20+ Backend Java Developer Interview Questions and Answers

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

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

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

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

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

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

An exception in Java is an event that disrupts the normal flow of a program's instructions.

• Exceptions are objects that are thrown when an abnormal condition occurs during the execution of a program.

• They can be caught and handled using try-catch blocks.

• Unchecked exceptions do not need to be declared in a method's throws clause, while checked exceptions do.

• Examples of exceptions in Java include NullPointerException, ArrayIndexOutOfBoundsException, and IOException.

Inheritance is a mechanism in object-oriented programming where a class inherits properties and behaviors from another class.

• Allows a class to inherit attributes and methods from another class

• Promotes code reusability and reduces redundancy

• Creates a parent-child relationship between classes

• Derived class can override inherited methods or add new methods

• Example: Class Car can inherit from class Vehicle

A singleton class is a class that can only have one instance created and provides a global point of access to that instance.

• Ensure the class has a private constructor to prevent external instantiation.

• Provide a static method to access the single instance of the class.

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

• Consider thread safety if the singleton will be accessed by multiple threads.

StreamsAPI is a feature in Java that allows for processing sequences of elements in a functional style.

• StreamsAPI provides a way to work with collections of objects in a more concise and declarative manner.

• It supports operations like filter, map, reduce, and collect.

• Example: List names = Arrays.asList("Alice", "Bob", "Charlie"); names.stream().filter(name -> name.startsWith("A")).forEach(System.out::println);

Design a REST API for a backend Java developer

• Define the resources and endpoints

• Use HTTP methods like GET, POST, PUT, DELETE

• Implement authentication and authorization

• Use JSON for data exchange

• Include error handling and status codes