Java concurrency using ConcurrentHashMap and synchronized blocks

Here is my main class to initialize and start 5 different threads:

public class Server implements Runnable {
    Server1 server1;
    Thread server1Thread;

    public Server() {}

    @Override
    public void run() {
        server1 = new Server1();
        server1Thread = new Thread(server1);
        server1Thread.start();
    }

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            Server s = new Server();
            s.run();
        }
    }
}

Here is my Server1Runnable:

import java.util.concurrent.ConcurrentHashMap;
public class Server1 implements Runnable {
    private ConcurrentHashMap<Integer, Integer> storage= new ConcurrentHashMap<>();

    public Server1() {}

    @Override
    public void run() {
        synchronized (this){
            for (int i = 0; i < 10; i++) {
                storage.put(i, (int)(Math.random()*100));
            }
            for (int i : storage.keySet()) {
                System.out.print("(" + i + "," + storage.get(i) + ") ");
            }
            System.out.println();
        }
    }
}

It ConcurrentHashMap storagestarts by putting in the key 0, 9and assigns a random value between 0and 100. After that, print it and print a new line at the end. I have the user synchronizedblock to make sure the thread itself accesses the key correctly, but it prints something like this:

(0,8) (0,87) (1,60) (1,14) (2,20) (2,70) (3,5) (0,74) (0,42) (1,22) (4,96) (0,85) (1,97) (2,75) (3,68) (4,3) (5,49) (6,3) (7,9) (8,47) (9,52) 
(3,2) (5,74) (2,86) (1,48) (3,5) (6,0) (4,0) (7,86) (4,22) (8,20) (2,17) (9,87) 
(5,96) (5,15) (6,15) (6,92) (7,48) (8,93) (9,67) 
(3,87) (7,43) (4,34) (5,48) (8,91) (9,64) 
(6,84) (7,75) (8,47) (9,87) 

Apparently this means that some thread can print out more than 10 keys that I have assigned to it. How can I make each thread print exactly the 10 keys and values ​​assigned to them and ensure concurrency here?

I'm not sure how to test.