public class MyProg {
    public static void main(String[] args) {
        Database myDB = new Database();
        Reader reader1 = new Reader(1, myDB);
        Reader reader2 = new Reader(2, myDB);
        Reader reader3 = new Reader(3, myDB);
        Reader reader4 = new Reader(4, myDB);
        Reader reader5 = new Reader(5, myDB);
        Writer writer1 = new Writer(1, myDB);
        Writer writer2 = new Writer(2, myDB);
        Writer writer3 = new Writer(3, myDB);
        Writer writer4 = new Writer(4, myDB);
        reader1.start();
        reader2.start();
        reader3.start();
        reader4.start();
        reader5.start();
        writer1.start();
        writer2.start();
        writer3.start();
        writer4.start();
    }
}


class Reader extends Thread {
    public Reader(int r, Database db) {
        readerNum = r;
        server = db;
    }

    public void run() {
        int c;
        for (int i = 1; i <= 3; i++) // It should be while (true)
        {
            napping();
            System.err.println("reader " + readerNum + " wants to read.");

            c = server.startRead();
            System.err.println("reader " + readerNum + " is reading. Reader Count = " + c);
            Database.something();

            c = server.endRead();
        }
    }

    public void napping() {
        try {
            sleep((int) (Math.random() * 5000));
        } catch (InterruptedException e) {
        }
    }

    private Database server;
    private int readerNum;
}


class Writer extends Thread {
    public Writer(int w, Database db) {
        writerNum = w;
        server = db;
    }

    public void run() {
        for (int i = 1; i <= 3; i++) // It should be while (true)
        {
            napping();
            System.err.println("writer " + writerNum + " wants to write.");
            server.startWrite();

            System.err.println("-------------- WRITER " + writerNum + " IS WRITING.");
            Database.something();

            System.err.println("               writer " + writerNum + " is done writing.");
            server.endWrite();
        }
    }

    public void napping() {
        try {
            sleep((int) (Math.random() * 5000));
        } catch (InterruptedException e) {
        }
    }

    private Database server;
    private int writerNum;
}


class Database {
    public Database() {
        readerCount = 0;
        semWriter = new Semaphore(1);
    }

    public static void something() {
        try {
            Thread.sleep((int) (Math.random() * 5000));
        } catch (InterruptedException e) {
        }
    }

    public int startRead() {
        if (readerCount == 0){
            semWriter.P();
        }
        readerCount++;
        return readerCount;
    }

    public int endRead() {
        --readerCount;
        System.err.println("Reader is done reading. Count = " + readerCount);
        if (readerCount == 0){
             semWriter.V();            // if I am the last reader tell all others
        }
        return readerCount;
    }

    public void startWrite() {
        semWriter.P();
     }

    public void endWrite() {
        semWriter.V();
    }

    private int readerCount;    // the number of active readers
    private Semaphore semWriter;
}

class Semaphore {
    private int value;

    Semaphore(int v) {
        value = v;
    }

    public void P() {
        while (value <= 0) ;
        value--;
    }

    public void V() {
        ++value;
    }
    public int getValue(){
        return value;
    }
}

class Semaphore
{
    public Semaphore(int v)
    { value = v; }

    public synchronized void P()
    { value--;
        if (value < 0)
            try
            {wait();}
            catch (InterruptedException e) {}
    }

    public synchronized void V()
    { ++value;
        if (value<=0)
            notify();
    }

    private int value;
}

 public void P()
    { value--;
        if (value < 0)
            try
            {wait();}
            catch (InterruptedException e) {}
    }