Variables and Data Types

## Learning Objectives - Declare and initialize variables - Understand primitive types - Work with reference types - Master type conversion ## Variables ### Declaration ```java int age; // Declaration age = 25; // Assignment int score = 100; // Declaration + Initialization ``` ### Naming Rules - Start with letter, underscore, or dollar sign - Can contain numbers (after first character) - Cannot use reserved words - Case-sensitive ```java int age; // Valid int _count; // Valid int $total; // Valid int 2ndPlace; // Invalid int class; // Invalid (reserved word) ``` ## Primitive Data Types ### Integer Types | Type | Size | Range | |------|------|-------| | byte | 1 byte | -128 to 127 | | short | 2 bytes | -32,768 to 32,767 | | int | 4 bytes | -2.1B to 2.1B | | long | 8 bytes | Very large | ```java byte b = 100; short s = 32000; int i = 2000000; long l = 9000000000L; // Note the L suffix ``` ### Floating-Point Types | Type | Size | |------|------| | float | 4 bytes | | double | 8 bytes | ```java float pi = 3.14f; // Note the f suffix double precise = 3.14159265359; ``` ### Character Type ```java char grade = 'A'; char symbol = '\u0041'; // Unicode ``` ### Boolean Type ```java boolean isActive = true; boolean hasPermission = false; ``` ## Reference Types ### Strings ```java String name = "Alice"; String greeting = new String("Hello"); ``` ### Arrays ```java int[] numbers = {1, 2, 3, 4, 5}; String[] names = new String[3]; ``` ### Classes ```java String name = "Alice"; // String is a class Scanner scanner = new Scanner(System.in); ``` ## Type Conversion ### Widening (Automatic) ```java int i = 100; long l = i; // int to long (automatic) double d = i; // int to double (automatic) ``` ### Narrowing (Explicit) ```java double d = 3.99; int i = (int) d; // Truncates to 3 ``` ### String to Number ```java String s = "123"; int i = Integer.parseInt(s); double d = Double.parseDouble(s); // With error handling try { int i = Integer.parseInt("abc"); } catch (NumberFormatException e) { System.out.println("Invalid number"); } ``` ### Number to String ```java int i = 42; String s = String.valueOf(i); String s2 = Integer.toString(i); String s3 = "" + i; // Concatenation ``` ## Constants ```java final double PI = 3.14159; final int MAX_SIZE = 100; // Attempting to modify results in error // PI = 3.14; // Compile error! ``` ## Variables Scope ```java public class Scope { int global = 10; // Instance variable public void method() { int local = 20; // Local variable for (int i = 0; i < 5; i++) { // Block variable // i only exists in this loop } } } ``` ## Primitive vs Reference ```java // Primitive - copy of value int a = 10; int b = a; b = 20; // a is still 10 // Reference - copy of reference int[] arr1 = {1, 2, 3}; int[] arr2 = arr1; arr2[0] = 99; // arr1[0] is also 99 (same array) ``` ## var Keyword (Java 10+) ```java var name = "Alice"; // Compiler infers String var age = 25; // Compiler infers int var list = new ArrayList(); // Long type inferred ``` ## Summary - Variables store data values - 8 primitive types: byte, short, int, long, float, double, char, boolean - Reference types: Strings, Arrays, Objects - Use `final` for constants - Widening is automatic; narrowing requires cast - `Integer.parseInt()` converts String to int