30+ Backend Java Developer Interview Questions and Answers

This program filters employees with a salary greater than 40,000 from a list, showcasing Java's collection framework capabilities.

• Use of Streams: Java 8 introduced streams, which allow for functional-style operations on collections, making filtering straightforward.

• Example Code: List<Employee> highEarners = employees.stream().filter(e -> e.getSalary() > 40000).collect(Collectors.toList());

• Lambda Expressions: The filter method utilizes lambda expressions for concise and readab...read more

Use Java Stream API to create a map of names and their corresponding vowel counts.

• Use Stream.of() to create a stream from the array of names.

• Map each name to its vowel count using a custom method.

• Collect the results into a Map using Collectors.toMap().

• Example: For names ['Alice', 'Bob'], the output would be {'Alice': 3, 'Bob': 1}.

Use Java streams to filter prime numbers from a list, find highest employee salary, and demonstrate basic SQL joins.

• Use Java streams to filter prime numbers from the list

• Use streams to find the highest salary of employees

• Demonstrate basic SQL queries for joins

Counting leaves on trees can be simplified using algorithms and data structures for efficiency.

• Use a recursive function to traverse the tree structure and count leaves.

• Implement a depth-first search (DFS) or breadth-first search (BFS) algorithm.

• For large trees, consider parallel processing to speed up counting.

• Example: In a binary tree, count leaves by checking if a node has no children.

Multithreading in Java allows concurrent execution of tasks, improving performance and resource utilization in applications.

• Concurrency: Multithreading enables multiple threads to run simultaneously, allowing for better CPU utilization and faster execution of tasks.

• Responsiveness: In GUI applications, multithreading keeps the interface responsive by performing background tasks without freezing the UI.

• Resource Sharing: Threads within the same process share memory and resources...read more

To find the second highest salary, we can use SQL queries that utilize subqueries or the DISTINCT keyword.

• Using DISTINCT: SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 1; This retrieves the second highest salary.

• Using Subquery: SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees); This finds the maximum salary less than the highest.

• Using ROW_NUMBER: SELECT salary FROM (SELECT salary, ROW_NUMBER() OVER (ORDER BY salar...read more

JDK is Java Development Kit, JRE is Java Runtime Environment, and JVM is Java Virtual Machine. public static void main (String [] args) is the entry point of a Java program. ArrayList and LinkedList are both implementations of the List interface in Java, but ArrayList uses a dynamic array for storing elements while LinkedList uses a doubly linked list.

• JDK (Java Development Kit) is a software development kit used for developing Java applications.

• JRE (Java Runtime Environment) ...read more

No, static methods or variables cannot be created inside static main() method.

• Static methods or variables cannot be created inside another static method.

• Static methods or variables can only be created at the class level, outside of any method.

• Example: public class MyClass { static int myVariable = 10; }

Spring Boot framework provides rapid development, easy configuration, and built-in features. Annotations simplify development. Primary key uniquely identifies a record in a table, foreign key establishes a relationship between tables. @Qualifier in Spring Boot is used to specify which bean to autowire.

• Spring Boot offers rapid development, easy configuration, and built-in features like embedded servers.

• Annotations in Spring or Spring Boot simplify development by reducing boile...read more

Rearrange an array to move all zeroes to the end while maintaining the order of non-zero elements.

• Use two pointers: one for iterating through the array and another for tracking the position to place non-zero elements.

• Iterate through the array, and for each non-zero element, place it at the 'write' pointer's position and increment the pointer.

• After processing all elements, fill the remaining positions in the array with zeroes.

• Example: For input [0, 1, 0, 3, 12], output should ...read more

Idempotent term in patch and put means that multiple identical requests have the same effect as a single request.

• Idempotent means that the result of a successful request is the same regardless of how many times it is repeated.

• In PATCH and PUT requests, idempotent means that sending the same request multiple times will not have any additional side effects.

• For example, if a PATCH request updates a resource with specific data, sending the same PATCH request multiple times will n...read more

JVM is a virtual machine that executes Java bytecode. JRE is a runtime environment for Java programs. Multithreading is the ability of a program to execute multiple threads concurrently. JDBC is a Java API for connecting to databases. Loop is a control structure that allows a program to repeat a set of instructions.

• JVM executes Java bytecode

• JRE provides a runtime environment for Java programs

• Multithreading allows for concurrent execution of threads

• JDBC is used for connecting ...read more

Overriding static methods in Java does not behave like instance method overriding; it is based on the reference type, not the object type.

• Static methods belong to the class, not instances, so they cannot be overridden in the same way as instance methods.

• If a subclass defines a static method with the same name and parameters as a static method in the superclass, it hides the superclass method.

• Example: If class A has a static method 'static void display()' and class B extends A...read more

Query param is used to pass parameters in the URL query string, while path param is used to define parameters in the URL path itself.

• Query param is specified after the '?' in the URL, like '?param=value'

• Path param is specified in the URL path itself, like '/{param}'

• Query params are optional and can be used for filtering or sorting data

• Path params are used to define variables in the URL path, like IDs or names

HashMap is a data structure that stores key-value pairs and uses hashing to efficiently retrieve values based on keys.

• HashMap internally uses an array of linked lists to store key-value pairs.

• When a key-value pair is added, the key is hashed to determine the index in the array where the pair will be stored.

• If multiple keys hash to the same index, a linked list is used to handle collisions.

• Retrieving a value involves hashing the key to find the index and then traversing the li...read more

This Java program identifies the student with the second highest rank from a list of students based on their scores.

• Data Structure: Use an array or list to store student names and their corresponding ranks or scores.

• Sorting: Sort the list of students based on their ranks in descending order to easily access the second highest.

• Unique Ranks: Ensure that the ranks are unique; if not, handle duplicates appropriately.

• Example: For students with ranks [1, 3, 2, 4], the second highes...read more

Design a notification system that efficiently delivers messages across various platforms and devices.

• Use a message broker (e.g., RabbitMQ, Kafka) for decoupling services.

• Implement a REST API for sending notifications.

• Support multiple channels: email, SMS, push notifications.

• Consider user preferences for notification types and timings.

• Ensure scalability by using microservices architecture.

Custom exceptions can be created by extending the Exception class in Java.

• Create a new class that extends the Exception class

• Override the constructors to provide custom messages and error codes

• Throw the custom exception using the 'throw' keyword

Java 8 introduced new methods for HashMap such as forEach, compute, merge, etc.

• Java 8 introduced new methods like forEach, compute, merge for HashMap operations

• Java 8 allows using lambda expressions for iterating over HashMap entries

• Java 8 introduced default methods in Map interface for HashMap

Abstract class can have both abstract and non-abstract methods, while interface can only have abstract methods.

• Abstract class can have constructors, fields, and methods, while interface cannot have any of these.

• A class can extend only one abstract class, but can implement multiple interfaces.

• Abstract classes are used to provide a common base for subclasses, while interfaces are used to define a contract for classes to implement.

• Example: Abstract class 'Animal' with abstract m...read more

LinkedHashMap is a Java collection that maintains insertion order while providing fast access and retrieval of key-value pairs.

• Maintains a doubly-linked list of entries to preserve insertion order.

• Allows null values and one null key.

• Provides O(1) time complexity for basic operations like get() and put().

• Example: LinkedHashMap<String, Integer> map = new LinkedHashMap<>();

• Useful for caching and maintaining order in data processing.

Arrays have fixed size, while ArrayLists can dynamically resize. ArrayLists are part of Java Collections framework.

• Arrays have a fixed size, while ArrayLists can dynamically resize.

• Arrays can hold primitive data types and objects, while ArrayLists can only hold objects.

• Arrays use square brackets [] for declaration, while ArrayLists use angle brackets <>.

• Arrays are faster than ArrayLists for accessing elements directly by index.

• Example: String[] names = new String[5]; ArrayLis...read more

Find the smallest integer in an unsorted array.

• Iterate through the array and keep track of the smallest integer found so far.

• Use a for loop or a stream to iterate through the array.

• If the array is empty, return null or throw an exception.

• If the array contains only one element, return that element.

A palindrome list is a list that reads the same backward as forward.

• Traverse the list and store the elements in an array

• Compare the first element with the last element, second with second last and so on

• If all elements match, then the list is a palindrome

Polymorphism is the ability of a single function or method to operate on different types of data.

• Polymorphism allows objects of different classes to be treated as objects of a common superclass.

• There are two types of polymorphism: compile-time (method overloading) and runtime (method overriding).

• Example: Inheritance in Java allows a subclass to override a method of its superclass, exhibiting polymorphic behavior.

Profile in Spring Boot is a set of configurations that can be used to customize the behavior of an application.

• Profiles allow developers to define different configurations for different environments (e.g. development, testing, production).

• Profiles can be activated using the 'spring.profiles.active' property in the application.properties file.

• Profiles can be used to specify different database connections, logging levels, or other settings based on the environment.

• Example: 'app...read more

Yes, hash map stores unique keys.

• Hash map in Java does not allow duplicate keys. If you try to insert a duplicate key, it will replace the existing value with the new one.

• However, hash map allows duplicate values.

• Example: HashMap<String, Integer> map = new HashMap<>(); map.put("John", 25); map.put("Jane", 30); map.put("John", 35); // 'John' key will now have value 35.