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

# I/O Bound vs CPU Bound
<p>We write programs to solve problems. Programs utilize various resources of the computer systems on which they run. For instance a program running on your machine will broadly require:</p>

<ul>
<li><p>CPU Time</p></li>
<li><p>Memory</p></li>
<li><p>Networking Resources</p></li>
<li><p>Disk Storage</p></li>
</ul>

<p>Depending on what a program does, it can require heavier use of one or more resources. For instance, a program that loads gigabytes of data from storage into main memory would hog the main memory of the machine it runs on. Another can be writing several gigabytes to permanent storage, requiring abnormally high disk i/o.</p>
</div>
</div>

# CPU Bound 
<p><span class="hi-yl">Programs which are compute-intensive i.e. program execution requires very high utilization of the CPU (close to 100%) are called CPU bound programs. Such programs primarily depend on improving CPU speed to decrease program completion time.</span> This could include programs such as data crunching, image processing, matrix multiplication etc.</p>
<p>If a CPU bound program is provided a more powerful CPU it can potentially complete faster. Eventually, there is a limit on how powerful a single CPU can be. At this point, the recourse is to harness the computing power of multiple CPUs and structure your program code in a way that can take advantage of the multiple CPU units available. Say we are trying to sum up the first 1 million natural numbers. A single-threaded program would sum in a single loop from 1 to 1000000. To cut down on execution time, we can create two threads and divide the range into two halves. The first thread sums the numbers from 1 to 500000 and the second sums the numbers from 500001 to 1000000. If there are two processors available on the machine, then each thread can independently run on a single CPU in parallel. In the end, we sum the results from the two threads to get the final result. Theoretically, the multithreaded program should finish in half the time that it takes for the single-threaded program. However, there will be a slight overhead of creating the two threads and merging the results from the two threads.</p>

<p>Multithreaded programs can improve performance in cases where the problem lends itself to being divided into smaller pieces that different threads can work on independently. This may not always be true though.</p>
</div>
</div>

# I/O Bound
<p>I/O bound programs are the opposite of CPU bound programs. <span class="hi-yl">Such programs spend most of their time waiting for input or output operations to complete while the CPU sits idle. I/O operations can consist of operations that write or read from main memory or network interfaces.</span> Because the CPU and main memory are physically separate a data bus exists between the two to transfer bits to and fro. Similarly, data needs to be moved between network interfaces and CPU/memory. Even though the physical distances are tiny, the time taken to move the data across is big enough for several thousand CPU cycles to go waste. This is why I/O bound programs would show relatively lower CPU utilization than CPU bound programs. </p>
</div>
</div>

# Caveats
<p>Both types of programs can benefit from concurrent architectures. If a program is CPU bound we can increase the number of processors and structure our program to spawn multiple threads that individually run on a dedicated or shared CPU. For I/O bound programs, it makes sense to have a thread give up CPU control if it is waiting for an I/O operation to complete so that another thread can get scheduled on the CPU and utilize CPU cycles. Different programming languages come with varying support for multithreading. For instance, Javascript is single-threaded, Java provides full-blown multithreading and Python is <i>sort</i> of multithreaded as it can only have a single thread in running state because of its global interpreter lock (GIL) limitation. However, all three languages support asynchronous programming models which is another way for programs to be concurrent (but not parallel).</p>

<p>For completeness we should mention that there are also memory-bound programs that depend on the amount of memory available to speed up execution.</p>
</div>
</div>


# I/O Bound vs CPU Bound
<p>We write programs to solve problems. Programs utilize various resources of the computer systems on which they run. For instance a program running on your machine will broadly require:</p>

<ul>
<li><p>CPU Time</p></li>
<li><p>Memory</p></li>
<li><p>Networking Resources</p></li>
<li><p>Disk Storage</p></li>
</ul>

<p>Depending on what a program does, it can require heavier use of one or more resources. For instance, a program that loads gigabytes of data from storage into main memory would hog the main memory of the machine it runs on. Another can be writing several gigabytes to permanent storage, requiring abnormally high disk i/o.</p>
</div>
</div>

# CPU Bound 
<p><span>Programs which are compute-intensive i.e. program execution requires very high utilization of the CPU (close to 100%) are called CPU bound programs. Such programs primarily depend on improving CPU speed to decrease program completion time.</span> This could include programs such as data crunching, image processing, matrix multiplication etc.</p>
<p>If a CPU bound program is provided a more powerful CPU it can potentially complete faster. Eventually, there is a limit on how powerful a single CPU can be. At this point, the recourse is to harness the computing power of multiple CPUs and structure your program code in a way that can take advantage of the multiple CPU units available. Say we are trying to sum up the first 1 million natural numbers. A single-threaded program would sum in a single loop from 1 to 1000000. To cut down on execution time, we can create two threads and divide the range into two halves. The first thread sums the numbers from 1 to 500000 and the second sums the numbers from 500001 to 1000000. If there are two processors available on the machine, then each thread can independently run on a single CPU in parallel. In the end, we sum the results from the two threads to get the final result. Theoretically, the multithreaded program should finish in half the time that it takes for the single-threaded program. However, there will be a slight overhead of creating the two threads and merging the results from the two threads.</p>

<p>Multithreaded programs can improve performance in cases where the problem lends itself to being divided into smaller pieces that different threads can work on independently. This may not always be true though.</p>
</div>
</div>

# I/O Bound
<p>I/O bound programs are the opposite of CPU bound programs. <span>Such programs spend most of their time waiting for input or output operations to complete while the CPU sits idle. I/O operations can consist of operations that write or read from main memory or network interfaces.</span> Because the CPU and main memory are physically separate a data bus exists between the two to transfer bits to and fro. Similarly, data needs to be moved between network interfaces and CPU/memory. Even though the physical distances are tiny, the time taken to move the data across is big enough for several thousand CPU cycles to go waste. This is why I/O bound programs would show relatively lower CPU utilization than CPU bound programs. </p>
</div>
</div>

# Caveats
<p>Both types of programs can benefit from concurrent architectures. If a program is CPU bound we can increase the number of processors and structure our program to spawn multiple threads that individually run on a dedicated or shared CPU. For I/O bound programs, it makes sense to have a thread give up CPU control if it is waiting for an I/O operation to complete so that another thread can get scheduled on the CPU and utilize CPU cycles. Different programming languages come with varying support for multithreading. For instance, Javascript is single-threaded, Java provides full-blown multithreading and Python is <i>sort</i> of multithreaded as it can only have a single thread in running state because of its global interpreter lock (GIL) limitation. However, all three languages support asynchronous programming models which is another way for programs to be concurrent (but not parallel).</p>

<p>For completeness we should mention that there are also memory-bound programs that depend on the amount of memory available to speed up execution.</p>
</div>
</div>


We delve into the characteristics of programs with different resource-use profiles and how that can affect program design choices.

The Basics

Multithreading in Java

Java's Memory Model

Interview Practice Problems

Bonus Questions

Beyond the Interview

Java Concurrency Reference

Revision & Quizzes

I/O Bound vs CPU Bound

I/O Bound vs CPU Bound