Wissen Technology Java Developer Interview Questions, Process, and Tips

It was on online platform consists of 7 questions, 2 coding each of 40 marks, 1 SQL query- 20 marks and 4 MCQ's 5 marks each

Q1. To merge the array and print it in sorted manner , time complexity of code what you have written, optimise it if required. More on Map, System design questions
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3 coding ques , basics of oops

3 coding ques , basics of oops

Q1. What are the ways to create the threads and all
Ans. 
There are two ways to create threads in Java: by extending the Thread class or by implementing the Runnable interface.
• Extending the Thread class: Create a new class that extends the Thread class and override the run() method.

• Implementing the Runnable interface: Create a new class that implements the Runnable interface and implement the run() method.

• Example using Thread class: MyThread extends Thread { public void run()...read more
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Q1. 1. Can we keep an object of a Class as key in HashMap?
Ans. 
Yes, we can keep an object of a Class as a key in HashMap.
• The key in a HashMap can be any non-null object.

• The key's class must override the hashCode() and equals() methods for proper functioning.

• If two objects have the same hashCode(), they will be stored in the same bucket and compared using equals().

• If the key is mutable, its state should not be modified after it is used as a key in the HashMap.
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Q2. 2. What will happen if hashcode only returns a constant? How will it affect the internal working of the HashMap?
Ans. 
If hashcode returns a constant, all elements will be stored in the same bucket, resulting in poor performance.
• All elements will be stored in the same bucket of the HashMap.

• This will lead to poor performance as the HashMap will essentially become a linked list.

• Retrieving elements will take longer as the HashMap will have to iterate through the linked list to find the desired element.
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Q3. 3. What if we do not override Equals method ? What is we do not override Hashcode method?
Ans. 
Not overriding equals method can lead to incorrect comparison of objects. Not overriding hashCode method can lead to incorrect behavior in hash-based data structures.
• If equals method is not overridden, the default implementation from Object class will be used which compares object references.

• This can lead to incorrect comparison of objects where two different objects with same content are considered unequal.

• If hashCode...read more
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Q4. 4. How to make a class immutable?
Ans. 
To make a class immutable, we need to ensure that its state cannot be modified after creation.
• Make all fields private and final

• Do not provide any setters

• Ensure that mutable objects are not returned from methods

• Make the class final or use a private constructor

• Consider using defensive copying
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Q5. 5. If we have a mutable object inside immutable class then how will you handle it?
Ans. 
To handle a mutable object inside an immutable class, make the object private and provide only read-only access methods.
• Make the mutable object private to encapsulate it within the immutable class.

• Provide read-only access methods to retrieve the state of the mutable object.

• Ensure that the mutable object cannot be modified from outside the class.

• If necessary, create defensive copies of the mutable object to prevent unin
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Q6. 6. Advantages and Disadvantages of immutable class?
Ans. 
Immutable classes have advantages like thread safety, security, and simplicity, but also have disadvantages like memory usage and performance overhead.
• Advantages of immutable classes:

• - Thread safety: Immutable objects can be safely shared among multiple threads without the need for synchronization.

• - Security: Immutable objects cannot be modified, which can prevent unauthorized changes to sensitive data.

• - Simplicity: Im...read more
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Q7. How to sort a list of students on the basis of their First name?
Ans. 
To sort a list of students on the basis of their First name, use the Collections.sort() method with a custom Comparator.
• Create a custom Comparator that compares the first names of the students.

• Implement the compare() method in the Comparator to compare the first names.

• Use the Collections.sort() method to sort the list of students using the custom Comparator.
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Q8. Object level Lock vs Class level lock?
Ans. 
Object level lock is used to synchronize access to an instance method or block, while class level lock is used to synchronize access to a static method or block.
• Object level lock is acquired on a per-object basis, while class level lock is acquired on a per-class basis.

• Object level lock is useful when multiple threads are accessing the same instance method or block, while class level lock is useful when multiple thread...read more
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Q9. If there is a class inside it we have a method - public void synchronized M1() - inside method - Thread.Sleep(500); We create 2 different object of the class and invoke the method. Ob1.M1() Ob2.M1() How ...read more
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Q10. Using Java 8 find the sum of squares of all the odd numbers in the arraylist.
Ans. 
Using Java 8, find the sum of squares of all the odd numbers in the arraylist.
• Use the stream() method to convert the ArrayList to a stream

• Use the filter() method to filter out the even numbers

• Use the mapToInt() method to square each odd number

• Use the sum() method to find the sum of the squared odd numbers
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Q11. LeetCode 1 - Two Sum.
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Q12. LeetCode 20 - Valid Parantheses
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Q13. Parent class has run() and walk() . Parent run() - calls walk() Child Class overrides Parent class Inside run() - calls super.run() Walk() - calls super.walk In main class Parent p = new child() p.ru...read more
Ans. 
The order of execution will be: parent's run() -> child's run() -> parent's walk()
• When p.run() is called, it will first execute the run() method of the parent class

• Inside the parent's run() method, it will call the walk() method of the parent class

• Since the child class overrides the parent class, the child's run() method will be executed next

• Inside the child's run() method, it will call the run() method of the parent c...read more
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Q14. LeetCode - 532 , K diff pairs
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Q15. What is Serialization?
Ans. 
Serialization is the process of converting an object into a stream of bytes to store or transmit it over a network.
• Serialization is used to save the state of an object and recreate it later.

• It is also used to send objects over a network as a byte stream.

• Java provides Serializable interface to implement serialization.

• Serialization can be used for deep cloning of objects.

• Example: Saving an object to a file or sending it
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Q16. Synchronized Map vs Concurrent hashmap
Ans. 
Synchronized Map is thread-safe but slower, while Concurrent HashMap is faster but not entirely thread-safe.
• Synchronized Map locks the entire map during write operations, causing other threads to wait.

• Concurrent HashMap allows multiple threads to read and write simultaneously, using a lock-free approach.

• Synchronized Map is useful for small maps or low-concurrency scenarios.

• Concurrent HashMap is better suited for large ...read more
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Q17. Write a query to find name of authors who have written more than 10 books. Table - Bookauthhor Column - Book Column - Author
Ans. 
Query to find authors who have written more than 10 books
• Use the GROUP BY clause to group the records by author

• Use the HAVING clause to filter the groups with a count greater than 10

• Join the Bookauthor table with the Author table to get the author names
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Q18. Write a program to return the length of the longest word from a string whose length is even?
Ans. 
This program returns the length of the longest word from a string whose length is even.
• Split the string into an array of words using a space as the delimiter.

• Iterate through the array and check if the length of each word is even.

• Keep track of the length of the longest even word encountered.

• Return the length of the longest even word.
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Q1. How much salary expectations?
Ans. 
Answer the salary expectations question in an interview for a Java Developer position.
• Research the average salary range for Java Developers in your location and level of experience.

• Consider your qualifications, skills, and experience when determining your salary expectations.

• Be realistic and flexible in your salary expectations, taking into account the company's budget and industry standards.

• Provide a salary range rath...read more
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Q2. Do you have location constraints?
Ans. 
No location constraints.
• I am open to working in any location.

• I am willing to relocate if required.

• I do not have any personal or family commitments that tie me to a specific location.
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Q3. Why are you leaving this company?
Ans. 
Seeking new challenges and growth opportunities.
• Looking for a company that aligns better with my career goals.

• Seeking a more collaborative and innovative work environment.

• Desire to work on larger and more complex projects.

• Want to expand my skill set and learn new technologies.

• Seeking better work-life balance.

• Company restructuring or downsizing.

• Limited career advancement opportunities in current company.
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Q1. Given hackerank platform to test code
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2 coding questions and 1 sql mcq

Couple of quants and technical mcqs and one coding question.

Q1. Internal implementation of hashmap.
Ans. 
HashMap is internally implemented using an array of linked lists.
• HashMap uses an array to store key-value pairs

• Each array index is called a bucket

• Each bucket can store multiple key-value pairs using a linked list

• Hashing is used to determine the index of the array for a given key

• If multiple keys hash to the same index, they are stored in the same bucket
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Q2. What is immutable class how can we create it?
Ans. 
An immutable class is a class whose objects cannot be modified after they are created.
• To create an immutable class, make the class final so that it cannot be extended.

• Make all the fields private and final, so they cannot be modified.

• Do not provide any setter methods, only provide getter methods.

• If the class contains mutable objects, make sure to return a copy of the object instead of the original.

• Ensure that the class ...read more
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