📘 PART 1: INTRODUCTION TO JAVA

🎯 General Objectives

• Get familiar with Java syntax and program organization.
• Learn how to declare variables, use data types, conditional statements, and loops.

🧑‍🏫 Lesson 1: Java Program Structure

Basic Java project organization

• Files with .java extension contain Java source code
• Each .java file contains at least one class
• File name must match the class name containing the main method

The main method

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

Explanation

• public class HelloWorld: Defines a class named HelloWorld
• public static void main(String[] args): The main method - the entry point of the program
• System.out.println(): Statement to print to console

Naming conventions

• Class: Capitalize the first letter of each word (PascalCase) - HelloWorld, StudentManager
• Variables and methods: First letter lowercase, subsequent words capitalized (camelCase) - studentName, calculateTotal
• Constants: All uppercase, words separated by underscores - MAX_SIZE, PI_VALUE

Running the program

• Use javac command to compile source code into bytecode
• Use java command to run the program

javac HelloWorld.java  # Compile
java HelloWorld        # Run the program

• The result will be:

Hello, World!

Compiling and running programs from VS Code

• If you have installed the Java Extension Pack, you can open a .java file and press Ctrl + F5 to compile and run the program.
• The result will be displayed in the VS Code integrated terminal.
• You can refer to debugging Java in VS Code to learn how to debug Java programs.

Reading input from keyboard

• Java provides the Scanner class from the java.util package to read input data:

import java.util.Scanner;

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

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

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

        System.out.println("Your name is: " + name);
        System.out.println("Your age is: " + age);

        scanner.close(); // Close Scanner to release resources
    }
}

Result:

Enter name: Nguyen Van A
Enter age: 25
Your name is: Nguyen Van A
Your age is: 25

Basic input methods

Method	Data Type	Description
nextInt()	int	Read integer
nextDouble()	double	Read floating-point number
nextBoolean()	boolean	Read boolean value (true/false)
next()	String	Read one word (until whitespace)
nextLine()	String	Read a complete line

Common issues and solutions

1. Buffer line feed issue:

   When using nextInt(), nextDouble() or similar methods, the newline character (\n) remains in the input buffer. If you then call nextLine(), it will read this newline character instead of new input.

   int number = scanner.nextInt();     // Enter number
   scanner.nextLine();                 // Read remaining newline character
   String text = scanner.nextLine();    // Enter new text

2. Reading multiple values on one line:

   // Method 1: Input string and split
   String input = scanner.nextLine();          // Example: "10 20 30"
   String[] values = input.split(" ");         // Split string into array
   int a = Integer.parseInt(values[0]);        // 10
   int b = Integer.parseInt(values[1]);        // 20
   
   // Method 2: Use next() consecutively
   int x = scanner.nextInt();                  // Read first number
   int y = scanner.nextInt();                  // Read next number

3. Exception handling:

   When user input doesn't match the expected format, handle it to prevent program crashes:

   try {
       int number = scanner.nextInt();
   } catch (InputMismatchException e) {
       System.out.println("Please enter an integer!");
       scanner.nextLine(); // Clear invalid input
   }

4. Safe input method:

   Scanner scanner = new Scanner(System.in);
   int number = 0;
   boolean validInput = false;
   
   while (!validInput) {
       System.out.print("Enter an integer: ");
       try {
           number = Integer.parseInt(scanner.nextLine());
           validInput = true;
       } catch (NumberFormatException e) {
           System.out.println("Error: Please enter a valid integer!");
       }
   }

Important note: Always close Scanner using the close() method when no longer needed to avoid resource leaks.

Displaying data to screen

Java provides multiple ways to display data to the console:

Basic display methods

// Print with newline
System.out.println("Hello World");    // Print "Hello World" and add newline

// Print without newline
System.out.print("Hello ");           // Print "Hello " without newline
System.out.print("World");            // Print "World" continues after "Hello "

// Formatted print (similar to printf in C)
System.out.printf("Hello %s, you are %d years old", "Nam", 25);

Displaying different data types

int age = 25;
double height = 1.75;
String name = "Nguyen Van A";

// String concatenation with +
System.out.println("Name: " + name + ", Age: " + age + ", Height: " + height + "m");

// Using String.format
String info = String.format("Name: %s, Age: %d, Height: %.2fm", name, age, height);
System.out.println(info);

// Using printf
System.out.printf("Name: %s, Age: %d, Height: %.2fm\n", name, age, height);

Special format specifiers

Format	Description	Example
%d	Integer	%d → 123
%f	Float	%f → 123.456000
%.2f	Float (limit decimal places)	%.2f → 123.46
%s	String	%s → Hello
%c	Character	%c → A
%b	Boolean	%b → true
%n	Newline	%n → newline
%%	Percent sign	%% → %

Advanced formatting examples

// Number formatting
System.out.printf("Integer: %d\n", 1234);             // 1234
System.out.printf("Integer (right-aligned 8 chars): %8d\n", 1234);  // "    1234"
System.out.printf("Integer (left-aligned 8 chars): %-8d\n", 1234); // "1234    "
System.out.printf("Integer (padded with zeros): %08d\n", 1234);  // "00001234"

// Float formatting
System.out.printf("Float: %f\n", 12.34);              // 12.340000
System.out.printf("Float (rounded to 2 decimals): %.2f\n", 12.34567); // 12.35
System.out.printf("Currency: %,.2f đ\n", 1234567.89);    // 1,234,567.89 đ

// String formatting
System.out.printf("String: %s\n", "Hello");              // Hello
System.out.printf("String (uppercase): %S\n", "Hello");   // HELLO
System.out.printf("String (right-aligned 10 chars): %10s\n", "Hello"); // "     Hello"
System.out.printf("String (left-aligned 10 chars): %-10s\n", "Hello"); // "Hello     "

Format Method

Java also provides the Formatter class for string formatting:

String formatted = String.format("Hello %s, you scored %.1f points", "Nam", 8.5);
System.out.println(formatted);  // Hello Nam, you scored 8.5 points

// Date formatting
import java.util.Date;
System.out.printf("Today is: %tD\n", new Date());  // MM/dd/yy format

Note: From Java 15 onwards, you can use text blocks (multi-line strings) with the """...""" syntax:

// From Java 15+
String html = """
              <html>
                  <body>
                      <h1>Hello!</h1>
                  </body>
              </html>
              """;
System.out.println(html);

🧑‍🏫 Lesson 2: Variables and Data Types

Variable concept in Java

• Variables are memory locations to store data
• Each variable has a name, data type, and value

Primitive data types

int age = 25;                // Integer
double salary = 5000.50;     // Floating-point
char grade = 'A';            // Character
boolean isActive = true;     // Boolean value

Reference data types

String name = "Nguyen Van A";  // String
int[] numbers = {1, 2, 3, 4};  // Integer array

Data storage in memory

For primitive types, the value is stored directly in the variable. For reference types, the variable stores an address pointing to the actual data.

┌─────────────┐
│  Variable: age  │
├─────────────┤
│     25      │  ◄── Value is stored in memory cell
└─────────────┘

┌───────────────┐
│ Variable: salary  │
├───────────────┤
│    5000.50    │
└───────────────┘

┌───────────────┐
│ Variable: name    │
├───────────────┤
│  0x12AB34CD   │  ◄── Address (reference) to another memory region
└───────────────┘
    │
    ▼
┌───────────────────────┐
│  "Nguyen Van A"       │  ◄── Actual data is located in another memory region
└───────────────────────┘

Variable declaration and initialization

// Declare and initialize later
int count;
count = 10;

// Declare and initialize together
double price = 19.99;

// Declare multiple variables of the same type
int x = 1, y = 2, z = 3;

Type casting

// Implicit casting (widening) - no data loss
int num = 10;
double numDouble = num;  // 10.0

// Explicit casting (narrowing) - may lose data
double pi = 3.14;
int wholePi = (int) pi;  // 3

🧑‍🏫 Lesson 3: Operators and Expressions

• Arithmetic operators: +, -, *, /, %
• Comparison operators: ==, !=, >, <, >=, <=
• Logical operators: &&, ||, !

Arithmetic operators

int a = 10, b = 3;
int sum = a + b;        // 13
int difference = a - b; // 7
int product = a * b;    // 30
int quotient = a / b;   // 3 (integer division)
int remainder = a % b;  // 1 (remainder)

// Increment/decrement operators
int i = 5;
i++;                   // i = 6 (post-increment)
++i;                   // i = 7 (pre-increment)
i--;                   // i = 6 (post-decrement)
--i;                   // i = 5 (pre-decrement)

Assignment operators

int x = 10;
x += 5;  // x = x + 5 = 15
x -= 3;  // x = x - 3 = 12
x *= 2;  // x = x * 2 = 24
x /= 4;  // x = x / 4 = 6
x %= 4;  // x = x % 4 = 2

Comparison operators

int p = 10, q = 20;
boolean isEqual = (p == q);       // false
boolean isNotEqual = (p != q);    // true
boolean isGreater = (p > q);      // false
boolean isLess = (p < q);         // true
boolean isGreaterOrEqual = (p >= q); // false
boolean isLessOrEqual = (p <= q);    // true

Logical operators

boolean condition1 = true;
boolean condition2 = false;

boolean andResult = condition1 && condition2;  // false
boolean orResult = condition1 || condition2;   // true
boolean notResult = !condition1;               // false

Operator precedence

• Like in mathematics, operators have different precedence levels:
  • Inside parentheses before outside
  • Multiplication and division before addition and subtraction

1. Increment/decrement (++, --), negation (!)
2. Multiplication, division, modulus (*, /, %)
3. Addition, subtraction (+, -)
4. Comparison operators (<, >, <=, >=)
5. Equality operators (==, !=)
6. Logical AND (&&)
7. Logical OR (||)
8. Assignment operators (=, +=, -=, *=, /=, %=)

Precedence examples

int result = 5 + 3 * 2;  // 5 + 6 = 11 (multiply first, then add)
int result2 = (5 + 3) * 2;  // 8 * 2 = 16 (parentheses have highest precedence)

🧑‍🏫 Lesson 4: Conditional Statements

If statement

int age = 18;

// Simple if statement
if (age >= 18) {
    System.out.println("You are eligible to vote");
}

// If-else statement
if (age >= 18) {
    System.out.println("You are eligible to vote");
} else {
    System.out.println("You are not eligible to vote");
}

// If-else if-else statement
int score = 75;
if (score >= 90) {
    System.out.println("Excellent");
} else if (score >= 80) {
    System.out.println("Very Good");
} else if (score >= 70) {
    System.out.println("Good");
} else if (score >= 60) {
    System.out.println("Average");
} else {
    System.out.println("Poor");
}

Switch-case statement

int day = 3;
String dayName;

switch (day) {
    case 1:
        dayName = "Sunday";
        break;
    case 2:
        dayName = "Monday";
        break;
    case 3:
        dayName = "Tuesday";
        break;
    case 4:
        dayName = "Wednesday";
        break;
    case 5:
        dayName = "Thursday";
        break;
    case 6:
        dayName = "Friday";
        break;
    case 7:
        dayName = "Saturday";
        break;
    default:
        dayName = "Invalid day";
}
System.out.println("Today is " + dayName);  // Today is Tuesday

Switch with Java 12+ (new syntax)

int day = 3;
String dayType = switch (day) {
    case 1, 7 -> "Weekend";
    case 2, 3, 4, 5, 6 -> "Weekday";
    default -> "Invalid day";
};

Complex conditional expressions

int age = 25;
boolean hasID = true;
boolean isResident = true;

// Using AND (&&)
if (age >= 18 && hasID) {
    System.out.println("You can vote");
}

// Using OR (||)
if (isResident || age >= 65) {
    System.out.println("You get a ticket discount");
}

// Combining multiple conditions
if ((age >= 18 && hasID) || (isResident && age >= 65)) {
    System.out.println("You have special privileges");
}

Ternary operator

int age = 20;
String status = (age >= 18) ? "Adult" : "Minor";
System.out.println(status);  // Adult

🧑‍🏫 Lesson 5: Loop Statements

For loop

// Print numbers from 1 to 5
for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

// Calculate sum of numbers from 1 to 10
int sum = 0;
for (int i = 1; i <= 10; i++) {
    sum += i;
}
System.out.println("Sum: " + sum);  // Sum: 55

// Enhanced for loop (for-each) - iterate array/collection
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

• Step-by-step explanation using for loop to calculate sum from 1 to 10:
• Initial values: sum = 0, i = 1
• Loop condition: i <= 10
  • Iteration 1: i = 1, sum = 0 + 1 = 1, i++
  • Iteration 2: i = 2, sum = 1 + 2 = 3, i++
  • Iteration 3: i = 3, sum = 3 + 3 = 6, i++
  • Iteration 4: i = 4, sum = 6 + 4 = 10, i++
  • Iteration 5: i = 5, sum = 10 + 5 = 15, i++
  • Iteration 6: i = 6, sum = 15 + 6 = 21, i++
  • Iteration 7: i = 7, sum = 21 + 7 = 28, i++
  • Iteration 8: i = 8, sum = 28 + 8 = 36, i++
  • Iteration 9: i = 9, sum = 36 + 9 = 45, i++
  • Iteration 10: i = 10, sum = 45 + 10 = 55, i++
  • Iteration 11: i = 11, condition not met, exit loop
• Final result: Sum = 55

While loop

// Print numbers from 1 to 5
int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

// Find first number divisible by both 3 and 5
int num = 1;
// Check condition first, then execute
while (num <= 100) {
    if (num % 3 == 0 && num % 5 == 0) {
        System.out.println("First number divisible by both 3 and 5: " + num);
        break;
    }
    num++;
}

Do-while loop

// Print numbers from 1 to 5
int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

// Simulate menu selection
int choice;
// Execute at least once, then check condition
do {
    System.out.println("\nMenu:");
    System.out.println("1. View list");
    System.out.println("2. Add new");
    System.out.println("3. Delete");
    System.out.println("0. Exit");

    choice = 1; // Assume user selects 1

    switch (choice) {
        case 1:
            System.out.println("Displaying list...");
            break;
        case 2:
            System.out.println("Adding new...");
            break;
        case 3:
            System.out.println("Deleting...");
            break;
        case 0:
            System.out.println("Exiting...");
            break;
        default:
            System.out.println("Invalid choice!");
    }
} while (choice != 0);

Break and continue keywords

// Using break to exit loop
for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;  // Exit loop when i = 5
    }
    System.out.println(i);  // Print: 1, 2, 3, 4
}

// Using continue to skip current iteration
for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;  // Skip iteration when i = 3
    }
    System.out.println(i);  // Print: 1, 2, 4, 5
}

// Nested loops with label
outerLoop: for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (i * j > 5) {
            break outerLoop;  // Exit outer loop
        }
        System.out.println(i + " * " + j + " = " + (i * j));
    }
}

🧪 Final Project: Student Grade Management

Problem Description

Write a program that allows users to:

• Enter student name and scores for 3 subjects (Math, Physics, Chemistry)
• Calculate average score
• Classify academic performance according to criteria:
  • AVG >= 8.0 → Excellent
  • 6.5 <= AVG < 8.0 → Very Good
  • 5.0 <= AVG < 6.5 → Average
  • < 5.0 → Poor
• Print student information table and classification result

Program Output (Example)

STUDENT GRADE MANAGEMENT PROGRAM
-----------------------------------
Enter student name: Nguyen Van A
Enter Math score: 8.5
Enter Physics score: 7.5
Enter Chemistry score: 9.0

CLASSIFICATION RESULT
-----------------------------------
Student: Nguyen Van A
Math score: 8.5
Physics score: 7.5
Chemistry score: 9.0
Average score: 8.33
Classification: Excellent