Object lifecycle

public class Main {
    static class Order {
        static int created = 0;
        final int id;
        Order() {
            created++;
            id = created;
        }
    }

    public static void main(String[] args) {
        Order a = new Order();
        for (int i = 0; i < 2; i++) {
            Order temp = new Order();
            System.out.println("Temporary order " + temp.id);
        }
        System.out.println("First order id: " + a.id);
        System.out.println("Orders created: " + Order.created);
    }
}

public class Main {
    static class Invoice {
        double gstRate = initRate();
        String status;

        {
            System.out.println("2. initializer block, status = " + status);
        }

        Invoice() {
            System.out.println("3. constructor body");
            status = "DRAFT";
        }

        double initRate() {
            System.out.println("1. field initializer, gstRate was " + gstRate);
            return 18.0;
        }
    }

    public static void main(String[] args) {
        Invoice inv = new Invoice();
        System.out.println("Ready: " + inv.status + " at " + inv.gstRate + "%");
    }
}

public class Main {
    static class Account {
        String number;
        Account(String number) {
            this.number = number;
            System.out.println("Account part ready: " + number);
        }
    }

    static class SavingsAccount extends Account {
        double rate;
        SavingsAccount(String number) {
            super(number);
            rate = 3.5;
            System.out.println("Savings part ready: rate " + rate + "%");
        }
    }

    public static void main(String[] args) {
        SavingsAccount s = new SavingsAccount("SB-101");
        System.out.println(s.number + " created");
    }
}

public class Main {
    static class Resource implements AutoCloseable {
        final String name;
        Resource(String name) {
            this.name = name;
            System.out.println("open " + name);
        }
        @Override
        public void close() {
            System.out.println("close " + name);
        }
    }

    public static void main(String[] args) {
        try (Resource db = new Resource("db");
             Resource file = new Resource("file")) {
            System.out.println("export orders from " + db.name + " to " + file.name);
        }
        System.out.println("both released");
    }
}

public class Main {
    static class Connection implements AutoCloseable {
        Connection() { System.out.println("connection opened"); }
        @Override
        public void close() { System.out.println("connection closed"); }
    }

    static void export(Connection c) {
        System.out.println("exporting row 1");
        throw new IllegalStateException("disk full");
    }

    public static void main(String[] args) {
        try (Connection c = new Connection()) {
            export(c);
        } catch (IllegalStateException e) {
            System.out.println("export failed: " + e.getMessage());
        }
    }
}

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] recent = new String[3];
        int next = 0;
        String[] views = {"Phone", "Cover", "Charger", "Earbuds", "Stand"};
        for (String product : views) {
            String dropped = recent[next];
            recent[next] = product;
            next = (next + 1) % recent.length;
            System.out.println(Arrays.toString(recent) + (dropped != null ? "  dropped " + dropped : ""));
        }
    }
}

public class Main {
    static class Ticket implements AutoCloseable {
        static int issued = 0;
        static int active = 0;
        final String pnr;

        Ticket() {
            issued++;
            active++;
            pnr = "PNR" + (1000 + issued);
        }

        @Override
        public void close() {
            active--;
            System.out.println(pnr + " released");
        }
    }

    public static void main(String[] args) {
        Ticket t1 = new Ticket();
        Ticket t2 = new Ticket();
        try (Ticket t3 = new Ticket()) {
            System.out.println("Using " + t3.pnr + ", active = " + Ticket.active);
        }
        t1.close();
        t1 = null;
        System.out.println("Issued: " + Ticket.issued + ", active: " + Ticket.active);
        System.out.println("Still holding " + t2.pnr);
    }
}

public class Main {
    static class GatewayClient {
        static int created = 0;
        final int number;
        int handled = 0;
        GatewayClient() {
            created++;
            number = created;
        }
        void pay(int amount) {
            handled++;
            System.out.println("client " + number + " paid Rs " + amount);
        }
    }

    public static void main(String[] args) {
        GatewayClient[] pool = {new GatewayClient(), new GatewayClient()};
        int[] payments = {499, 1250, 80, 3000, 640};
        for (int i = 0; i < payments.length; i++) {
            GatewayClient c = pool[i % pool.length];   // borrow
            c.pay(payments[i]);
        }
        System.out.println("Clients created: " + GatewayClient.created);
        System.out.println("Client 1 handled " + pool[0].handled + ", client 2 handled " + pool[1].handled);
    }
}