📖 Tutorial Guide

The Java Bible

A from-scratch, syllabus-mapped walkthrough of Java — from your very first Hello World to full object-oriented programs.

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👋 1. Hello, World!

Every Java program lives inside a class, and every runnable program needs exactly one main method — that's where execution starts.

Save this as Main.java, then compile it with javac Main.java and run it with java Main.

java

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Tip:The filename must match the public class name exactly (Main.java for class Main) — Java enforces this.

📦 2. Variables & Data Types

Java is statically typed — you declare the type of every variable before you use it. The IEB syllabus focuses on int, double, boolean, char and String.

java

int age = 17;
double average = 84.5;
boolean passed = true;
char grade = 'A';
String name = "Amahle";

System.out.println(name + " is " + age + " and scored " + average);

Tip:int holds whole numbers, double holds decimals. Mixing them in an expression (e.g. int / double) automatically promotes the result to double.

⌨️ 3. Reading User Input

Scanner reads from the keyboard (System.in). Always import it, and match the read method to the type you expect.

java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your mark: ");
        int mark = sc.nextInt();
        System.out.println("You scored " + mark);
    }
}

Tip:nextInt()/nextDouble() leave the trailing newline in the buffer — call sc.nextLine() once afterwards if you need to read a full line of text next.

➕ 4. Operators & Expressions

Arithmetic (+ - * / %), relational (== != < > <= >=) and logical (&& || !) operators are the backbone of every algorithm question.

java

int a = 17, b = 5;
System.out.println(a / b);   // 3  (integer division — the remainder is dropped)
System.out.println(a % b);   // 2  (modulus — the remainder)
System.out.println(a / (double) b); // 3.4 — casting forces a decimal answer

boolean canVote = (a >= 18) && (b > 0);

Tip:Integer division truncates, it doesn't round. 7 / 2 is 3, not 3.5 — a very common IEB paper trap.

🔀 5. Selection: if / else / switch

Selection statements let a program choose between different paths depending on a condition.

java

int mark = 72;

if (mark >= 80) {
    System.out.println("Distinction");
} else if (mark >= 50) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

// switch works well for a fixed set of exact values
char grade = 'B';
switch (grade) {
    case 'A': System.out.println("Excellent"); break;
    case 'B': System.out.println("Good"); break;
    default:  System.out.println("Keep working"); break;
}

🔁 6. Iteration: for, while, do-while

Use for when you know how many times to repeat, while when you don't, and do-while when the loop body must run at least once.

java

// for — counting loop
for (int i = 1; i <= 5; i++) {
    System.out.println("Line " + i);
}

// while — condition checked first
int n = 10;
while (n > 0) {
    n = n / 2;
}

// do-while — body always runs once
int choice;
Scanner sc = new Scanner(System.in);
do {
    System.out.print("Enter 0 to quit: ");
    choice = sc.nextInt();
} while (choice != 0);

🗂️ 7. Arrays

An array holds a fixed number of values of the same type, accessed by index starting at 0.

java

int[] marks = {78, 65, 91, 55, 88};

int total = 0;
for (int i = 0; i < marks.length; i++) {
    total += marks[i];
}
double average = total / (double) marks.length;

System.out.println("Average: " + average);

Tip:marks.length has no parentheses — it's a field, not a method. String's length() DOES have parentheses. This trips up almost everyone at first.

🧩 8. Methods

A method groups reusable logic behind a name. static methods belong to the class itself and don't need an object to call them.

java

public class Main {
    public static void main(String[] args) {
        System.out.println(square(6));
    }

    static int square(int n) {
        return n * n;
    }
}

🏗️ 9. OOP — Classes & Objects

A class is a blueprint; an object is an instance built from it. Fields store an object's state, methods define its behaviour.

This is the foundation of the IEB Grade 12 OOP section — cycle tests almost always start here.

java

public class Learner {
    String name;
    int mark;

    void printReport() {
        System.out.println(name + " scored " + mark);
    }
}

public class Main {
    public static void main(String[] args) {
        Learner l = new Learner();
        l.name = "Sipho";
        l.mark = 72;
        l.printReport();
    }
}

🔒 10. OOP — Constructors & Encapsulation

A constructor initialises an object when it's created. Encapsulation means keeping fields private and only exposing them through getters/setters — protecting the object's internal state.

java

public class Learner {
    private String name;
    private int mark;

    public Learner(String name, int mark) {
        this.name = name;
        this.mark = mark;
    }

    public int getMark() {
        return mark;
    }

    public void setMark(int mark) {
        if (mark >= 0 && mark <= 100) {
            this.mark = mark;
        }
    }
}

Tip:this.mark = mark refers to the object's field on the left, and the constructor parameter on the right — the this. is what tells them apart.

🌳 11. OOP — Inheritance

A subclass (extends) reuses and extends a superclass's fields and methods — modelling an "is-a" relationship.

java

public class Person {
    protected String name;

    public Person(String name) {
        this.name = name;
    }

    public void greet() {
        System.out.println("Hi, I'm " + name);
    }
}

public class Learner extends Person {
    private int grade;

    public Learner(String name, int grade) {
        super(name);
        this.grade = grade;
    }
}

Tip:super(name) must be the first line in the subclass constructor — it calls the parent's constructor before anything else runs.

🎭 12. OOP — Polymorphism

A subclass can override a method to give it a more specific implementation. The correct version runs automatically based on the object's real type.

java

public class Person {
    public void greet() {
        System.out.println("Hello.");
    }
}

public class Learner extends Person {
    @Override
    public void greet() {
        System.out.println("Hey, I'm a learner!");
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Learner(); // declared as Person, actually a Learner
        p.greet(); // prints "Hey, I'm a learner!"
    }
}

⚠️ 13. Exception Handling

try/catch lets your program recover from runtime errors — like bad user input — instead of crashing.

java

Scanner sc = new Scanner(System.in);
try {
    System.out.print("Enter a number: ");
    int n = sc.nextInt();
    System.out.println(100 / n);
} catch (ArithmeticException e) {
    System.out.println("Can't divide by zero!");
} catch (Exception e) {
    System.out.println("Something went wrong: " + e.getMessage());
}

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