Stack vs heap memory

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = a;          // copies the value 10
        b = 20;

        int[] scores = {50, 60};
        int[] alias = scores;   // copies the reference
        alias[0] = 99;

        System.out.println("a = " + a + ", b = " + b);
        System.out.println("scores = " + Arrays.toString(scores));
    }
}

public class Main {
    static class Student {
        String name;
        int marks;
        boolean passed;
        double fee;
    }

    public static void main(String[] args) {
        Student s = new Student();
        System.out.println(s.name + " " + s.marks + " " + s.passed + " " + s.fee);
        // int local; System.out.println(local);  -> compile error
    }
}

import java.util.Arrays;

public class Main {
    static void swap(int x, int y) {
        int t = x;
        x = y;
        y = t;
    }

    static void swap(int[] pair) {
        int t = pair[0];
        pair[0] = pair[1];
        pair[1] = t;
    }

    public static void main(String[] args) {
        int p = 5, q = 9;
        swap(p, q);
        System.out.println("p = " + p + ", q = " + q);

        int[] pair = {5, 9};
        swap(pair);
        System.out.println("pair = " + Arrays.toString(pair));
    }
}

public class Main {
    static int depth = 0;
    static int maxDepth = 0;

    static long factorial(int n) {
        depth++;
        if (depth > maxDepth) maxDepth = depth;
        long result = (n <= 1) ? 1 : n * factorial(n - 1);
        depth--;
        return result;
    }

    public static void main(String[] args) {
        System.out.println("5! = " + factorial(5));
        System.out.println("Deepest stack: " + maxDepth + " factorial frames (+ main)");
        System.out.println("Depth after return: " + depth);
    }
}

public class Main {
    // Recursive idea (do NOT use for large n):
    // static long sum(int n) { return n == 0 ? 0 : n + sum(n - 1); }

    static long sumLoop(int n) {
        long total = 0;
        for (int i = 1; i <= n; i++) {
            total += i;
        }
        return total;
    }

    public static void main(String[] args) {
        int n = 100_000;
        System.out.println("Sum 1.." + n + " = " + sumLoop(n));
        System.out.println("Formula check  = " + (long) n * (n + 1) / 2);
    }
}

public class Main {
    static int[] parent = {1, 0};   // 0 -> 1 -> 0 -> 1 ... never reaches -1

    static int findRoot(int c) {
        if (parent[c] == -1) return c;
        return findRoot(parent[c]);
    }

    public static void main(String[] args) {
        try {
            System.out.println("Root: " + findRoot(0));
        } catch (StackOverflowError e) {
            System.out.println("Category tree has a cycle: " + e.getClass().getSimpleName());
        }
        System.out.println("Program continues");
    }
}

public class Main {
    public static void main(String[] args) {
        int[] amounts = {1200, 450, 980, 300, 2200, 150, 640, 875, 410, 990};
        int[] buffer = new int[4];          // one object on the heap, reused
        int grand = 0, batchNo = 0;
        for (int start = 0; start < amounts.length; start += buffer.length) {
            int count = Math.min(buffer.length, amounts.length - start);
            System.arraycopy(amounts, start, buffer, 0, count);
            int batchTotal = 0;
            for (int i = 0; i < count; i++) batchTotal += buffer[i];
            batchNo++;
            grand += batchTotal;
            System.out.println("Batch " + batchNo + " (" + count + " orders): Rs " + batchTotal);
        }
        System.out.println("Grand total: Rs " + grand);
    }
}

public class Main {
    static double balanceAfter(double principal, double monthlyRate, int months) {
        double balance = principal;
        for (int m = 1; m <= months; m++) {
            balance += balance * monthlyRate / 100;
        }
        return balance;
    }

    public static void main(String[] args) {
        double b = balanceAfter(100000, 0.5, 12);
        System.out.printf("Balance after 12 months: Rs %.2f%n", b);
        System.out.printf("Interest earned: Rs %.2f%n", b - 100000);
    }
}

public class Main {
    static class Cart {
        int items;
        int total;
        void add(int price) {
            items++;
            total += price;
        }
    }

    public static void main(String[] args) {
        Cart cart = new Cart();
        Cart cartScreen = cart;       // both screens get the same reference
        Cart checkoutScreen = cart;

        cartScreen.add(499);
        checkoutScreen.add(99);

        System.out.println("Cart screen sees: " + cartScreen.items + " items, Rs " + cartScreen.total);
        System.out.println("Checkout sees:    " + checkoutScreen.items + " items, Rs " + checkoutScreen.total);
        System.out.println("Objects on the heap for carts: " + (cartScreen == checkoutScreen ? 1 : 2));
    }
}