Java 26-Day Course - Day 2: Variables and Data Types

Day 2: Variables and Data Types

A variable is a name attached to a memory space that stores data. Java is a strongly typed language, so you must specify the data type when declaring a variable. Types are broadly divided into 8 primitive types and reference types.

Primitive Types

Java’s 8 primitive types store the value itself directly in memory.

public class PrimitiveTypes {
    public static void main(String[] args) {
        // Integer types
        byte smallNum = 127;           // 1 byte, -128 to 127
        short mediumNum = 32000;       // 2 bytes
        int number = 2_000_000_000;    // 4 bytes (most commonly used)
        long bigNum = 9_000_000_000L;  // 8 bytes, L suffix required

        // Floating-point types
        float pi = 3.14f;             // 4 bytes, f suffix required
        double precise = 3.141592653; // 8 bytes (default floating-point)

        // Character type
        char letter = 'A';            // 2 bytes, Unicode
        char korean = '\uAC00';       // Korean characters supported

        // Boolean type
        boolean isJavaFun = true;     // true or false

        System.out.println("Integer: " + number);
        System.out.println("Double: " + precise);
        System.out.println("Char: " + letter);
        System.out.println("Boolean: " + isJavaFun);
    }
}

Variable Declaration and Initialization

Variables can be initialized at the time of declaration or assigned a value later.

public class VariableExample {
    public static void main(String[] args) {
        // Declaration with initialization
        int age = 25;
        String name = "John";

        // Declaration first, initialization later
        double salary;
        salary = 3500.50;

        // Constants: use the final keyword (value cannot be changed)
        final int MAX_SCORE = 100;
        final String COMPANY = "NITEU";
        // MAX_SCORE = 200; // Compile error!

        // var keyword (Java 10+): type inference
        var message = "Type is inferred"; // Inferred as String
        var count = 42;                    // Inferred as int

        System.out.println(name + "'s age: " + age);
        System.out.println("Salary: " + salary);
        System.out.println("Max score: " + MAX_SCORE);
    }
}

Reference Types and String

Reference types store the memory address of an object. String is the most commonly used reference type.

public class ReferenceTypeExample {
    public static void main(String[] args) {
        // String is a reference type but gets special treatment
        String greeting = "Hello";
        String language = new String("Java");

        // String comparison
        String a = "hello";
        String b = "hello";
        String c = new String("hello");

        System.out.println(a == b);       // true (same literal pool)
        System.out.println(a == c);       // false (different objects)
        System.out.println(a.equals(c));  // true (content comparison)

        // null: default value for reference types (points to no object)
        String empty = null;
        // empty.length(); // NullPointerException!
    }
}

Variable Scope

A variable is only valid within the block {} where it is declared.

public class ScopeExample {
    // Class variable (static): shared across all instances
    static int classVar = 10;

    // Instance variable: separate for each object
    int instanceVar = 20;

    public static void main(String[] args) {
        // Local variable: valid only within the method
        int localVar = 30;

        {
            // Block variable: valid only within this block
            int blockVar = 40;
            System.out.println(blockVar); // Accessible
        }
        // System.out.println(blockVar); // Compile error!

        System.out.println("Class variable: " + classVar);
        System.out.println("Local variable: " + localVar);
    }
}

Today’s Exercises

1. Personal Information Storage: Write a program that stores a name (String), age (int), height (double), and marital status (boolean) in variables and prints each on a separate line.

2. Data Type Size Comparison: Write a program that prints Byte.MAX_VALUE, Short.MAX_VALUE, Integer.MAX_VALUE, and Long.MAX_VALUE to compare the maximum values of each integer type.

3. String Equality Experiment: Experiment with the difference between == and .equals(). Create two objects using new String("test"), compare them with both == and .equals(), print the results, and explain why they differ in comments.