Java/Android Interview @ BirdEye

Hi Friends,

In this post, I'm sharing Java interview questions asked in BirdEye.



Question 1:

Print all unique permutations on a String?


Answer:

Question 2:

Write Algorithm for custom BlockingQueue.

Answer:

Algorithm steps:

• Define an array to store elements for queue. Specify the initial size for that array.

• Use Lock and conditions objects to create custom blocking queue.

• Define two methods , put() and take().

• While putting the elements in queue, check the size of array. If it is full, then the producer will wait for the queue to have some space.

• While consuming element from queue, if the queue is empty, then consumer will wait for the queue to have some objects in it. 

Packages to be included:

java.util.concurrent.locks.Condition;
java.util.concurrent.locks.Lock;
java.util.concurrent.locks.ReentrantLock;


Question 3:

Singleton and Synchronization question:

If Thread1 calls synchronized method in Singleton class, then can another thread call getInstance() method [If synchronize(Singleton.class) is 1st statement in getInstance() method] of singleton class?


Answer 3:

Yes, another thread can call getInstance() method of singleton class. It is because, this time thread will acquire lock on Class object [Singleton.class].

So first thread acquired lock on Singleton instance and this another thread will acquire lock on Singleton Class's object.



Question 4:

Print all edge/corner nodes of a binary tree.

Answer:

Follow Level order traversal of binary tree. So , while doing level order traversal, if the current node happens to be the first node or last node in current level, print it.

void print(Node *root){

    //return if tree is empty.
    if(root == null)
        return;

    // Create an empty queue to store tree nodes.

    Queue<Node*> q;

    //enqueue root node
    q.push(root);

    // run till queue is not empty

    while(!q.empty()){

        //get size of current level
        int size = q.size();
        int n = size;

        //Process all noes present in current level
        while(n--){
         
             Node* node = q.front();
             q.pop();

              // If corner node found, print it.
              if(n == size-1 || n == 0)
                  println(node);

              //enqueue left and right child of current node
              if(node-> left != null)
                  q.push(node->left);

              if(node -> right != null)
                  q.push(node->right);
        }
        //terminate level by printing newline
        println();

    }
}




Question 5:

How locking mechanism is implemented by JVM?


Answer:

The implementation of locking mechanism in java is specific to the instruction set of the java platform.
For example with x86, it might use the CMPXCHG instruction - atomic compare and exchange  - at the lowest level to implement the fast path of the lock.

The CMPXCHG instruction is a compare-and-swap instruction that guarantees atomic memory access at the hardware level.

If the thread cannot acquire the lock immediately , then it could "spinlock" or it could perform a syscall to schedule a different thread. Different strategies are used depending on the platform , JVM Switches.


Question 6:

Is Java pass-by-value or pass-by-reference?


Answer:


Java is always pass-by-value.  Whenever we pass an object to some method, then we actually send a reference variable that points to the actual object. Means that reference variable will be containing the memory address as value. That's why Java is called as pass-by-value.

Let's take an example:

public static void main(String[] args){

    Book book = new Book("Java");
    Book bookNew = book;

    change(book);
    book.getName(); //Prints Java
}

public void change(Book book){
    book.getName() // Prints Java
 
    book = new Book("Angular");
    book.getName(); // Prints Anguar
}

So, here what we see, book name doesn't change after change() call , because passed book reference value points to new Book object [Angular].

Now, lets look at another example:

public static void main(String[] args){
    Book book = new Book("Java");
    Book bookNew = book;

    change(book);
    book.getName(); //Prints .Net

}

public void change(Book book){

    book.getName(); //Prints Java
    book.setName(".Net");
}

Here, what we see, value of book object gets changed after change() method call. Because book reference value in change() method still contains the address of actual Book object and it will act upon it only.




That's all for this post.

Hope these interview questions help everybody.

Thanks for reading.