use join in threads loop

Question

I'm learning to use threads in java, since I want to speed up a task to reduce time. I try to see how long it takes to finish the threads but since I don't know how to use join in this case:

public class MyThread1 extends Thread {
    int inicio;
    int fin;
    public MyThread1(int _inicio, int _fin) {
        this.inicio = _inicio;
        this.fin = _fin;            
    }

    @Override
    public void run() {
        String pass;        
        for(int i = inicio; i < fin; i++){
            pass = Integer.toString(i);
            System.out.println(pass);
        }
    }
}


public static void main(String arg[]) throws UnknownHostException {
    long previousTime;
    previousTime = System.currentTimeMillis();
        
    int threads = 10; // Number of threads
        
    for (int i = 0; i < threads; i++) {
        MyThread1 object = new MyThread1(0,10);
        object.start();
    }
        
    long currentTime = System.currentTimeMillis();
    double elapsedTime = (currentTime - previousTime) / 1000.0;
    System.out.println("Time in seconds : " + String.valueOf(elapsedTime));
    
}

Answer 1

Add MyThread1 object to an array that is declared before the loop. After the loop, have another loop which will join on all objects.

for (int i = 0; i < threads; i++) {
       MyThread1 object = new MyThread1(0,10);
       arr[i] = object;
       object.start();
}

for (int i = 0; i < threads; i++) {
       arr[i].join();
}