In this post, we will explore some of the most frequently asked basic Java interview questions. It contains interview questions from different Java topics like strings, threads, servlets, OOPs etc. These questions are easy to answer and it will also enhance your confidence and help you ace your Java interviews.
Mostly Asked Java Basic Interview Questions:
1. What is Java?
Java is a high-level, object-oriented, secure, and widely used programming language that is used to develope standalone applications, mobile applications, web and distributed applications. It allows you to write code that can run on any device with the Java Virtual Machine (JVM).
2. Explain the main features of Java.
Key features include platform independence, object-oriented, simplicity, security, and robustness.
3. Differentiate between JDK, JRE, and JVM.
- JDK (Java Development Kit) is a software development kit which provides a set of development tools needed to create, compile and debug java applications.
- JRE (Java Runtime Environment) provides the libraries, Java Virtual Machine (JVM), and other components to run applications written in Java.
- JVM (Java Virtual Machine) is an abstract machine that enables the execution of Java bytecode.
4. What is the main difference between == and .equals() method in Java?
- "==" used to compare object references, to check if they are pointing to the same memory location.
- ".equals()" method compares the actual content of two objects.
5. Explain the concept of Object-Oriented Programming (OOP) in Java.
Object oriented programming is a way to organize and structure the code. It is used to increase code readability and reusability. OOPs in java provides 4 important concepts called inheritance, encapsulation, polymorphism and abstraction.
6. What is the significance of the "static" keyword in Java?
When we make variables or methods static, they belong to the class rather than a particular object of the class. i.e, if a variable or method is static, it is the same for every instance of the class, and you can use it without creating an object.
Let's take a quick example: If you have a static method, it's like a helper that everyone in the class can call without creating a special member.
7. How does Java support multithreading?
Java supports multithreading by extending the Thread class or implementing the Runnable interface.
8. Explain the difference between the "final," "finally," and "finalize" keywords.
- "final" is used to declare constants, make methods not overrideable, or prevent inheritance.
- "finally" is a block of code that is always executed, whether an exception is thrown or not.
- "finalize" is a method called by the garbage collector before reclaiming an object's memory.
9. What is the purpose of the "main" method in Java?
The "main" method is the entry point of a Java program. It is where the program starts its execution.
10. How does exception handling work in Java?
Exception handling in Java is done through try, catch, and finally blocks. Code that might throw exceptions is placed inside the "try" block, and the exception is caught and handled in the "catch" block. The "finally" block is optional and is executed regardless of whether an exception is thrown.
11. Can you explain why we use "this" keyword in Java?
"this" keyword is used to refer to the current instance of a class. It is used to differentiate instance variables from local variables when they share the same name.
For example: If our class contains a constructor with parameters declared with the same names as the class fields, then we can use the "this" keyword to differentiate between the class field variables and the constructor parameters.
class Student{
//class fields
String name;
int rollNumber;
//Constructor with the parameters having same name as a class field names
public Student(String name, int rollNumber){
this.name = name; //here this.name refers to the class fields and the value without this refers to the constructor parameter
this.rollNumber = rollNumber;
}
}
12. What is method overloading?
Method overloading is the ability to define multiple methods in the same class with the same name but different parameters.
13. Differentiate between method overloading and method overriding.
Method Overloading: means within a class having methods with the same name but different parameters.
Method Overriding: involves a child class providing a specific implementation for a method already defined in its parent class.
14. How does Java support multiple inheritance?
Java supports multiple inheritance through interfaces. A class can implement multiple interfaces, allowing it to inherit methods from all the implemented interfaces.
15. What is the purpose of the "super" keyword in Java?
The "super" keyword is used to refer to the parent class. It is often used to invoke the parent class's methods, access parent class fields, or call the parent class's constructor.
16. What is JDBC?
JDBC, or Java Database Connectivity, is like the bridge between Java programs and databases. Imagine it as a friendly messenger that helps Java talk to databases. With JDBC, Java applications can send requests to databases, get the needed information, and do things like storing or retrieving data.
17. Explain the concept of abstraction in Java.
Abstraction is the process of hiding the implementation details and showing only the necessary details of an object. Abstract classes and interfaces are used to achieve abstraction in Java.
18. How does garbage collection work in Java?
Java's garbage collector automatically identifies and removes objects that are no longer reachable or in use, freeing up memory.
19. Why String is immutable in Java?
In Java, a string is immutable, which means that once it is created, it cannot be changed. When you modify a string, a new string is created instead of altering the original one. This immutability helps make strings secure and efficient. For example, if multiple parts of a program share or use the same string, we don't have to worry about one part changing the string and affecting others. It also makes strings thread-safe, meaning they can be safely used in multi-threaded environments without causing unexpected issues.
20. How does the "break" statement work in Java?
The "break" statement is used to terminate the loop or switch statement it appears in. It transfers control to the statement immediately following the loop or switch.
21. What is the role of the "static" block in Java?
The "static" block is used to initialize static variables. It is executed only once when the class is loaded into memory.
22. Can you explain the concept of polymorphism in Java?
Polymorphism means "many forms", which allows us to perform one action in different ways. It is achieved through method overloading and method overriding.
23. How does the "implements" keyword differ from "extends" in Java?
"implements" is used for implementing interfaces, while "extends" is used for inheriting from a class (either an abstract class or a concrete class).
24. What is the purpose of the "volatile" keyword in Java?
The "volatile" keyword is used to indicate that a variable's value may be changed by multiple threads simultaneously.
25. How can you prevent a class from being inherited in Java?
We can prevent a class from being inherited by declaring it as "final."
26. What is a constructor in Java?
A constructor is a special type of method that is called when an object is created. The constructor has the same name as the class and is used to initialize object attributes.
27. What is a Servlet?
Think of a Servlet as a mini Java program that lives on a web server. Its job is to handle requests from web browsers and do things behind the scenes. For example, when you fill out a form on a website, a Servlet might be the one processing that information and making things happen on the server side.
28. What is a JSP Page?
JSP, or JavaServer Pages, is like a magical way to mix regular HTML with Java code. Imagine building a webpage where you can use Java to make parts of it dynamic. JSP pages make it easy to create these dynamic web pages without getting too tangled up in complicated code.
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