300+ Java Developer Interview Questions and Answers for Freshers

The function sorts an integer array containing only 0s and 1s in linear time complexity.

• Use two pointers, one starting from the beginning and the other from the end of the array.

• Swap the elements at the pointers if the element at the beginning pointer is 1 and the element at the end pointer is 0.

• Continue moving the pointers towards each other until they meet in the middle of the array.

The task is to convert a Binary Search Tree into a Greater Sum Tree.

• Traverse the BST in reverse inorder (right, root, left) to visit nodes in descending order.

• Keep track of the sum of all greater nodes encountered so far.

• Update the value of each node by adding the sum of greater nodes and update the sum.

• Continue this process until all nodes have been visited.

• Return the modified BST.

Interfaces can be used to connect two classes in Java.

• Interfaces define a contract that implementing classes must adhere to.

• A class can implement multiple interfaces.

• Interfaces can be used to achieve loose coupling and abstraction.

• Example: Connecting a Car class with a Fuelable interface.

No, I don't have any Java certificate.

• No, I don't possess any Java certification.

• Unfortunately, I haven't obtained any Java certificate.

• Regrettably, I do not hold any Java certification.

Memory is allocated when an instance of a class is created.

• Memory is allocated dynamically when an object is instantiated.

• Each instance of a class has its own memory allocation.

• Memory allocation occurs at runtime, not during class creation.

Local, static, and instance variables differ in their scope and lifetime.

• Local variables are declared inside a method and have a limited scope.

• Static variables belong to the class and are shared among all instances.

• Instance variables are unique to each object and can be accessed using the object reference.

Session in Java is a way to store information about a user across multiple requests.

• Session is used to maintain stateful information about a user.

• It is created when a user first accesses a web application and remains active until the user logs out or the session times out.

• Session data is stored on the server and can be accessed by multiple requests from the same user.

• It is commonly used for user authentication, shopping carts, and personalization.

• The HttpSession interface in ...read more

Implicit objects in JSP servlets are predefined objects that can be accessed without any declaration or initialization.

• Implicit objects are automatically available in JSP servlets without the need for explicit declaration or initialization.

• These objects provide access to various functionalities and information related to the JSP environment.

• Some commonly used implicit objects include request, response, session, application, out, pageContext, and exception.

• For example, the req...read more

Java is a high-level programming language known for its portability and object-oriented programming support. OOPs stands for Object-Oriented Programming. HashMap and HashSet are both data structures in Java, but HashMap is used to store key-value pairs while HashSet is used to store unique elements.

• Java is a high-level programming language used for developing applications and software.

• OOPs (Object-Oriented Programming) is a programming paradigm based on the concept of objects...read more

The question is about exception handling, OOPs, and the difference between abstraction and encapsulation in a project.

• Exception handling is a mechanism to handle runtime errors and prevent program termination.

• Abstraction is the process of hiding implementation details and showing only necessary information to the user.

• Encapsulation is the process of binding data and methods together in a single unit.

• In a project, abstraction can be achieved by using interfaces and abstract cl...read more

An object in Java is an instance of a class that encapsulates data and behavior.

• Objects have state and behavior

• They are created from classes

• They can be used to represent real-world entities or concepts

• Objects can interact with each other through method calls

HashMap and Hashtable are both used to store key-value pairs, but there are some differences.

• HashMap is not synchronized, while Hashtable is synchronized.

• HashMap allows null values and one null key, while Hashtable does not allow null values or keys.

• HashMap is generally preferred over Hashtable for non-thread-safe applications.

• HashMap is faster than Hashtable as it is not synchronized.

• Example: HashMap map = new HashMap<>();

An interface is a blueprint for classes, defining methods that must be implemented. A class is a blueprint for objects.

• An interface can only have abstract methods, while a class can have both abstract and concrete methods.

• A class can be instantiated to create objects, while an interface cannot be instantiated.

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

• Interfaces are used to achieve multiple inheritance in Java.

• Classes can have fields, construc...read more

Access modifiers in Java control the visibility and accessibility of classes, methods, and variables.

• There are four types of access modifiers in Java: public, private, protected, and default.

• Public access modifier allows unrestricted access from any class or package.

• Private access modifier restricts access to only within the same class.

• Protected access modifier allows access within the same class, package, and subclasses.

• Default access modifier (no keyword) restricts access t...read more

Abstract class is a class that cannot be instantiated and can have both abstract and non-abstract methods. Interface is a blueprint for classes to implement.

• Abstract class can have constructors, while interface cannot.

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

• Abstract class can have instance variables, while interface cannot.

• Abstract class can provide default implementation for methods, while interface cannot.

• Interfaces are used to achi...read more

Method overloading allows creating multiple methods with the same name but different arguments.

• Method signature must differ in number, type, or order of parameters

• Return type can be different but not the only difference

• Example: public void print(int num), public void print(String str), public void print(int[] arr)

• Overloading improves code readability and reusability

Composition is a way to combine objects to create complex structures, while inheritance is a way to create new classes based on existing ones.

• Composition is a 'has-a' relationship, where an object contains other objects as its parts.

• Inheritance is an 'is-a' relationship, where a subclass inherits properties and behaviors from its superclass.

• Composition allows for greater flexibility and modularity in design, as objects can be easily swapped out or added.

• Inheritance can lead t...read more

No, subclass cannot access private methods of the parent class.

• Private methods in a class are only accessible within that class.

• Subclasses can only access public and protected methods of the parent class.

• Private methods are not inherited by subclasses.

Different types of collections in Java include List, Set, Queue, and Map.

• List: Ordered collection that allows duplicate elements (e.g. ArrayList, LinkedList)

• Set: Unordered collection that does not allow duplicate elements (e.g. HashSet, TreeSet)

• Queue: Collection that holds elements prior to processing (e.g. LinkedList, PriorityQueue)

• Map: Collection that maps unique keys to values (e.g. HashMap, TreeMap)

Answers to common Java interview questions on super keyword, inheritance, and abstract classes.

• The super keyword is used to call a method or constructor in the parent class.

• Inheritance is a mechanism in which one class acquires the properties and methods of another class.

• Abstract classes are classes that cannot be instantiated and are used as a base for other classes to inherit from.

• Abstract methods are methods that do not have a body and must be implemented by the subclass.

• P...read more

No, a subclass cannot inherit the properties of a parent class if the parent class is not loaded.

• In Java, inheritance allows a subclass to inherit the properties and methods of its parent class.

• However, for a subclass to inherit the properties of its parent class, the parent class must be loaded.

• If the parent class is not loaded, the subclass will not have access to its properties.

• To ensure that a subclass can inherit the properties of its parent class, the parent class must ...read more

HashMap is a key-value pair collection while HashSet is a set of unique elements.

• HashMap allows duplicate values but not duplicate keys

• HashSet does not allow duplicate values

• HashMap implements Map interface while HashSet implements Set interface

• HashMap uses put() method to add elements while HashSet uses add() method

• Example: HashMap<String, Integer> map = new HashMap<>(); HashSet<String> set = new HashSet<>();

Hibernate is an ORM framework for Java that simplifies database interactions, and it integrates seamlessly with Spring Boot.

• Hibernate is an Object-Relational Mapping (ORM) tool that maps Java objects to database tables.

• It provides a framework for managing database operations using Java objects, reducing boilerplate code.

• Spring Boot simplifies the configuration of Hibernate through application properties.

• Example: Use @Entity annotation to define a model class, and @Repository ...read more

Thread in Java is a lightweight sub-process that executes a set of instructions independently.

• Thread can be defined by extending the Thread class or implementing the Runnable interface.

• Thread class provides various methods to control the execution of threads like start(), sleep(), join(), etc.

• Runnable interface has only one method run() which needs to be implemented.

• Threads can be created and started using the start() method.

• Example: public class MyThread extends Thread { pub...read more

throw is used to explicitly throw an exception, while throws is used to declare that a method may throw an exception.

• throw is used within a method to throw an exception explicitly

• throws is used in the method signature to declare that the method may throw an exception

• throw is followed by an instance of an exception class

• throws is followed by the list of exception classes that the method may throw

HashSet uses HashMap internally to store unique elements with null values allowed.

• HashSet is backed by a HashMap, where each element is stored as a key.

• The value associated with each key in HashMap is a constant (usually 'PRESENT').

• HashSet allows only unique elements; duplicates are ignored based on the hash code.

• Example: Adding 'A' and 'B' to HashSet results in two entries in HashMap.

• HashSet allows one null value, which is stored as a key in HashMap.

HTML stands for HyperText Markup Language, used for creating and structuring web pages.

• HTML is a markup language used to create the structure of web pages.

• It consists of elements enclosed in tags, such as <html>, <head>, <body>.

• Attributes can be added to elements to provide additional information or functionality, like <img src='image.jpg'>.

• HTML is the foundation of web development and is often used in conjunction with CSS and JavaScript.