Capgemini is a global technology leader, and Java is one of the most widely used programming languages in software development. To help you ace your interview for the Java Developer role, here are the Top 20 Java Developer Interview Questions with their answers.
1. What are the main features of Java?
Java is known for being object-oriented, platform-independent, secure, robust, multithreaded, and dynamic. The key features include:
- Object-Oriented: Everything is based on objects and classes.
- Platform-Independent: Java bytecode can run on any platform with a JVM (Java Virtual Machine).
- Multithreaded: Java supports multithreading and concurrency.
2. What is the difference between JDK, JRE, and JVM?
- JDK (Java Development Kit): It is the development environment for building Java applications.
- JRE (Java Runtime Environment): It is the environment where Java programs run.
- JVM (Java Virtual Machine): It is the component that runs Java bytecode on any platform.
3. What are the four pillars of Object-Oriented Programming (OOP)?
The four pillars of OOP are:
- Encapsulation: Bundling data and methods that operate on the data within a single unit (class).
- Abstraction: Hiding complex implementation details and showing only essential features.
- Inheritance: Acquiring properties and behavior from a parent class.
- Polymorphism: Allowing objects to be treated as instances of their parent class.
4. Explain the difference between ‘==’ and ‘equals()’ in Java.
- ‘==’: It checks for reference equality (whether two references point to the same object).
- ‘equals()’: It checks for value equality (whether two objects have the same value or data).
5. What is a constructor in Java?
A constructor is a special method used to initialize objects. It is called when an object of a class is created and has the same name as the class. Constructors do not have a return type.
6. What is the difference between method overloading and method overriding?
- Method Overloading: Defining multiple methods with the same name but different parameter lists within the same class.
- Method Overriding: Redefining a method in a subclass that is already defined in the parent class.
7. What is a static keyword in Java?
The static keyword is used to define class-level variables and methods. It means the variable or method belongs to the class itself rather than instances of the class.
8. What is the difference between an interface and an abstract class?
- Interface: An interface can only declare methods, and all methods are implicitly abstract (before Java 8). A class can implement multiple interfaces.
- Abstract Class: An abstract class can have both abstract and non-abstract methods. A class can extend only one abstract class.
9. What are exceptions in Java, and how are they handled?
An exception is an event that disrupts the normal flow of the program. Exceptions are handled using try, catch, finally, and throw keywords.
10. What is the difference between checked and unchecked exceptions?
- Checked Exceptions: These are checked at compile time. The programmer must handle these exceptions (e.g.,
IOException). - Unchecked Exceptions: These are checked at runtime (e.g.,
ArrayIndexOutOfBoundsException).
11. What is the use of the ‘final’ keyword in Java?
The final keyword is used to declare constants, prevent method overriding, and prevent inheritance. A final class cannot be subclassed, and a final method cannot be overridden.
12. What is the Java Collection Framework?
The Java Collection Framework provides classes and interfaces to store and manipulate groups of data. Common interfaces include List, Set, Map, and Queue.
13. What is a HashMap in Java?
A HashMap is a part of the Java Collection Framework and stores data in key-value pairs. It allows null values and keys and does not maintain any order of elements.
14. What is the difference between ArrayList and LinkedList?
- ArrayList: It uses a dynamic array to store elements and provides fast access to elements but is slower in adding/removing elements in the middle.
- LinkedList: It uses a doubly linked list, allowing fast insertion and deletion of elements but slower access time.
15. What is multithreading in Java?
Multithreading is a feature that allows concurrent execution of two or more threads. It is used to perform multiple tasks simultaneously and is implemented by using the Thread class or implementing the Runnable interface.
16. What is the difference between process and thread?
- Process: A process is an independent program in execution, with its own memory space.
- Thread: A thread is a lightweight sub-process and shares the same memory space with other threads of the same process.
17. How does Java achieve platform independence?
Java achieves platform independence through the Java Virtual Machine (JVM). The Java compiler generates bytecode, which can be executed on any machine with a JVM, regardless of the underlying hardware and operating system.
18. What is garbage collection in Java?
Garbage collection in Java is the process of automatically reclaiming memory by destroying objects that are no longer in use. It is managed by the JVM.
19. Explain the difference between String, StringBuilder, and StringBuffer.
- String: Immutable class, meaning once created, it cannot be modified.
- StringBuilder: A mutable class, used for creating modifiable strings without synchronization.
- StringBuffer: Similar to
StringBuilder, but thread-safe and synchronized.
20. What is the difference between == operator and equals() method?
- ‘==’: It compares object references, checking if two variables point to the same memory location.
- ‘equals()`: It checks for equality in the values of two objects.
Conclusion
These questions cover a broad range of Java concepts that are often asked in Capgemini interviews. Make sure to review these topics thoroughly, as they can help you build a strong foundation for your interview. Good luck with your preparation!
Good Luck!
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