Core Java Interview Questions and Answers (2024)

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1. What is Java?

Java is a high-level programming language created by James Gosling in 1991. It enables developers to create software that runs across various platforms and devices, including websites and mobile applications.

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2. What are the Features of Java?

• Simple: Java is easy to learn, making it accessible for developers of all skill levels.
• Object-Oriented: Everything in Java is treated as an object, with properties and behaviors. Developers can manipulate these through classes and objects.
• Platform-Independent: Write once, run anywhere. Java code runs on all operating systems like Windows, Linux, Unix, and macOS.
• Portable: Java bytecode can run on any system, ensuring code portability across platforms.
• Secure: Java uses internal pointers, which hide object memory locations for enhanced security.
• Robust: Java’s strong memory management and automatic garbage collection make it a stable language.
• Multithreading: Java supports multiple tasks running concurrently with shared memory.
• Architecture-Neutral: Java is independent of system architecture.
• Distributed: Java supports distributed systems like RMI and EJB.

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3. What is OOP?

OOP stands for Object-Oriented Programming. It focuses on creating programs using classes and objects. Here are its main concepts:

• Object: An instance that contains data (state) and behavior. Examples include chairs, pens, and bicycles.
• Class: A blueprint for objects that contains variables, methods, and constructors.
• Inheritance: Child classes inherit properties and methods from parent classes.
• Polymorphism: Performing a single task in different ways, such as method overloading and overriding.
• Abstraction: Hiding implementation details while showing only functionality, like using a phone to call without knowing how it works internally.
• Encapsulation: Binding data and code into a single unit, such as a Model class in Java.

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4. What is a Constructor?

A constructor is a special method used to initialize objects in Java. It shares the class name, has no return type, and cannot use keywords like abstract, static, or final.

Types of Constructors:

• Default Constructor: Parameter-less constructor that initializes default values.
• Parameterized Constructor: Accepts parameters to initialize objects with specific values.

Constructor Chaining:

This refers to one constructor calling another. It can happen within the same class using the this keyword or between different classes using the super keyword.

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5. What is an Abstract Class?
An abstract class in Java is a class that contains abstract methods (methods without a body) as well as concrete methods. It provides partial abstraction and is used to define common behavior for subclasses.

Achieving Abstraction:

• Abstract Class: Offers 0-100% abstraction.
• Interface: Offers 100% abstraction.

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6. What is Association and Composition?

Association refers to relationships between two or more objects. It comes in two types:

• Aggregation: A weak relationship where objects can exist independently (e.g., Student and College).

       Program Example:

         class Student {

int id;

String name;

String school_name;

//create constructor

System.out.println(“\nStudent name is “ + name);

System.out.println(“Student Id is “ + id);

System.out.println(“Student belongs to the “ + school_name + “School”);

}}

class school {

String schoolName;

int noOfStudents;

// create constructor

}

public class AggregationClass

public static void main(String[] args) {

Student n1 = new Student(1, “Shekher”, “Galgotia”);

Student n2 = new Student(2, “Trilok”, “IEC”);

Student n3 = new Student(3, “Aryan”, “ABC”)

}}

• Composition: A strong relationship where one object cannot exist without the other (e.g., Department and College).

Benefits of Association:

• Code reusability
• Cost-effectiveness
• Reducing redundancy

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7. What is JDK?

The JDK (Java Development Kit) is a software development kit used to develop Java applications. It includes tools like the Java compiler, Java Runtime Environment (JRE), and APIs needed to create and run Java programs.