90+ Java Software Developer Interview Questions and Answers

Explaining Java technical skills and analyzing code snippets for output prediction.

• Understand Java syntax: Familiarity with variables, data types, and operators.

• Control structures: Know how if-else, switch, and loops (for, while) work.

• Object-oriented principles: Grasp concepts like inheritance, polymorphism, and encapsulation.

• Exception handling: Be able to explain try-catch blocks and custom exceptions.

• Collections framework: Understand lists, sets, maps, and their methods.

To convert list to arraylist, use ArrayList constructor. To convert arraylist to list, use List constructor.

• To convert list to arraylist, use ArrayList constructor and pass the list as parameter.

• To convert arraylist to list, use List constructor and pass the arraylist as parameter.

• Example: List<String> list = Arrays.asList("one", "two", "three");

• ArrayList<String> arrayList = new ArrayList<>(list);

• Example: ArrayList<String> arrayList = new ArrayList<>(Arrays.asList("one", "two...read more

Accessing the same ArrayList object by two threads can cause race conditions and lead to unpredictable outcomes.

• Race conditions can occur when two threads try to modify the same element in the array at the same time.

• Synchronization can be used to prevent race conditions and ensure thread safety.

• Using a concurrent collection like CopyOnWriteArrayList can also prevent race conditions.

• If one thread is only reading from the array and the other is modifying it, then synchronizatio...read more

Functional interfaces are interfaces with only one abstract method. They are used for lambda expressions and method references.

• Functional interfaces are used for functional programming in Java.

• They are used for lambda expressions and method references.

• They have only one abstract method.

• Examples of functional interfaces are Runnable, Callable, and Comparator.

• Functional interfaces can be annotated with @FunctionalInterface to ensure they have only one abstract method.

Exceptions in Rest APIs are handled using try-catch blocks and appropriate error responses.

• Use try-catch blocks to catch exceptions that may occur during API execution.

• Handle different types of exceptions separately to provide specific error responses.

• Return appropriate HTTP status codes and error messages in the response.

• Log the exception details for debugging purposes.

• Consider using a global exception handler to centralize exception handling logic.

Interprocess Communication in Microservices architecture is typically achieved through lightweight protocols like HTTP or messaging queues.

• Microservices communicate with each other using RESTful APIs over HTTP.

• Message brokers like Kafka or RabbitMQ are used for asynchronous communication between microservices.

• Service mesh tools like Istio can be used to manage communication between microservices.

• gRPC can be used for high-performance, low-latency communication between microser...read more

Explaining CopyOnWriteArrayList and difference between synchronized and concurrent collections.

• CopyOnWriteArrayList is a thread-safe variant of ArrayList where all mutative operations are implemented by making a new copy of the underlying array.

• Synchronized collections use a single lock to synchronize access to the collection, while concurrent collections use multiple locks to allow concurrent access.

• Concurrent collections are designed to handle high-concurrency scenarios, wh...read more

Design a rate limiter service to control the number of requests a user can make in a given time frame.

• Use token bucket or leaky bucket algorithms for rate limiting.

• Implement a sliding window mechanism for more granular control.

• Store user request counts in a distributed cache like Redis.

• Set limits per user or IP address, e.g., 100 requests per minute.

• Provide feedback to users when limits are exceeded, e.g., HTTP 429 status.

Java 8 introduced new features like lambda expressions, Stream API, default methods, and more.

• Lambda expressions allow functional programming in Java.

• Stream API provides a way to work with collections in a more functional style.

• Default methods allow interfaces to have method implementations.

• Java 8 also introduced the new Date and Time API, Nashorn JavaScript engine, and more.

• MetaSpace in Java 8 is a replacement for the permanent generation in Java 7, used for storing class me...read more

Java 8 introduced several enhancements to HashMap including performance improvements and new methods.

• Java 8 introduced the 'compute', 'computeIfAbsent', and 'computeIfPresent' methods for HashMap.

• The 'forEach' method was added to HashMap in Java 8 for iterating over key-value pairs.

• Java 8 also introduced the 'merge' method for combining values in case of duplicate keys.

Race condition occurs when multiple threads try to access and modify the same resource simultaneously, while deadlock occurs when two or more threads are waiting for each other to release resources.

• Race condition is a situation in which the outcome of a program depends on the order of execution of its threads.

• Deadlock is a situation where two or more threads are blocked forever, waiting for each other to release resources.

• Race condition can lead to unpredictable behavior and ...read more

I use lambda expressions, streams, and default methods in my current project.

• Lambda expressions: I use them to write more concise and functional code.

• Streams: I use them for processing collections of data in a declarative way.

• Default methods: I use them to provide default implementations in interfaces.

Yes, unit testing is essential in my projects to ensure code quality and identify bugs early on.

• Yes, I have experience with unit testing in my projects.

• Unit testing helps in identifying bugs early in the development process.

• It ensures code quality and helps in maintaining code integrity.

• Unit tests also serve as documentation for the codebase.

• Examples: JUnit, Mockito, TestNG.

SpringBootApplication is a combination of three annotations in Spring Boot.

• SpringBootApplication is a meta-annotation that combines @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations.

• @Configuration annotation indicates that the class is a source of bean definitions.

• @EnableAutoConfiguration annotation enables Spring Boot to automatically configure the application based on the dependencies added to the classpath.

• @ComponentScan annotation tells Spring to sc...read more

The main difference between @EnableMock and @Spy in Junit5 is that @EnableMock creates a mock object while @Spy creates a spy object.

• The @EnableMock annotation is used to create a mock object for a class or interface, allowing you to define its behavior using Mockito.

• The @Spy annotation is used to create a spy object, which is a real object with some mock behavior. It allows you to mock specific methods while calling the real methods of the object.

• Mock objects created with @E...read more

ConcurrentModificationException is thrown when an object is modified concurrently while iterating over it.

• Occurs when a collection is modified while being iterated over using an iterator

• Can be avoided by using ConcurrentHashMap or CopyOnWriteArrayList

• Example: ArrayList being modified while iterating over it

The main difference between @PutMapping and @PatchMapping is the level of data that is updated.

• PutMapping is used to update an entire resource, while PatchMapping is used to update only specific fields of a resource.

• PutMapping replaces the entire resource with the new data provided, while PatchMapping updates only the specified fields.

• PutMapping is idempotent, meaning multiple identical requests will have the same effect as a single request, while PatchMapping may not be idem...read more

Streams in Java are a sequence of elements that can be processed in parallel or sequentially.

• Streams provide a concise and functional way to perform operations on collections in Java.

• They can be used to filter, map, reduce, and perform other operations on data.

• Streams can be processed in parallel, improving performance on multi-core systems.

• They are part of the Java 8 Stream API and can be used with collections, arrays, or I/O channels.

• Example: List numbers = Arrays.asList(1,...read more

ResponseState and ServerState are different states in a software system that represent different aspects of the system's functionality.

• ResponseState typically refers to the state of a response object in a software system, indicating whether the response was successful, failed, or pending.

• ServerState, on the other hand, refers to the state of the server in a software system, indicating whether the server is running, stopped, or experiencing issues.

• ResponseState is more focused...read more

Polymorphism is the ability of a single function or method to operate on different data types. Types include compile-time and runtime polymorphism.

• Polymorphism allows a single function to work with different data types.

• Compile-time polymorphism is achieved through method overloading.

• Runtime polymorphism is achieved through method overriding.

• Example: Overloading a method with different parameter types.

• Example: Overriding a method in a subclass to provide specific implementatio...read more

SQL stands for Structured Query Language. DDL commands are used to define the structure of a database. RDBS stands for Relational Database Management System.

• SQL stands for Structured Query Language

• DDL commands are used to define the structure of a database

• RDBS stands for Relational Database Management System

The Java program counts the length of a string without using foreach and length function.

• Iterate through each character of the string using a for loop

• Increment a counter variable for each character encountered

• Return the counter variable as the length of the string

Yes, RESTful web services use HTTP methods to implement the concept of REST.

• HTTP methods like GET, POST, PUT, DELETE are used to perform CRUD operations on resources.

• GET is used to retrieve a resource, POST is used to create a new resource, PUT is used to update an existing resource, and DELETE is used to delete a resource.

• RESTful web services use these HTTP methods to provide a uniform interface for accessing and manipulating resources.

• For example, a RESTful web service for ...read more

Inner join returns only the rows that have matching values in both tables, while left join returns all rows from the left table and the matched rows from the right table.

• Inner join: returns rows where there is a match in both tables

• Left join: returns all rows from the left table and the matched rows from the right table

• Example: Inner join - SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id

• Example: Left join - SELECT * FROM table1 LEFT JOIN table2 ON table1.id = ...read more

The main method serves as the entry point for Java applications, enabling the JVM to execute the program.

• Entry Point: The main method is where the Java Virtual Machine (JVM) starts executing the program. For example, 'public static void main(String[] args)'.

• Static Method: It is declared as static, meaning it can be called without creating an instance of the class.

• String Array Parameter: The main method accepts a String array as an argument, allowing command-line arguments to ...read more

Program to find largest number in array with increasing then decreasing numbers.

• Find the peak element in the array using binary search.

• If peak element is at index 0 or last index, return it.

• Else, search for largest element in left and right subarrays.

OOP principles include encapsulation, inheritance, polymorphism, and abstraction, essential for modular and maintainable code.

• Encapsulation: Bundling data and methods. Example: A class 'Car' with attributes like 'speed' and methods like 'accelerate()'.

• Inheritance: Deriving new classes from existing ones. Example: 'ElectricCar' inherits from 'Car', adding features like 'batteryCapacity'.

• Polymorphism: Methods behaving differently based on the object. Example: A method 'start()'...read more