Wissen Technology Java Developer Interview Questions, Process, and Tips

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. Implement a custom stack that allows fetching the minimum element in constant time, O(1).
Ans. 
Custom stack with constant time access to minimum element
• Use two stacks - one to store elements and another to store minimum values

• When pushing an element, compare with top of min stack and push the smaller value

• When popping an element, pop from both stacks

• Access minimum element in O(1) time by peeking at top of min stack
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Q2. How can one create an immutable class named Employee with fields for name and hobbies defined as List?
Ans. 
To create an immutable class named Employee with fields for name and hobbies defined as List<String>, use private final fields and return new instances in getters.
• Use private final fields for name and hobbies in the Employee class

• Provide a constructor to initialize the fields

• Return new instances or unmodifiable lists in the getters to ensure immutability
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Q3. What changes are required in the Employee class to use it as a key for a HashMap?
Ans. 
Add hashCode() and equals() methods to Employee class.
• Override hashCode() method to generate a unique hash code for each Employee object.

• Override equals() method to compare Employee objects based on their attributes.

• Ensure that the attributes used in hashCode() and equals() methods are immutable.

• Example: Add id, name, and department attributes to Employee class and override hashCode() and equals() methods based on thes
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Q4. What is the implementation of the singleton design pattern?
Ans. 
Singleton design pattern ensures a class has only one instance and provides a global point of access to it.
• Create a private static instance of the class within the class itself.

• Provide a public static method to access the instance, creating it if necessary.

• Ensure the constructor of the class is private to prevent instantiation from outside the class.

• Use lazy initialization to create the instance only when needed.

• Thread...read more
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Q5. Different ways to create a thread
Ans. 
Ways to create a thread in Java include extending the Thread class, implementing the Runnable interface, and using Java 8's lambda expressions.
• Extend the Thread class and override the run() method

• Implement the Runnable interface and pass it to a Thread object

• Use Java 8's lambda expressions with the Executor framework
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Q6. What are the SOLID principles in software design, and can you provide examples for each?
Ans. 
SOLID principles are a set of five design principles in object-oriented programming to make software more maintainable, flexible, and scalable.
• Single Responsibility Principle (SRP) - A class should have only one reason to change. Example: A class that handles user authentication should not also handle database operations.

• Open/Closed Principle (OCP) - Software entities should be open for extension but closed for modific...read more
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Q1. What is the concept of Object-Oriented Programming (OOP) and can you provide an example?
Ans. 
OOP is a programming paradigm based on the concept of objects, which can contain data in the form of fields and code in the form of procedures.
• OOP focuses on creating objects that interact with each other to solve a problem.

• Encapsulation is a key concept in OOP, where data is kept within an object and only accessible through its methods.

• Inheritance allows objects to inherit attributes and methods from parent classes.

• Po...read more
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Q2. How do threads communicate with each other in Java?
Ans. 
Threads communicate in Java through shared memory or message passing.
• Threads can communicate through shared variables in memory.

• Threads can communicate through synchronized methods or blocks.

• Threads can communicate through wait(), notify(), and notifyAll() methods.

• Threads can communicate through message passing using classes like BlockingQueue or ExecutorService.
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Q3. Write a program to reverse a linked list.
Ans. 
Program to reverse a linked list
• Create a new linked list to store the reversed elements

• Traverse the original linked list and insert each node at the beginning of the new list

• Update the head of the new list as the last node of the original list
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Q4. What is the difference between string literals and the string pool?
Ans. 
String literals are constant strings defined in code, while the string pool is a memory area where unique string objects are stored.
• String literals are created using double quotes, while string pool objects are created using the 'new' keyword.

• String literals are stored in the string pool to conserve memory and improve performance.

• String literals are automatically interned by the JVM, while string pool objects need to b
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Q5. What is the program to print the 90-degree rotated view of a 2D array?
Ans. 
Program to print the 90-degree rotated view of a 2D array.
• Iterate through each column in reverse order and print the corresponding row elements.

• Repeat this process for all columns to get the rotated view of the 2D array.
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The assessment consists of basic coding and SQL-related questions, including two challenge questions. The total test duration is 1 hour and 30 minutes, featuring a total of 7 questions, with a passing score of 50% out of 120 points.

Q1. Can you provide an introduction about yourself, including your current place of residence?
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Q2. What is a HashMap in Java, and how does it function internally?
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Q3. I was asked to solve a question related to a 2D array that involved iterating through the array and storing the elements in a new array.
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Q4. Find second highest salary from employee table?
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There were 3 to 4 questions related to the camera that needed to be solved within the given time.

Q1. Postmortem of Hashmap
Ans. 
Hashmap postmortem involves analyzing performance, memory usage, collisions, and resizing.
• Analyze performance: Check for time complexity of operations like get, put, and remove.

• Memory usage: Evaluate memory footprint and potential memory leaks.

• Collisions: Investigate collision resolution strategies like separate chaining or open addressing.

• Resizing: Examine load factor and rehashing process for efficient resizing.

• Examp...read more
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Q2. Locking mechanism in multithreading
Ans. 
Locking mechanism in multithreading is used to control access to shared resources by multiple threads.
• Locks are used to prevent multiple threads from accessing shared resources simultaneously

• Types of locks include synchronized blocks, ReentrantLock, and ReadWriteLock

• Locks help prevent race conditions and ensure data consistency in multithreaded applications
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Q3. Find highest salaried emp from each dept
Ans. 
Use SQL query to group by department and find employee with highest salary in each department
• Write a SQL query to group by department and select max salary for each department

• Join the result with employee table to get employee details

• Example: SELECT dept, emp_name, MAX(salary) FROM employees GROUP BY dept
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Q4. Sorting algorithms
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