ReaderWriter Lock, My code just gives exceptions - multithreading

Question

I'm trying to implement the Reader Writer Problem .. I don't know what is wrong with it?
can any one help me figure out why?
...................................................
package readerWriterController;
import java.util.ArrayList;
import java.util.Scanner;
public class Main {
public class ReaderThread extends Thread{
String threadName;
public ReaderThread(String threadName){
super(threadName);
}
public void run(){
System.out.println(getName() + " started reading!");
p();
System.out.println(getName() + " Read : " + Main.buffer);
System.out.println(getName() + " finished reading!");
v();
}
private void p() {
semaphore++;
if(isBlocked){ // if writing
try{
System.out.println(getName() + " is blocked!!");
wait();
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
private void v() {
--semaphore;
if(semaphore == 0 )notify();
}
}
public class WriterThread extends Thread{
private String text;
public WriterThread(String threadName, String text){
super(threadName);
this.text = text;
}
public void run(){
System.out.println(getName() +" start writing");
if(!isBlocked && semaphore == 0){
isBlocked = true;
synchronized (buffer) {
System.out.println(getName() +" Writing : " + Main.buffer + " " + text);
Main.buffer += text;
System.out.println(getName() +" finished writing!!" );
notify();
}
isBlocked=false;
}else if(isBlocked || semaphore != 0){ // just else !
System.out.println(getName() + " is blocked!");
try{
wait();
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static String buffer;
public static int readerCounter = 0;
public static int semaphore ;
private volatile boolean isBlocked=false;
//public boolean semaphore ;
public static void main(String[] args) {
//System.out.println(readerCounter);
Scanner input = new Scanner(System.in);
System.out.println("Enter the buffer:");
buffer = input.next();
int Nr, Nw;
System.out.println("Enter the number of readers");
Nr = input.nextInt();
semaphore = 0;
System.out.println("Enter the number of writers");
Nw = input.nextInt();
// for the reader threads ...
String name;
ArrayList<Thread> list = new ArrayList<Thread>();
for(int i = 0 ; i < Nr ; i++){
// get reader info.
System.out.println("Enter reader "+ (i+1) + " name :");
name = input.next();
// pass to thread!!
list.add(new Main().new ReaderThread(name));
}
// for the writer thread ...
String text;
for(int i = 0 ; i < Nw ; i++){
// get reader info.
System.out.println("Enter writer "+ (i+1) + " name :");
name = input.next();
System.out.println("Enter the text to be written by this writer:");
text = input.next();
// pass to thread!!
list.add(new Main().new WriterThread(name, text));
}
for(Thread t : list){
t.start();
}
}
}
it just gives me more and more exceptions !!!
is there a problem if I called notify() while there is no thread waiting ?
I think that can not call wait() on constant String's or global objects.!
I have seen this here!