Syncronize thread to wait for multiple threads

Question

I have four threads makes processing on four files, then I want a thread to concatenate these files.

My solution is to make a fifth thread (thread1) to concatenate.

sum = new Thread() {
            public void run() {
                if (thread1.isAlive()) {
                synchronized (lock1) {
                        while (thread1.isAlive()) {
                            try {
                                lock1.wait();
                            } catch (InterruptedException ex) {
                                Logger.getLogger(mainClass.class.getName()).log(Level.SEVERE, null, ex);
                            }
                        }
                    }
                }

                if (thread2.isAlive()) {
                synchronized (lock2) {

                        while (thread2.isAlive()) {
                            try {
                                lock2.wait();
                            } catch (InterruptedException ex) {
                                Logger.getLogger(mainClass.class.getName()).log(Level.SEVERE, null, ex);
                            }
                        }
                    }
                }

                if (thread3.isAlive()) {
                synchronized (lock3) {
                        while (thread3.isAlive()) {
                            try {
                                lock3.wait();
                            } catch (InterruptedException ex) {
                                Logger.getLogger(mainClass.class.getName()).log(Level.SEVERE, null, ex);
                            }
                        }
                    }
                }

                if (thread4.isAlive()) {
                synchronized (lock4) {
                        while (thread4.isAlive()) {
                            try {
                                lock4.wait();
                            } catch (InterruptedException ex) {
                                Logger.getLogger(mainClass.class.getName()).log(Level.SEVERE, null, ex);
                            }
                        }
                    }
                }   //                    new MakeDataSet(path + "f4", "d4.arff");

The problem occurred if the threads don't finish in the order of their indices (like thread3 finishes before thread2 or thread1 or when thread4 finished before thread3/thread2/thread1) in which case the program never ended.

Answer 1

java.lang.Thread's method join() allows you to wait for thread to exit. You can try using something like this:

//wait for all threads to finish
try {
    thread1.join();
    thread2.join();
    thread3.join();
} catch (InterruptedException e) {
    e.printStackTrace();
}