Learn Java Programming

public class Calculator {

    // Method with parameters and return value
    public static int add(int a, int b) {
        return a + b;
    }

    // Method with no return value (void)
    public static void greet(String name) {
        System.out.println("Hello, " + name + "!");
    }

    // Method with multiple return points
    public static String getGrade(int score) {
        if (score >= 90) return "A";
        if (score >= 80) return "B";
        if (score >= 70) return "C";
        if (score >= 60) return "D";
        return "F";
    }

    public static void main(String[] args) {
        int sum = add(5, 3);          // 8
        greet("Alice");                // Hello, Alice!
        String grade = getGrade(85);   // "B"
        System.out.println(sum + " " + grade);
    }
}

public class MathUtils {

    // Same method name, different parameter lists
    public static int multiply(int a, int b) {
        return a * b;
    }

    public static double multiply(double a, double b) {
        return a * b;
    }

    public static int multiply(int a, int b, int c) {
        return a * b * c;
    }

    public static void main(String[] args) {
        System.out.println(multiply(3, 4));       // 12 (int version)
        System.out.println(multiply(2.5, 3.0));   // 7.5 (double version)
        System.out.println(multiply(2, 3, 4));    // 24 (three-param version)
    }
}

public class VarargsDemo {

    // Accepts any number of int arguments
    public static int sum(int... numbers) {
        int total = 0;
        for (int n : numbers) {
            total += n;
        }
        return total;
    }

    // Varargs must be the LAST parameter
    public static String format(String prefix, String... items) {
        return prefix + ": " + String.join(", ", items);
    }

    public static void main(String[] args) {
        System.out.println(sum(1, 2, 3));         // 6
        System.out.println(sum(10, 20, 30, 40));  // 100
        System.out.println(sum());                 // 0 (empty is valid)

        System.out.println(format("Fruits", "Apple", "Banana", "Cherry"));
        // Fruits: Apple, Banana, Cherry
    }
}

public class BankAccount {
    private String owner;
    private double balance;
    private static int totalAccounts = 0;  // shared across all instances

    public BankAccount(String owner, double initialBalance) {
        this.owner = owner;
        this.balance = initialBalance;
        totalAccounts++;
    }

    // Instance method — operates on THIS object's data
    public void deposit(double amount) {
        if (amount > 0) {
            this.balance += amount;
        }
    }

    // Instance method with return value
    public double getBalance() {
        return this.balance;
    }

    // Static method — belongs to the CLASS, not any specific object
    public static int getTotalAccounts() {
        return totalAccounts;
    }

    public static void main(String[] args) {
        BankAccount acc1 = new BankAccount("Alice", 1000);
        BankAccount acc2 = new BankAccount("Bob", 500);

        acc1.deposit(200);  // instance method on acc1
        System.out.println(acc1.getBalance());  // 1200.0

        // Static method called on the class
        System.out.println(BankAccount.getTotalAccounts());  // 2
    }
}

public class Recursion {

    // Recursive factorial: n! = n * (n-1)!
    public static long factorial(int n) {
        if (n <= 1) return 1;       // base case
        return n * factorial(n - 1); // recursive call
    }

    // Recursive Fibonacci
    public static int fibonacci(int n) {
        if (n <= 0) return 0;
        if (n == 1) return 1;
        return fibonacci(n - 1) + fibonacci(n - 2);
    }

    public static void main(String[] args) {
        System.out.println(factorial(5));   // 120
        System.out.println(fibonacci(10));  // 55
    }
}