Why is synchronized not working properly?

Question

Here\'s my code:

private int count = 0;

  public synchronized void increment() {
      count++;
  }

 public void doWork() throws InterruptedException {

          


        

Answer 1

One alternative solution without using synchronized.

Since your use case is simple ( just incrimenting the counter and print the value, AtomicInteger is better choice.

import java.util.concurrent.atomic.AtomicInteger;

public class TestCounter{
    private AtomicInteger count = new AtomicInteger(0);

    public void doWork() throws InterruptedException {

        Thread t1 = new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 5; i++) {
                    System.out.println(""+Thread.currentThread().getName()+":"+count.incrementAndGet());
                }}});

        Thread t2 = new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 5; i++) {
                    System.out.println(""+Thread.currentThread().getName()+":"+count.incrementAndGet());
                }}});

        t1.start();
        t2.start();
    }

    public static void main(String args[]) throws Exception{
        TestCounter tc = new TestCounter();
        tc.doWork();
    }
}

output:

Thread-0:1
Thread-0:3
Thread-0:4
Thread-0:5
Thread-0:6
Thread-1:2
Thread-1:7
Thread-1:8
Thread-1:9
Thread-1:10

Refer to @fabian answer for why these numbers are nor not printed in sequence.

If you expect the sequence in series of numbers in ascending order from 1-10, Threads are not required.