Top 20 Java Interview Questions and Answers
1. What is Java?
Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is platform-independent, meaning code written in Java can run on any device that has a Java Virtual Machine (JVM).
2. What are the main features of Java?
- Object-Oriented: Everything in Java is an object.
- Platform-Independent: Write Once, Run Anywhere (WORA).
- Simple and Secure: Easy to learn, with a focus on security.
- Multithreaded: Supports multithreading, which allows multiple tasks to run in parallel.
- High Performance: Java is faster than traditional interpreted programming languages because of the Just-In-Time (JIT) compiler.
3. What is the difference between JDK, JRE, and JVM?
- JDK (Java Development Kit): A complete environment for developing, compiling, and running Java applications.
- JRE (Java Runtime Environment): A package of libraries and other files that JVM uses to run applications.
- JVM (Java Virtual Machine): A virtual machine that runs Java bytecode, providing platform independence.
4. What is a Class and an Object in Java?
- Class: A blueprint or template from which objects are created. It defines the properties (fields) and behaviors (methods) that objects of the class will have.
- Object: An instance of a class. It is created using the new keyword.
5. What is Inheritance in Java?
Inheritance allows one class to inherit fields and methods from another class, promoting code reusability. The class that inherits is called the subclass (or derived class), and the class from which it inherits is the superclass (or base class).
6. What is Polymorphism in Java?
Polymorphism means "many forms" and allows objects to be treated as instances of their parent class. In Java, it is mainly achieved through method overloading (compile-time polymorphism) and method overriding (runtime polymorphism).
7. What is the difference between Method Overloading and Method Overriding?
- Method Overloading: Defining multiple methods with the same name but different parameters in the same class.
- Method Overriding: Redefining a method from a superclass in a subclass with the same method signature.
8. What is an Interface in Java?
An interface is a reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It is used to achieve abstraction and multiple inheritance.
9. What is the difference between Abstract Class and Interface?
- Abstract Class: Can have both abstract methods (without a body) and concrete methods (with a body). It can have state (fields).
- Interface: Cannot have any method implementations (except default and static methods) and cannot have state. It is implemented by classes to provide a common contract.
10. What is a Constructor in Java?
A constructor is a special method that is used to initialize objects. It is called when an object of a class is created. It has the same name as the class and does not have a return type.
11. What are the types of Constructors in Java?
- Default Constructor: A no-argument constructor automatically provided by the compiler if no constructors are defined in the class.
- Parameterized Constructor: A constructor that accepts arguments to initialize the object's fields.
12. What is the this keyword in Java?
The this keyword is a reference to the current object, which is used to access instance variables, methods, and constructors. It is particularly useful when field names are hidden by method or constructor parameters.
13. What is the super keyword in Java?
The super keyword is used to refer to the immediate parent class object. It is often used to call the superclass's constructor or methods that have been overridden in the subclass.
14. What is Exception Handling in Java?
Exception handling is a mechanism in Java to handle runtime errors, allowing the normal flow of the application to be maintained. It uses try, catch, finally, and throw blocks to handle exceptions.
15. What is the difference between Checked and Unchecked Exceptions?
- Checked Exceptions: Exceptions that are checked at compile-time. Examples include IOException and SQLException.
- Unchecked Exceptions: Exceptions that are not checked at compile-time. Examples include ArrayIndexOutOfBoundsException and NullPointerException.
16. What is Multithreading in Java?
Multithreading is a feature in Java that allows the concurrent execution of two or more threads for maximum utilization of CPU. Each thread runs in parallel, performing separate tasks simultaneously.
17. What is Synchronization in Java?
Synchronization is the capability to control the access of multiple threads to shared resources. It ensures that only one thread can access a resource at a time, preventing data inconsistency.
18. What is the difference between ArrayList and LinkedList in Java?
- ArrayList: A resizable array implementation of the List interface. It allows random access but is slower for insertions and deletions compared to LinkedList.
- LinkedList: A doubly-linked list implementation of the List interface. It is faster for insertions and deletions but does not allow random access.
19. What is the use of the final keyword in Java?
The final keyword in Java is used to restrict the usage of variables, methods, and classes. A final variable cannot be changed once assigned, a final method cannot be overridden, and a final class cannot be subclassed.
20. What are Java Annotations?
Java Annotations are a form of metadata that provide data about a program but are not part of the program itself. They are used to give information to the compiler, detect errors, and suppress warnings. Examples include @Override, @Deprecated, and @SuppressWarnings.
