I have this JUnit test:

package com.baeldung.concurrent;

import static org.junit.Assert.assertEquals;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.junit.Test;

/**
 * This is defined as a manual test because it tries to simulate the race conditions
 * in a concurrent program that is poorly designed and hence may fail nondeterministically.
 * This will help the CI jobs to ignore these tests and a developer to run them manually.
 *
 */
public class MyCounterSimpleManualTest {

    @Test
    public void testCounter() {
        MyCounter counter = new MyCounter();
        for (int i = 0; i < 500; i++)
            counter.increment();
        assertEquals(500, counter.getCount());
    }

    @Test
    public void testCounterWithConcurrency() throws InterruptedException {
        int numberOfThreads = 100;
        ExecutorService service = Executors.newFixedThreadPool(10);
        CountDownLatch latch = new CountDownLatch(numberOfThreads);
        MyCounter counter = new MyCounter();
        for (int i = 0; i < numberOfThreads; i++) {
            service.execute(() -> {
                counter.increment();
                latch.countDown();
            });
        }
        latch.await();
        assertEquals(numberOfThreads, counter.getCount());
    }

    @Test
    public void testSummationWithConcurrencyAndWait() throws InterruptedException {
        int numberOfThreads = 2;
        ExecutorService service = Executors.newFixedThreadPool(10);
        CountDownLatch latch = new CountDownLatch(numberOfThreads);
        MyCounter counter = new MyCounter();
        for (int i = 0; i < numberOfThreads; i++) {
            service.submit(() -> {
                try {
                    counter.incrementWithWait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            });
        }
        latch.await();
        assertEquals(numberOfThreads, counter.getCount());
    }

}

This is my MyCounter (version 1):

package com.baeldung.concurrent;

public class MyCounter {

    private int count;

    public void increment() {
        int tmp = count;
        count = tmp + 1;
    }

    public synchronized void incrementWithWait() throws InterruptedException {
        int tmp = count;
        wait(100);
        count = tmp + 1;
    }

    public int getCount() {
        return count;
    }

}

Here is stacktrace with JUnit fail:

 java.lang.AssertionError: expected:<2> but was:<1>
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.failNotEquals(Assert.java:835)
        at org.junit.Assert.assertEquals(Assert.java:647)
        at org.junit.Assert.assertEquals(Assert.java:633)
        at com.baeldung.concurrent.MyCounterSimpleManualTest.testSummationWithConcurrencyAndWait(MyCounterSimpleManualTest.java:60)

When I change MyCounter to this, JUnit works (version 2):

package com.baeldung.concurrent;

public class MyCounter {

    private int count;

    public void increment() {
        count++;
    }

    public synchronized void incrementWithWait() throws InterruptedException {
        wait(100);
        count++;
    }

    public int getCount() {
        return count;
    }

}    

Can someone explain me why first version of MyCounter doesnt work. Maybe there is some race condition issue but I dont understand why becouse method incrementWithWait() is synchronized.Thank you for help