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

# Joining Threads
A thread is always created by another thread except for the main application thread. Study the following code snippet. The `innerThread` is created by the thread which executes the `main` method. You may wonder what happens to the `innerThread` if the main thread finishes execution before the `innerThread` is done?

class Demonstration {
    public static void main( String args[] ) throws InterruptedException {
        
        ExecuteMe executeMe = new ExecuteMe();
        Thread innerThread = new Thread(executeMe);
        innerThread.setDaemon(true);
        innerThread.start();
    }
}

class ExecuteMe implements Runnable {
 
  public void run() {
    while (true) {
      System.out.println("Say Hello over and over again.");
      try {
        Thread.sleep(500);
      } catch (InterruptedException ie) {
        // swallow interrupted exception
      }
    }
  }
}


If you execute the above code, you'll see no output. That is because the main thread exits right after starting the `innerThread`. Once it exits, the JVM also kills the spawned thread. On <b>line 6</b> we mark the `innerThread` thread as a *daemon* thread, which we'll talk about shortly, and is responsible for innerThread being killed as soon as the main thread completes execution. Do bear in mind, that if the main thread context switches just after executing **Line 7**, we may see some output from the `innerThread`, till the main thread is context switched back in and exits.

If we want the main thread to wait for the `innerThread` to finish before proceeding forward, we can direct the main thread to suspend its execution by calling <code **join** method on the `innerThread` object right after we **start** the `innerThread`. The change would look like the following.


```java
        Thread innerThread = new Thread(executeMe);
        innerThread.start();
        innerThread.join();
```

If we didn't execute **join** on `innerThread` and let the main thread continue after `innerThread` was spawned then the `innerThread` may get killed by the JVM upon `main` thread's completion.

This lesson shows various thread handling methods with examples.

The Basics

Multithreading in Java

Java's Memory Model

Interview Practice Problems

Bonus Questions

Beyond the Interview

Java Concurrency Reference

Revision & Quizzes

Basic Thread Handling

Joining Threads