ReadersWriters.java

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ReadersWriters {
    boolean writer = false;
    int readers = 0;
    boolean wantToWrite = false;

    public synchronized void ReaderLock() throws InterruptedException{
        while(writer || wantToWrite){
            wait();
        }
        readers++;
    }

    public synchronized void ReaderUnlock() throws InterruptedException{
        readers--;
        if(readers==0){
            notifyAll();
        }
    }

    public synchronized void WriterLock() throws InterruptedException{
        wantToWrite = true;
        while(readers > 0 || writer){
            wait();
        }
        writer = true;
        wantToWrite = false;
    }

    public synchronized void WriterUnlock() throws InterruptedException{
        writer=false;
        notifyAll();
    }

    public ReadersWriters() {

        final int numReadersWriters = 10;

        for (int i = 0; i < numReadersWriters; i++) {

            new Thread(() -> {
                try{
                    ReaderLock();
                    System.out.println(" Reader " + Thread.currentThread().getId() + " started reading");
                    // read
                    System.out.println(" Reader " + Thread.currentThread().getId() + " stopped reading");
                    ReaderUnlock();
                }catch(InterruptedException e){
                    System.out.println(e);
                }
            }).start();

            new Thread(() -> {
                try{
                    WriterLock();
                    System.out.println(" Writer " + Thread.currentThread().getId() + " started writing");
                    // write
                    System.out.println(" Writer " + Thread.currentThread().getId() + " stopped writing");
                    WriterUnlock();
                }catch(InterruptedException e){
                    System.out.println(e);
                }
            }).start();

        }
    }

    public static void main(String[] args) {
        new ReadersWriters();
    }

}