Essential Java interview questions with answers:
Essential Java interview questions with answers
1. What is Java?
Java is a high-level, object-oriented programming language known for its portability across platforms due to the Java Virtual Machine (JVM).
2. What are the main features of Java?
- Platform-independent
- Object-oriented
- Robust and secure
- Multithreaded
- High performance
3. What is the difference between JDK, JRE, and JVM?
JDK (Java Development Kit): Includes tools for developing Java applications.
JRE (Java Runtime Environment): Provides libraries and JVM for running applications.
JVM (Java Virtual Machine):* Executes Java bytecode on any platform.
4. Explain the concept of Object-Oriented Programming (OOP) in Java.
Java follows OOP principles: Encapsulation, Inheritance, Polymorphism, and Abstraction.
5. What is the difference between '=='' and '.equals()' in Java?
- '==' checks for reference equality.
'equals()' checks for value equality.
6. What is the purpose of the 'static' keyword?
'static' denotes that a member belongs to the class rather than instances.
7. What is inheritance in Java?
Inheritance allows a class to acquire properties and behaviors of another class using the 'extends' keyword.
8. What is polymorphism in Java?
Polymorphism allows methods to perform differently based on the object, achieved through method overloading and overriding.
9. What is encapsulation in Java?
Encapsulation hides internal state and requires all interaction to be performed through an object's methods.
10. What is abstraction in Java?
Abstraction hides complex implementation details and shows only essential features.
11. What are exceptions in Java?
Exceptions are events that disrupt normal program flow. They are handled using try-catch blocks.
12. What is the difference between checked and unchecked exceptions?
Checked exceptions: Checked at compile-time.
Unchecked exceptions: Occur at runtime.
13. What is multithreading in Java?
Multithreading allows concurrent execution of two or more threads for maximum CPU utilization.
14. What is the difference between ArrayList and LinkedList?
ArrayList: Better for storing and accessing data.
LinkedList: Better for manipulating data.
15. What is the difference between HashMap and TreeMap?
HashMap: Unsorted, allows null keys.
TreeMap: Sorted, does not allow null keys.
16. What is the purpose of the 'final' keyword in Java?
'final' is used to declare constants, prevent method overriding, and inheritance.
17. What is the difference between abstract classes and interfaces?
Abstract classes: Can have method implementations.
Interfaces: Cannot have method implementations (prior to Java 8).
18. What is the difference between 'this' and 'super' keywords?
this: Refers to the current class instance.
super: Refers to the parent class instance.
19. What is method overloading and overriding?
Overloading: Same method name with different parameters.
Overriding: Subclass provides specific implementation of a method.
20. What is the purpose of constructors in Java?
Constructors initialize new objects. They have the same name as the class and no return type.
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