Java Interview @ Dew Solutions

Hi  Friends,

In this post , I'm sharing the Java interview questions asked in Dew Solutions recently.


Question 1:

Java is pass by value or reference.


Answer:

Everything in java is pass-by-value.
When we pass object reference to some method, we actually pass address of that reference which in turn contains memory address of actual object.
And as  we know, memory address is a value, so means, we always use pass-by-value in java.

e.g.:

public static void main(String[] args){

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

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

public void change(Book book){
    book.getName(); //Prints Java

    book = new Book("Angular");
    book.getName();// Prints Angular
}
So here, what we see Book name doesn't change after change() method, because passed book reference in change() method now points to new Book Object with value "Angular".


Now, lets look at another example:

public static void main(String[] args){

    Book book = new Book("Java");
    Book newBook = 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 after change() method call has been changed to .Net. It is because book reference in change() method still contains memory address of Book object with java as value.



Question 2:

We have multiple Employee objects and we need to store these objects in TreeMap. What problems we can face while storing Employee objects?

Answer:

We will get error at runtime : "No suitable method found for sort(List<Employee>)".
It is because Employee class doesn't implement the Comparable interface so the sort() method cannot compare the objects.

As TreeMap stores object in Ascending order by default using natural ordering. So, each object which needs to be stored in TreeMap should implement Comparable interface or  Comparator interface.



Question 3:

What is Callable interface and how it is similar/different to/from Runnable?

Answer:

Callable interface is just like Runnable interface.

Difference between Callable and Runnable is given below:

• Runnable is defined in Java 1.0  . While Callable was introduced in Java 5.
• Callable includes call() method which returns object and also throws Checked exception. On the other hand , Runnable's run() method neither return any value nor throws any Checked exception. 
• Runnable instance can be passed to a thread. While Callable instance can't be passed to a thread.

Similarities between Runnable and Callable:

• Both are used to encapsulate code which needs to be executed parallely in separate thread.
• Both interfaces can be used with Executor framework introduced in java 5.
• Both includes single abstract method. Means , both can be used in Lambda expressions in java 8.

Question 4:

Can HashSet store duplicate objects?


Answer:

No, HashSet cannot store duplicate objects. As HashSet is implementation of Set interface and Set interface is meant to store unique elements. That's why HashSet doesn't store duplicate objects.


Question 5:

SQL query to find employee with highest salary and corresponding department.


Answer:

select department , max(salary) from employee
                               group by department;


Question 6:

Design the architecture of Vending machine?

Answer:

Question 7:

Describe one problem that you have solved in production build?

Answer:

In my previous company, client complained about slowness in the system performance.
We tracked the logs and found there was problem with some memory leaks happening.

We just immediately asked client to increase the JVM heap size for the moment so that it worked  and later on we corrected the problem by removing all the memory leaks in the code.



Question 8: Why to use Lock in multithreading? What are the locking mechanisms available in java?


Answer:

Lock is required in multithreading to properly distribute access of Monitor associated with an object among multiple threads.
If we don't use Locks, then multiple threads will try to acquire shared resources and we will get corrupted outputs from threads.


Locking mechanisms available in java are:

• synchronized block
• synchronized methods
• Read/Write lock
• Atomic classes
• final classes

That's all about this interview.

Hope this post helps everybody in interviews.

Thanks for reading.