Basic Java Syntax and Data Types¶

Now that your environment is set up, let's learn the fundamental building blocks of Java programs: variables, data types, and basic syntax.

Java Program Structure¶

Every Java program follows this basic structure:

public class ClassName {
    public static void main(String[] args) {
        // Your code goes here
    }
}

Let's break this down:

public class ClassName - Defines a class (the filename must match)
public static void main(String[] args) - The starting point of your program
{} - Curly braces group code together
// - Single line comments (ignored when running)

Variables and Data Types¶

Variables store data that your program can use and manipulate.

Declaring Variables¶

// Pattern: type variableName = value;
int age = 25;
String name = "Alice";
double price = 19.99;
boolean isActive = true;

Primitive Data Types¶

Java has eight primitive (basic) data types:

Type	Purpose	Example	Range
`int`	Whole numbers	`42`	-2.1 billion to 2.1 billion
`double`	Decimal numbers	`3.14`	Very large range with decimals
`boolean`	True/false values	`true`	`true` or `false`
`char`	Single characters	`'A'`	Any Unicode character
`byte`	Small whole numbers	`127`	-128 to 127
`short`	Medium whole numbers	`32000`	-32,768 to 32,767
`long`	Large whole numbers	`123456789L`	Very large range
`float`	Small decimal numbers	`3.14f`	Smaller range than double

String (Reference Type)¶

String is not a primitive type, but it's used so frequently it feels like one:

String message = "Hello, World!";
String empty = "";
String multiWord = "Java is fun";

Hands-on Practice: Variables¶

Create a file called Variables.java:

public class Variables {
    public static void main(String[] args) {
        // Declare and initialize variables
        int studentAge = 16;
        double gpa = 3.85;
        String studentName = "Emma";
        boolean isHonorStudent = true;
        char grade = 'A';

        // Print the variables
        System.out.println("Student Information:");
        System.out.println("Name: " + studentName);
        System.out.println("Age: " + studentAge);
        System.out.println("GPA: " + gpa);
        System.out.println("Honor Student: " + isHonorStudent);
        System.out.println("Grade: " + grade);
    }
}

Compile and run:

javac Variables.java
java Variables

Basic Operations¶

Arithmetic Operations¶

int a = 10;
int b = 3;

int sum = a + b;        // Addition: 13
int difference = a - b; // Subtraction: 7
int product = a * b;    // Multiplication: 30
int quotient = a / b;   // Division: 3 (integer division)
int remainder = a % b;  // Modulus (remainder): 1

String Operations¶

String firstName = "John";
String lastName = "Doe";

// String concatenation
String fullName = firstName + " " + lastName;
System.out.println(fullName); // Prints: John Doe

// String length
int nameLength = fullName.length();
System.out.println("Name has " + nameLength + " characters");

Comparison Operations¶

int x = 5;
int y = 10;

boolean isEqual = (x == y);      // false
boolean isNotEqual = (x != y);   // true
boolean isLess = (x < y);        // true
boolean isGreater = (x > y);     // false
boolean isLessOrEqual = (x <= y); // true

Input and Output¶

Output with System.out.println()¶

System.out.println("This prints with a new line");
System.out.print("This prints without a new line");
System.out.println(" - continuing on same line");

// Print variables
int number = 42;
System.out.println("The answer is: " + number);

Input with Scanner¶

import java.util.Scanner;

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

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

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

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

        scanner.close();
    }
}

Hands-on Practice: Simple Calculator¶

Create Calculator.java:

import java.util.Scanner;

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

        System.out.println("Simple Calculator");
        System.out.print("Enter first number: ");
        double num1 = scanner.nextDouble();

        System.out.print("Enter second number: ");
        double num2 = scanner.nextDouble();

        // Perform calculations
        double sum = num1 + num2;
        double difference = num1 - num2;
        double product = num1 * num2;
        double quotient = num1 / num2;

        // Display results
        System.out.println("Results:");
        System.out.println(num1 + " + " + num2 + " = " + sum);
        System.out.println(num1 + " - " + num2 + " = " + difference);
        System.out.println(num1 + " * " + num2 + " = " + product);
        System.out.println(num1 + " / " + num2 + " = " + quotient);

        scanner.close();
    }
}

Control Flow Basics¶

If Statements¶

int score = 85;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else if (score >= 70) {
    System.out.println("Grade: C");
} else {
    System.out.println("Grade: F");
}

For Loops¶

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

While Loops¶

int count = 1;
while (count <= 3) {
    System.out.println("Loop iteration: " + count);
    count++;
}

Common Patterns and Examples¶

Temperature Converter¶

import java.util.Scanner;

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

        System.out.print("Enter temperature in Celsius: ");
        double celsius = scanner.nextDouble();

        double fahrenheit = (celsius * 9.0 / 5.0) + 32;

        System.out.println(celsius + "°C = " + fahrenheit + "°F");

        scanner.close();
    }
}

Number Guessing Game¶

import java.util.Scanner;
import java.util.Random;

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

        int secretNumber = random.nextInt(10) + 1; // 1-10

        System.out.println("Guess a number between 1 and 10:");
        int guess = scanner.nextInt();

        if (guess == secretNumber) {
            System.out.println("Correct! The number was " + secretNumber);
        } else {
            System.out.println("Wrong! The number was " + secretNumber);
        }

        scanner.close();
    }
}

Key Concepts to Remember¶

Case Sensitivity: Java is case-sensitive (myVariable ≠ MyVariable)
Semicolons: Every statement ends with a semicolon (;)
Curly Braces: Use {} to group code in blocks
Naming Conventions:
Variables and methods: camelCase
Classes: PascalCase
Constants: UPPER_CASE

Common Mistakes to Avoid¶

❌ Forgetting semicolons at the end of statements
❌ Mismatching variable types (int age = "twenty")
❌ Not closing Scanner objects
❌ Using = instead of == for comparison
✅ Reading error messages carefully - they tell you exactly what's wrong

What's Next?¶

Great! You now understand Java's basic syntax and data types. Next, we'll explore Java's built-in data structures that help you work with collections of data.

Next: Data Structures →

Practice Exercises¶

Try these exercises to reinforce your learning:

Personal Info Program: Create a program that asks for and displays personal information (name, age, favorite color)
BMI Calculator: Calculate Body Mass Index using weight and height inputs
Simple Interest Calculator: Calculate simple interest given principal, rate, and time
Grade Calculator: Input multiple test scores and calculate the average grade