Q:

I want to generate 5 random numbers using 5 Threads(each thread generates 1 number) and then to sum up the numbers and print the sum of them

this is my class that generates the numbers and adding the numbers
im getting a random number and adding returning it in the "summing" function
public class summing implements Runnable
{ 
int a;
public summing(int a){
    this.a = a;
}

public void run()
{
    add(a);
}

public void add(int a)
{
    Random rand = new Random();
    int  n = rand.nextInt(10) + 1;
    System.out.println("nmber generated:" + n); 
    synchronized (this)
    {
        a += n;   
    }
}

and this is my main class
i am making 5 threads and i want to add here the random numbers to 
the sum variable but it does not working.
public static void main(String[] args)
{
    int sum = 0;
    summing sum2 = new summing(sum);
    Thread t1 = new Thread(sum2);
    Thread t2 = new Thread(sum2);
    Thread t3 = new Thread(sum2);
    Thread t4 = new Thread(sum2);
    Thread t5 = new Thread(sum2);
    t1.start();
    t2.start();
    t3.start();
    t4.start();
    t5.start();
    }

A:

Try this
public class Summing implements Runnable {

    int a;

    public Summing(int a) {
        this.a = a;
    }

    public void run() {
        addRondom();
    }

    public void addRondom() {
        Random rand = new Random();
        int n = rand.nextInt(10) + 1;
        System.out.println("number generated: " + n);
        synchronized (this) {
            a += n;
        }
    }
}

and then
public static void main(String[] args) {
    int base = 0;

    Summing sum2 = new Summing(base);

    Thread t1 = new Thread(sum2);
    Thread t2 = new Thread(sum2);
    Thread t3 = new Thread(sum2);
    Thread t4 = new Thread(sum2);
    Thread t5 = new Thread(sum2);

    t1.start();
    t2.start();
    t3.start();
    t4.start();
    t5.start();

    try {
        t1.join();
        t2.join();
        t3.join();
        t4.join();
        t5.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.print("final result: " + sum2.a);
}

It works well
number generated: 1
number generated: 3
number generated: 7
number generated: 4
number generated: 5
final result: 20
Process finished with exit code 0

