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

## Callable Interface
In the previous sections we used the <code class="intf">Runnable</code> interface as the abstraction for tasks that were submitted to the executor service. The <code class="intf">Runnable</code> interface's sole <code class="mthd">run</code> method doesn't return a value, which is a handicap for tasks that don't want to write results to global or shared datastructures. The interface <code class="intf">Callable</code> allows such tasks to return results. Let's see the definition of the interface first.


```
public interface Callable<V> {
 /**
 * Computes a result, or throws an exception if unable to do so.
 *
 * @return computed result
 * @throws Exception if unable to compute a result
 */
 V call() throws Exception;
}
```

Note the interface also allows a task to throw an exception. A task goes through the various stages of its life which include the following:
<ul>
<li>created</li>
<li>submitted</li>
<li>started</li>
<li>completed</li>
</ul>

Let's say we want to compute the sum of numbers from 1 to n. Our task should accept an integer n and spit out the sum. Below are two ways to implement our task.


```java
 class SumTask implements Callable<Integer> {

 int n;

 public SumTask(int n) {
 this.n = n;
 }

 public Integer call() throws Exception {

 if (n <= 0)
 return 0;

 int sum = 0;
 for (int i = 1; i <= n; i++) {
 sum += i;
 }
 
 return sum;
 }
 }

```

Or we could take advantage of the anonymous class feature in the Java language to declare our task like so:


```java
 final int n = 10
 Callable<Integer> sumTask = new Callable<Integer>() {
 
 public Integer call() throws Exception {
 int sum = 0;
 for (int i = 1; i <= n; i++)
 sum += i;
 return sum;
 }
 };
```

Now we know how to represent our tasks using the <code class="intf">Callable</code> interface. In the next section we'll explore the <code class="intf">Future</code> interface which will help us manage a task's lifecycle as well as retrieve results from it.

# Callable Interface
In the previous sections we used the <code class="intf">Runnable</code> interface as the abstraction for tasks that were submitted to the executor service. The <code class="intf">Runnable</code> interface's sole <code class="mthd">run</code> method doesn't return a value, which is a handicap for tasks that don't want to write results to global or shared datastructures. The interface <code class="intf">Callable</code> allows such tasks to return results. Let's see the definition of the interface first.


```
public interface Callable<V> {
 /**
 * Computes a result, or throws an exception if unable to do so.
 *
 * @return computed result
 * @throws Exception if unable to compute a result
 */
 V call() throws Exception;
}
```

Note the interface also allows a task to throw an exception. A task goes through the various stages of its life which include the following:
<ul>
<li>created</li>
<li>submitted</li>
<li>started</li>
<li>completed</li>
</ul>

Let's say we want to compute the sum of numbers from 1 to n. Our task should accept an integer n and spit out the sum. Below are two ways to implement our task.


```java
 class SumTask implements Callable<Integer> {

 int n;

 public SumTask(int n) {
 this.n = n;
 }

 public Integer call() throws Exception {

 if (n <= 0)
 return 0;

 int sum = 0;
 for (int i = 1; i <= n; i++) {
 sum += i;
 }
 
 return sum;
 }
 }

```

Or we could take advantage of the anonymous class feature in the Java language to declare our task like so:


```java
 final int n = 10
 Callable<Integer> sumTask = new Callable<Integer>() {
 
 public Integer call() throws Exception {
 int sum = 0;
 for (int i = 1; i <= n; i++)
 sum += i;
 return sum;
 }
 };
```

Now we know how to represent our tasks using the <code class="intf">Callable</code> interface. In the next section we'll explore the <code class="intf">Future</code> interface which will help us manage a task's lifecycle as well as retrieve results from it.

# Callable Interface
In the previous sections we used the <code>Runnable</code> interface as the abstraction for tasks that were submitted to the executor service. The <code>Runnable</code> interface's sole <code>run</code> method doesn't return a value, which is a handicap for tasks that don't want to write results to global or shared datastructures. The interface <code>Callable</code> allows such tasks to return results. Let's see the definition of the interface first.


```
public interface Callable<V> {
 /**
 * Computes a result, or throws an exception if unable to do so.
 *
 * @return computed result
 * @throws Exception if unable to compute a result
 */
 V call() throws Exception;
}
```

Note the interface also allows a task to throw an exception. A task goes through the various stages of its life which include the following:
<ul>
<li>created</li>
<li>submitted</li>
<li>started</li>
<li>completed</li>
</ul>

Let's say we want to compute the sum of numbers from 1 to n. Our task should accept an integer n and spit out the sum. Below are two ways to implement our task.


```java
 class SumTask implements Callable<Integer> {

 int n;

 public SumTask(int n) {
 this.n = n;
 }

 public Integer call() throws Exception {

 if (n <= 0)
 return 0;

 int sum = 0;
 for (int i = 1; i <= n; i++) {
 sum += i;
 }
 
 return sum;
 }
 }

```

Or we could take advantage of the anonymous class feature in the Java language to declare our task like so:


```java
 final int n = 10
 Callable<Integer> sumTask = new Callable<Integer>() {
 
 public Integer call() throws Exception {
 int sum = 0;
 for (int i = 1; i <= n; i++)
 sum += i;
 return sum;
 }
 };
```

Now we know how to represent our tasks using the <code>Callable</code> interface. In the next section we'll explore the <code>Future</code> interface which will help us manage a task's lifecycle as well as retrieve results from it.

This lesson discusses the Callable interface.

The Basics

Multithreading in Java

Java's Memory Model

Interview Practice Problems

Bonus Questions

Beyond the Interview

Java Concurrency Reference

Revision & Quizzes

Callable Interface

Callable Interface