The following screenshot shows the output of the new version of the example. No matter how many times you execute it, you will always obtain the correct result:

Let's see the different uses of the synchronized keyword in the example:

• First, we protected the vehiclePay() method. If two or more Sensor tasks call this method at the same time, only one will execute it and the rest will wait for their turn; therefore, the final amount will always be correct.
• We used two different objects to control access to the car and motorcycle counters. This way, one Sensor task can modify the numberCars attribute and another Sensor task can modify the numberMotorcycles attribute at the same time; however, no two Sensor tasks will be able to modify the same attribute at the same time, so the final value of the counters will always be correct.

Finally, we also synchronized the getNumberCars() and getNumberMotorcycles() methods. Using the synchronized keyword, we can guarantee correct access to shared data in concurrent applications.

As mentioned at the introduction of this recipe, only one thread can access the methods of an object that uses the synchronized keyword in their declaration. If thread (A) is executing a synchronized method and thread (B) wants to execute another synchronized method of the same object, it will be blocked until thread (A) is finished. But if thread (B) has access to different objects of the same class, none of them will be blocked.

When you use the synchronized keyword to protect a block of code, you use an object as a parameter. JVM guarantees that only one thread can have access to all the blocks of code protected with this object (note that we always talk about objects, not classes).

We used the TimeUnit class as well. The TimeUnit class is an enumeration with the following constants: DAYS, HOURS, MICROSECONDS, MILLISECONDS, MINUTES, NANOSECONDS, and SECONDS. These indicate the units of time we pass to the sleep method. In our case, we let the thread sleep for 50 milliseconds.