Learn multithreading and concurrency concepts with the help of some related interview problems.

Ace the Java Coding Interview

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 module lays the foundations of advanced concurrency and multithreading and explains concepts such as monitors and deferred callbacks in depth. In the end, it builds simple and complete solutions to popular concurrency problems that can be asked in interviews, like the Reader-writer Problem and the Dining Philosopher Problem.

Concurrency and Multithreading

# Problem Statement
Suppose there are two threads t1 and t2. t1 prints **Foo** and t2 prints **Bar**. You are required to write a program which takes a user input n. Then the two threads print Foo and Bar alternately n number of times. The code for the class is as follows:

```java
class PrintFooBar {  
    
    public void PrintFoo() {    
        for (int i = 1 i <= n;  i++){
        System.out.print("Foo");    
        }  
    }

    public void PrintBar() {    
        for (int i = 1; i <= n; i++) {      
        System.out.print("Bar");    
        }  
    }
}
```

The two threads will run sequentially. You have to synchronize the two threads so that the functions `PrintFoo()` and `PrintBar()` are executed in an order. The workflow is shown below:


# Solution

We will solve this problem using the basic utilities of `wait()` and `notifyAll()` in Java.  The basic structure of `FooBar` class is given below:

```java
class FooBar {
    private int n;
    private int flag = 0;

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

    public void foo() { 
    }

    public void bar() {
    }
}

```
Two private instances of the class are integers `n`, and `flag`. 

`n` is the user input that tells how many times "Foo" and "Bar" should be printed. 
`flag` is an integer based on which the words are printed. When the value of `flag` is 0, the word "Foo" will be printed and it will be incremented. This way "Bar" can be printed next. `flag` is initialized with 0 because the printing has to start with "Foo".  The class consists of two methods `foo()` and `bar()` and their structures are given below:


```java
public void foo() {
        
        for (int i = 1; i <= n; i++) {
            synchronized(this) {     
                while (flag == 1) {
                    try {
                        this.wait();
                    }
                    catch (Exception e) {
                    }
                }
        	    System.out.print("Foo");
                flag = 1;
                this.notifyall();
            }    
        }    
}
```

In `foo()`, a loop is iterated `n`(user input) number of times. For synchronization purpose, the printing operation is locked in `synchronize(this)` block. This is done to ensure proper sequence of printing. If `flag` is 0 then “Foo” is printed, then `flag` is set to 1 and any waiting threads are notified via `notifyAll()`. While the value of `flag` is 1, then `wait()` blocks the calling thread.

```java
public void bar() {
        
        for (int i = 1; i <= n; i++) {
            synchronized(this) { 
                while (flag == 0) {
                    try {
                        this.wait();
                    }
                    catch (Exception e) {       
                    }
                }
                System.out.print("Bar");
                flag = 0;
        	    this.notifyAll();
            }
        }
}
```

Similarly in `bar()`, the loop is iterated `n` times. In every iteration, the while loop checks if the value of `flag` is 0. If it is, then it means it is not yet bar's turn to be printed and the calling thread will `wait()`.
When the value of `flag` changes to 1, the waiting thread can resume execution. "Bar" is printed and `flag` is changed to 0 for "Foo" to be printed next. All the waiting threads are then notified via `notifyAll()` that this thread has finished its work.

We will create a new class `FooBarThread` that extends Thread. This enables us to run `FooBar` methods in separate threads concurrently. The class consists of a `FooBar` object along with a string `method` which holds the name of the function to be called. If `method` matches "foo" then `fooBar.foo()` is called. If `method` matches "bar", then `fooBar.bar()` is called.

```java
class FooBarThread extends Thread {

    FooBar fooBar;
    String method;
    
    public FooBarThread(FooBar fooBar, String method){
        this.fooBar = fooBar;
        this.method = method;
    }
    
    public void run() {
        if ("foo".equals(method)) {
           fooBar.foo();
        }
        else if ("bar".equals(method)) {
           fooBar.bar();
        }
    }
}
```

To test our code, We will create two threads; **t1** and **t2**. An object of `FooBar` is initialized with `3`. Both threads will be passed the same object of `FooBar`. **t1** calls `foo()` & **t2** calls `bar()`.

Learn how to execute threads in a specific order for a user specified number of iterations.

Prelude

Interview Practice Problems

Quizzes

Conclusion

Printing Foo Bar n Times

Problem Statement