Wissen Technology Java Developer Interview Questions and Answers

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...read more
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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
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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...read more
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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?
Ans. 
I am a Java Developer currently residing in New York City.
• Experienced Java Developer

• Residing in New York City

• Strong knowledge of Java programming language
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Q2. What is a HashMap in Java, and how does it function internally?
Ans. 
HashMap in Java is a data structure that stores key-value pairs and uses hashing to efficiently retrieve values.
• HashMap is part of the Java Collections framework.

• It allows null keys and values.

• Internally, it uses an array of linked lists to handle collisions.

• The key's hash code is used to determine the index in the array where the key-value pair is stored.

• Example: HashMap<String, Integer> map = new HashMap<>...read more
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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
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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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