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public class MethodBasics {

// Static utility method: belongs to the class
    public static int add(int a, int b) {
        return a + b;
    }

// Static method returning a boolean
    public static boolean isPrime(int n) {
        if (n < 2) return false;
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) return false;
        }
        return true;
    }

// Instance method using instance state
    private String name;

public MethodBasics(String name) {
        this.name = name;
    }

public String greet(String greeting) {
        return greeting + ", " + this.name + "!";
    }

// Pass-by-value demo
    static void tryToChange(int x, int[] arr) {
        x = 999;          // changes local copy, not caller's variable
        arr[0] = 999;     // mutates the actual array object
    }

public static void main(String[] args) {
        // Static method calls
        System.out.println("3 + 4 = " + add(3, 4));

System.out.print("Primes up to 20: ");
        for (int i = 2; i <= 20; i++) {
            if (isPrime(i)) System.out.print(i + " ");
        }
        System.out.println();

// Instance method call
        MethodBasics obj = new MethodBasics("Alice");
        System.out.println(obj.greet("Good morning"));

// Pass-by-value demo
        int num = 10;
        int[] data = {1, 2, 3};
        tryToChange(num, data);
        System.out.println("num after tryToChange: " + num);      // still 10
        System.out.println("data[0] after tryToChange: " + data[0]); // 999
    }
}