Java Threads - Call threads in order

I want the threads to print out their name in order. The ouput should look something like this:
Thread-0
Thread-1
Thread-2
Thread-3
Thread-4
Thread-5
Thread-1
Thread-2
Thread-3

public class NameOutput extends Thread {
static final int N = 5;
static int activeThread = 0;

public static void main(String args){

for (int i = 1; i <= N; i++) {
new NameOutput().start();
}
}

@Override
public void run(){
while(true){
this.printThreadName();
}

}
private synchronized void printThreadName(){
String threadName = "Thread-"+ activeThread;
//Check if thread is in the right order
if(!threadName.equals(this.getName())){
try{
wait();
}catch (InterruptedException e){
System.out.println(this.getName()+" was interrupted!");
}
}else{
System.out.println(this.getName());
activeThread = (activeThread + 1) % N;
notifyAll();
}
}
}


Somehow the output is if Thread-0 gets selected first just Thread-0. The programm stops after the first thread waits.

synchronized void printThreadName

void printThreadName

synchronized

this

1 Answer
1

The problem is you created multiple NameOutputs, and they use their own monitor lock, which means there will be no 'communication'(signal) between them.

NameOutput

Instead, you need a shared monitor lock:

public class NameOutput extends Thread {
static final int N = 5;
static int activeThread = 0;
static final Object lock = new Object(); // shared monitor lock

public static void main(String args){

for (int i = 1; i <= N; i++) {
new NameOutput().start();
}
}

@Override
public void run(){
while(true){
this.printThreadName();
}

}
private void printThreadName(){
synchronized (lock) {
String threadName = "Thread-"+ activeThread;
//Check if thread is in the right order
if(!threadName.equals(this.getName())){
try{
lock.wait();
}catch (InterruptedException e){
System.out.println(this.getName()+" was interrupted!");
}
}else{
System.out.println(this.getName());
activeThread = (activeThread + 1) % N;
lock.notifyAll();
}
}
}
}


Another problem, wait should always be used in while loop, you can do a little more:

wait

while

private void printThreadName(){
synchronized (lock) {
while (!("Thread-"+ activeThread).equals(this.getName())) {
try{
lock.wait();
}catch (InterruptedException e){
System.out.println(this.getName()+" was interrupted!");
}
}
System.out.println(this.getName());
activeThread = (activeThread + 1) % N;
lock.notifyAll();
}
}


output:

Thread-0
Thread-1
Thread-2
Thread-3
Thread-4
Thread-0
Thread-1
Thread-2
Thread-3


By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.