Module 02 — Java Fundamentals

Parts of a Java Program

Every Java program has a specific structure. Even the simplest program follows rules about how it’s organized. Let’s look at the classic first program:

// My first Java program
public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

// My first Java program — A comment. Java ignores this line. It’s for humans reading your code.

public class Hello — Defines a class named Hello. Every Java program needs at least one class, and the filename must match: Hello.java.

{ } — Curly braces mark the beginning and end of a block of code. The outer pair belongs to the class; the inner pair belongs to the method.

public static void main(String[] args) — The main method. This is where Java starts running your program. Every Java application must have one.

System.out.println("Hello, World!"); — Prints text to the screen, then moves the cursor to the next line.

Printing Output

Java gives you two ways to print text to the screen:

Example: println vs. print

System.out.println("First line");
System.out.println("Second line");
System.out.print("Stay ");
System.out.print("on same line");

First line
Second line
Stay on same line

Escape Sequences

Sometimes you need to include special characters in your output. Java uses escape sequences — a backslash followed by a character — to represent them:

System.out.println("Name:\tJava");
System.out.println("She said \"hello\"");
System.out.println("Line one\nLine two");

Name:	Java
She said "hello"
Line one
Line two

Variables and Data Types

A variable is a named storage location in the computer’s memory. Before you can use a variable in Java, you must declare it — tell Java its type and name.

dataType variableName;

int age = 20;

Primitive Data Types

Java has eight primitive (built-in) data types. Here are the ones you’ll use most often:

*The JVM typically uses more memory to store a boolean, but logically it represents a single bit of information.

Declaring Variables — Examples

int score = 95;
double price = 19.99;
char grade = 'A';
boolean passed = true;

System.out.println("Score: " + score);
System.out.println("Price: $" + price);
System.out.println("Grade: " + grade);
System.out.println("Passed: " + passed);

Score: 95
Price: $19.99
Grade: A
Passed: true

Naming Rules

Java has rules for variable names:

• Must start with a letter, underscore (_), or dollar sign ($)
• After the first character, you can also use digits
• Cannot contain spaces
• Cannot be a Java reserved word (int, class, public, etc.)
• Case-sensitive — Score and score are different variables

Arithmetic Operators

Java can do math. Here are the arithmetic operators you’ll use:

int result = 15 / 4;   // result is 3, not 3.75

double result = 15.0 / 4;   // result is 3.75

Operator Precedence

Java follows the same order of operations you learned in math class:

1. Parentheses () — evaluated first
2. Multiplication, Division, Modulus * / % — left to right
3. Addition, Subtraction + - — left to right

int answer = 2 + 3 * 4;        // answer is 14 (not 20)
int answer2 = (2 + 3) * 4;      // answer2 is 20

Combined Assignment Operators

Java provides shortcuts for updating a variable using its current value:

int score = 100;
score += 25;    // score is now 125
score -= 10;    // score is now 115
score *= 2;     // score is now 230
System.out.println("Final score: " + score);

Final score: 230

Constants

A constant is a variable whose value cannot change after it’s been assigned. In Java, you declare a constant using the final keyword:

final double TAX_RATE = 0.075;
final int MAX_ATTEMPTS = 3;

final double TAX_RATE = 0.075;
double subtotal = 50.00;
double tax = subtotal * TAX_RATE;
double total = subtotal + tax;

System.out.println("Subtotal: $" + subtotal);
System.out.println("Tax:      $" + tax);
System.out.println("Total:    $" + total);

Subtotal: $50.0
Tax:      $3.75
Total:    $53.75

final int MAX = 10;
MAX = 20;  // ERROR: cannot assign a value to final variable

The String Class

A String is a sequence of characters — a word, a sentence, or any text. Unlike the primitive types, String is a class, so it starts with a capital S.

String firstName = "Alice";
String lastName = "Smith";
String fullName = firstName + " " + lastName;
System.out.println("Hello, " + fullName + "!");

Hello, Alice Smith!

The + operator joins (concatenates) strings together.

int age = 20;
System.out.println("Age: " + age);  // prints: Age: 20

Reading Input with Scanner

Most programs need to get information from the user. Java’s Scanner class reads input from the keyboard.

Three Steps to Use Scanner

Step 1: Import it at the top of your file (before the class):

import java.util.Scanner;

Step 2: Create a Scanner object inside your method:

Scanner keyboard = new Scanner(System.in);

Step 3: Use it to read different types of input:

Complete Example: Getting User Input

import java.util.Scanner;

public class Greeting {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = keyboard.nextLine();

        System.out.print("Enter your age: ");
        int age = keyboard.nextInt();

        System.out.println("Hello, " + name + "! You are " + age + " years old.");
    }
}

Enter your name: Maria
Enter your age: 19
Hello, Maria! You are 19 years old.

Comments

Comments are notes in your code that Java ignores. They help other people (and future you) understand what the code does.

// This program calculates a tip
public class TipCalc {
    public static void main(String[] args) {
        double bill = 45.00;      // original bill amount
        double tipRate = 0.20;    // 20% tip
        double tip = bill * tipRate;

        /* Display the results
           to the user */
        System.out.println("Tip: $" + tip);
        System.out.println("Total: $" + (bill + tip));
    }
}

📋 Module Summary

Vocabulary Review

Module 02 — Knowledge Check

Answer the following questions. All answers can be found in this document.

System.out.print("Hello ");
System.out.println("World");

int result = 2 + 3 * 4;

✅ Answer Key