[Print alternatively using alternate Threads] - multithreading

Question

I am trying to implement this:
First Thread prints 1,Second Thread prints 2,Third Thread prints 3,First Thread prints 4 and so on:
I did this and its working :
class Display implements Runnable
{
int threadId;
static int v=1;
static int turn=1;//init
static Object monitor=new Object();
Display(int id)
{
this.threadId=id;
}
public void run() {// TODO Auto-generated method stub
print();
}
private void print() {
// TODO Auto-generated method stub
while(v<100)
{
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
synchronized(monitor)
{
if(threadId==1)
{
if(turn!=1)
{
try {
monitor.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
else
{
System.out.println(threadId + ":"+v);
v++;
turn=2;
monitor.notifyAll();
}
}
else if(threadId==2)
{
if(turn!=2)
{
try {
monitor.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
else
{
System.out.println(threadId+":"+v);
v++;
turn=3;
monitor.notifyAll();
}
}
else
{
if(turn!=3)
{
try {
monitor.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
else
{
System.out.println(threadId+":"+v);
v++;
turn=1;
monitor.notifyAll();
}
}
}
}
}
}
public class Print {
public static void main(String[] ar)
{
Thread t1= new Thread(new Display(1));
Thread t2= new Thread(new Display(2));
Thread t3= new Thread(new Display(3));
t1.start();
t2.start();
t3.start();
}
}
Output is like this:
1:1
2:2
3:3
1:4
2:5
It achieves the purpose but if I am having two more Threads to print alternatively,then I have to use more If conditions.
Can Anyone suggest to write this code in a better form to achieve that task in a more clean way so that it is extendable if more Threads are added.

Answer 1

You will need the highest Id given to your threads. After this, you can simplify your code using the modulo operator.
class Display implements Runnable {
int threadId;
static int v = 1;
static int higehstId = 0;
static Object monitor = new Object();
Display(int id) {
this.threadId = id;
}
public void run() {
print();
}
private void print() {
while (v < 100) {
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
synchronized (monitor) {
if ((v - 1) % higehstId == threadId - 1) {
System.out.println(threadId + ":" + v);
v++;
monitor.notifyAll();
} else {
try {
monitor.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
public static void main(String[] ar) {
Thread t1 = new Thread(new Display(1));
Thread t2 = new Thread(new Display(2));
Thread t3 = new Thread(new Display(3));
Thread t4 = new Thread(new Display(4));
Thread t5 = new Thread(new Display(5));
Display.higehstId = 5;
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}
If you initialize v to 0 and start numbering the Threads from 0, you could simplify the if statement to this:
if(v % highestId == threadId)