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

<div class="card-quiz">
   # Question # 1
    <div class="qtn">
        <p>Is the following class thread-safe? </p>

```java
public class Sum {

    int count = 0;

    int sum(int... vals) {

        count++;

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        System.out.println(count);
    }
}
```
   </div>
</div>


<div class="ans">
# Show Explanation
<div style="display:none"><p class="exp">The class isn't thread-safe because it has state that can be mutated by different threads without synchronization amongst them. The state consists of the variable <code class="mthd">count</code></p>
</div>
</div>


<div class="card">
   # Question # 2
    <div class="qtn">
        <p>What are the different ways in which we can make the <code class="clazz">Sum</code> class thread-safe?</p>
    </div>
    <div class="ans">
        <p>We can use an instance of the <code class="clazz">AtomicInteger</code> for keeping the count of invocations. The thread-safe code will be as follows:</p>
## Using Atomic Integer

```java
public class SumFixed {

    AtomicInteger count = new AtomicInteger(0);

    int sum(int... vals) {

        count.getAndIncrement();

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        System.out.println(count.get());
    }
}
```
<p>We can also fix the sum class by using synchronizing on the object instance.</p>
<h5 class="code-hd">Using Synchronization on <code class="clazz">this</code></h5>

```java
public class SumFixed {

    int count = 0;

    synchronized int sum(int... vals) {

        count++;

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    synchronized void printInvocations() {
        System.out.println(count);
    }
}
```
<p>We could also use another object other than <code class="clazz">this</code> for synchronization. The code would then be as follows:</p>

```java
public class SumFixed {

    int count = 0;
    Object lock = new Object();

    int sum(int... vals) {

        synchronized (lock) {
            count++;
        }

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        synchronized (lock) {
            System.out.println(count);
        }
    }
}

```
</div>
</div>


<div>
   # Question # 1
    <div>
        <p>Is the following class thread-safe? </p>

```java
public class Sum {

    int count = 0;

    int sum(int... vals) {

        count++;

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        System.out.println(count);
    }
}
```
   </div>
</div>


<div>
# Show Explanation
<div><p>The class isn't thread-safe because it has state that can be mutated by different threads without synchronization amongst them. The state consists of the variable <code>count</code></p>
</div>
</div>


<div>
   # Question # 2
    <div>
        <p>What are the different ways in which we can make the <code>Sum</code> class thread-safe?</p>
    </div>
    <div>
        <p>We can use an instance of the <code>AtomicInteger</code> for keeping the count of invocations. The thread-safe code will be as follows:</p>
## Using Atomic Integer

```java
public class SumFixed {

    AtomicInteger count = new AtomicInteger(0);

    int sum(int... vals) {

        count.getAndIncrement();

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        System.out.println(count.get());
    }
}
```
<p>We can also fix the sum class by using synchronizing on the object instance.</p>
<h5>Using Synchronization on <code>this</code></h5>

```java
public class SumFixed {

    int count = 0;

    synchronized int sum(int... vals) {

        count++;

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    synchronized void printInvocations() {
        System.out.println(count);
    }
}
```
<p>We could also use another object other than <code>this</code> for synchronization. The code would then be as follows:</p>

```java
public class SumFixed {

    int count = 0;
    Object lock = new Object();

    int sum(int... vals) {

        synchronized (lock) {
            count++;
        }

        int total = 0;
        for (int i = 0; i < vals.length; i++) {
            total += vals[i];
        }
        return total;
    }

    void printInvocations() {
        synchronized (lock) {
            System.out.println(count);
        }
    }
}

```
</div>
</div>


Exercise on how to make classes thread-safe

The Basics

Multithreading in Java

Java's Memory Model

Interview Practice Problems

Bonus Questions

Beyond the Interview

Java Concurrency Reference

Revision & Quizzes

Quiz 5

Question # 1

Show Explanation