Variables in Java

// Variable declaration and initialization
int age;           // Declaration: creates a box labeled "age"
age = 15;          // Initialization: puts value 15 in the box
// Or do both at once
int score = 100;   // Declare and initialize together
// You can change the value later
score = 95;        // Changes the value from 100 to 95
score = score + 5; // Changes to 100 (95 + 5)

// ✅ GOOD - Clear and descriptive
int studentAge = 18;
String firstName = "Alice";
double averageScore = 85.5;
boolean isRegistered = true;
// ❌ BAD - Unclear or breaking rules
int a = 18;              // Too short, unclear
String 1name = "Bob";    // Starts with number (ERROR!)
double avg$ = 85.5;      // $ is allowed but not recommended
boolean x = true;        // Too vague
// ✅ GOOD - Following conventions
final int MAX_STUDENTS = 100;  // Constant in UPPERCASE
int totalCount = 0;            // camelCase for variables
String userName = "john";      // camelCase for multi-word
// ❌ BAD - Not following conventions
int Max_Students = 100;        // Wrong style for constant
int TotalCount = 0;            // Should start with lowercase
String user_name = "john";     // Should use camelCase not snake_case

public class Student {
    // INSTANCE VARIABLES (different for each student)
    String name;           // Each student has their own name
    int age;               // Each student has their own age
    double gpa;            // Each student has their own GPA
    // STATIC VARIABLE (shared by all students)
    static String schoolName = "Oak Valley High";  // Same for everyone!
    static int studentCount = 0;                   // Counts all students
    // Constructor
    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
        studentCount++;    // Increases for each new student
    }
    public void displayInfo() {
        // LOCAL VARIABLES (only exist in this method)
        String message = "Student Info:";  // Created when method starts
        int currentYear = 2024;            // Only accessible here
        System.out.println(message);
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("School: " + schoolName);
        // message and currentYear are destroyed when method ends
    }
    public static void main(String[] args) {
        // Creating students
        Student alice = new Student("Alice", 16, 3.8);
        Student bob = new Student("Bob", 17, 3.5);
        alice.displayInfo();
        bob.displayInfo();
        // Static variable - same for all
        System.out.println("Total students: " + Student.studentCount);  // 2
        System.out.println("School: " + Student.schoolName);  // Oak Valley High
        // Each student has different instance variables
        System.out.println("Alice's GPA: " + alice.gpa);  // 3.8
        System.out.println("Bob's GPA: " + bob.gpa);      // 3.5
    }
}

public class ScopeDemo {
    // CLASS SCOPE - accessible throughout the class
    int classVariable = 100;
    public void method1() {
        // METHOD SCOPE - only accessible in this method
        int methodVariable = 200;
        System.out.println(classVariable);    // ✓ Can access
        System.out.println(methodVariable);   // ✓ Can access
        // BLOCK SCOPE - only inside the if block
        if (classVariable > 50) {
            int blockVariable = 300;
            System.out.println(blockVariable);  // ✓ Can access here
        }
        // System.out.println(blockVariable);  // ❌ ERROR! Out of scope
    }
    public void method2() {
        System.out.println(classVariable);    // ✓ Can access
        // System.out.println(methodVariable);  // ❌ ERROR! Different method
    }
    public static void main(String[] args) {
        // Loop variable scope
        for (int i = 0; i < 5; i++) {
            int loopVar = i * 2;
            System.out.println("i = " + i + ", loopVar = " + loopVar);
        }
        // System.out.println(i);        // ❌ ERROR! i only exists in loop
        // System.out.println(loopVar);  // ❌ ERROR! loopVar only in loop
        // Block scope example
        {
            int x = 10;
            System.out.println(x);  // ✓ Works here
        }
        // System.out.println(x);  // ❌ ERROR! x only exists in block above
    }
}

public class FinalDemo {
    // Final variable - cannot be changed after initialization
    final int DAYS_IN_WEEK = 7;
    final double PI = 3.14159;
    final String SCHOOL_NAME = "Oak Valley High";
    public static void main(String[] args) {
        // Local final variable
        final int MAX_SCORE = 100;
        System.out.println("Max score: " + MAX_SCORE);
        // MAX_SCORE = 200;  // ❌ ERROR! Cannot change final variable
        // Final reference variable
        final int[] numbers = {1, 2, 3, 4, 5};
        numbers[0] = 99;  // ✓ Can modify array contents
        System.out.println(numbers[0]);  // 99
        // numbers = new int[]{6, 7, 8};  // ❌ ERROR! Cannot reassign final reference
        // Convention: Use UPPERCASE for constants
        final double GRAVITY = 9.8;
        final int SPEED_OF_LIGHT = 299792458;
    }
}