The only guide you’ll need on concurrency & multithreading for senior engineers. Pressure test your skills with real-world practice problems from top companies.

Concurrency in Java is one of the most complex and advanced topics brought up during senior engineering interviews. Knowledge of concurrency and multithreading can put interviewees at a considerable advantage. This course lays the foundations of advanced concurrency and multithreading and explains concepts such as Monitors and Deferred Callbacks in depth. It also builds simple and complete solutions to popular concurrency problems that can be asked about in interviews like the Reader-Writer Problem and the Dining Philosopher Problem. While prior knowledge of concurrency is not required to follow through with this course, familiarity with the very basics of concurrency would be helpful.

Java Multithreading for Senior Engineering Interviews

We discussed the abstract concept of a monitor in the previous section and now let's see the working of a concrete implementation of it in Java.

# Java's Monitor
 
In Java every object is a condition variable and has an associated lock that is hidden from the developer. Each java object exposes <code class="mthd">wait()</code> and <code class="mthd">notify()</code> methods.

Before we execute <code class="mthd">wait()</code> on a java object we need to lock its hidden mutex. That is done implicitly through the synchronized keyword. If you attempt to call <code class="mthd">wait()</code> or <code class="mthd">notify()</code> outside of a synchronized block, an <code class="clazz">IllegalMonitorStateException</code> would occur. It's Java reminding the developer that the mutex wasn't acquired before wait on the condition variable was invoked. <code class="mthd">wait()</code> and <code class="mthd">notify()</code> can only be called on an object once the calling thread becomes the owner of the monitor. The ownership of the monitor can be achieved in the following ways:

<ul>
 <li>the method the thread is executing has synchronized in its signature
 </li>
 <li>the thread is executing a block that is synchronized on the object on which wait or notify will be called
 </li>
 <li>
 in case of a class, the thread is executing a static method which is synchronized.
 </li>
</ul>
 </div>
</div>

# Java's Monitor
 
In Java every object is a condition variable and has an associated lock that is hidden from the developer. Each java object exposes <code>wait()</code> and <code>notify()</code> methods.

Before we execute <code>wait()</code> on a java object we need to lock its hidden mutex. That is done implicitly through the synchronized keyword. If you attempt to call <code>wait()</code> or <code>notify()</code> outside of a synchronized block, an <code>IllegalMonitorStateException</code> would occur. It's Java reminding the developer that the mutex wasn't acquired before wait on the condition variable was invoked. <code>wait()</code> and <code>notify()</code> can only be called on an object once the calling thread becomes the owner of the monitor. The ownership of the monitor can be achieved in the following ways:

<ul>
 <li>the method the thread is executing has synchronized in its signature
 </li>
 <li>the thread is executing a block that is synchronized on the object on which wait or notify will be called
 </li>
 <li>
 in case of a class, the thread is executing a static method which is synchronized.
 </li>
</ul>
 </div>
</div>

Continues the discussion of the differences between a mutex and a monitor and also looks at Java's implementation of the monitor.

The Basics

Multithreading in Java

Java's Memory Model

Interview Practice Problems

Bonus Questions

Beyond the Interview

Java Concurrency Reference

Revision & Quizzes

Java's Monitor & Hoare vs Mesa Monitors

Java’s Monitor