise code into reusable methods with parameters, return values, overloads, and clear responsibilities.

Learning objectives

Declare and call a method.
Pass information through parameters.
Return a result to the caller.
Understand the difference between void and value-returning methods.
Use method overloading appropriately.

1. Why use methods?

Methods break a large program into smaller named tasks. Good methods reduce duplication and make code easier to test and understand.

Example: a void method

public static void printWelcome() {
    System.out.println("Welcome to the Student System");
}

public static void main(String[] args) {
    printWelcome();
}

The method performs an action but returns no value, so its return type is void.

2. Parameters

Parameters allow the caller to supply values.

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

public static void main(String[] args) {
    greet("Amina");
    greet("Daniel");
}

3. Return values

public static double calculateTotal(double price, int quantity) {
    return price * quantity;
}

public static void main(String[] args) {
    double total = calculateTotal(19.90, 3);
    System.out.printf("Total: RM %.2f%n", total);
}

4. Method overloading

Methods may share a name when their parameter lists differ.

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

public static double add(double a, double b) {
    return a + b;
}

Mini project: reusable grade calculator

public class GradeCalculator {
    public static String getGrade(int mark) {
        if (mark >= 80) return "A";
        if (mark >= 70) return "B";
        if (mark >= 60) return "C";
        if (mark >= 50) return "D";
        return "F";
    }

    public static boolean isValidMark(int mark) {
        return mark >= 0 && mark <= 100;
    }

    public static void main(String[] args) {
        int mark = 76;

        if (isValidMark(mark)) {
            System.out.println("Grade: " + getGrade(mark));
        } else {
            System.out.println("Invalid mark.");
        }
    }
}

Common mistakes

Problem	Why it happens	How to fix it
Missing return statement	A non-void method must return the declared type on every required path.	Return a value or change the method to `void`.
Arguments in wrong order	Parameters are positional.	Check the method signature and pass values in matching order.
One giant method	Too many responsibilities are mixed together.	Extract meaningful operations into smaller methods.

Summary

Methods are the foundation of reusable program structure. In Lesson 14, you will place data and related methods together inside classes and create objects from those classes.

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