For a Java programming interview, you can expect questions that test your knowledge of core Java concepts, object-oriented programming (OOP), data structures, and problem-solving skills. Here are some of the most important Java questions to prepare for:
### Core Java Concepts
1. **What are the features of Java?**
- Focus on: Object-Oriented, Platform Independent, Simple, Secure, Robust, Multithreaded, etc.
2. **What is the difference between JDK, JRE, and JVM?**
- JDK: Java Development Kit (used for development).
- JRE: Java Runtime Environment (used for running programs).
- JVM: Java Virtual Machine (executes Java bytecode).
3. **What is a class and an object in Java?**
- Explain how a **class** is a blueprint and an **object** is an instance of a class.
4. **What is inheritance in Java? Explain with examples.**
- Focus on how one class can inherit fields and methods from another (e.g., `extends` keyword).
5. **Explain the difference between method overloading and method overriding.**
- **Overloading**: Same method name, different parameters.
- **Overriding**: Subclass provides a specific implementation of a method already defined in its parent class.
6. **What is an interface in Java? How is it different from an abstract class?**
- **Interface**: A contract for what a class can do, with no method implementations (prior to Java 8).
- **Abstract Class**: Can have both abstract methods and concrete methods.
7. **What is the difference between a constructor and a method?**
- **Constructor**: Initializes an object, no return type, same name as the class.
- **Method**: Performs a specific task, has a return type, can have any name.
### Exception Handling
8. **What is the difference between `throw` and `throws` in Java?**
- **`throw`**: Used to explicitly throw an exception in a method.
- **`throws`**: Declares that a method can throw an exception, handled by the calling code.
9. **What are checked and unchecked exceptions?**
- **Checked Exceptions**: Checked at compile time (e.g., `IOException`).
- **Unchecked Exceptions**: Not checked at compile time (e.g., `NullPointerException`, `ArrayIndexOutOfBoundsException`).
10. **What is a try-catch-finally block?**
- **try**: Code that may throw an exception.
- **catch**: Handles the exception.
- **finally**: Executes code whether an exception occurs or not.
### Multithreading and Concurrency
11. **What is a thread in Java? How do you create a thread?**
- Focus on two ways:
1. Implementing `Runnable` interface.
2. Extending `Thread` class.
12. **What is the difference between `synchronized` and `volatile`?**
- **`synchronized`**: Used to lock an object for mutual exclusive access by threads.
- **`volatile`**: Ensures visibility of changes to variables across threads.
13. **What is the difference between a process and a thread?**
- **Process**: Independent execution environment.
- **Thread**: Lightweight process within a process.
### Java Collections Framework
14. **What is the difference between List, Set, and Map in Java?**
- **List**: Ordered collection, allows duplicates (e.g., `ArrayList`).
- **Set**: Unordered collection, no duplicates (e.g., `HashSet`).
- **Map**: Collection of key-value pairs (e.g., `HashMap`).
15. **What is the difference between `HashMap` and `Hashtable`?**
- **HashMap**: Not synchronized, allows `null` keys and values.
- **Hashtable**: Synchronized, doesn’t allow `null` keys or values.
16. **Explain the difference between `ArrayList` and `LinkedList`.**
- **ArrayList**: Implements dynamic arrays, faster for accessing elements.
- **LinkedList**: Implements doubly linked list, faster for inserting and deleting elements.
17. **How does the `HashMap` work internally in Java?**
- Understand **hashing**, **bucket**, and the role of **equals()** and **hashCode()** methods.
### Java Memory Management
18. **What is garbage collection in Java? How does it work?**
- Explain the **automatic memory management** process in Java and how it reclaims memory.
19. **What are stack and heap memory in Java?**
- **Stack**: Stores method calls and local variables.
- **Heap**: Stores objects and their data.
20. **What is the `finalize()` method in Java?**
- This method is called by the garbage collector before an object is destroyed.
### Object-Oriented Programming (OOP)
21. **What are the four pillars of OOP? Explain each.**
- **Encapsulation**: Bundling of data with methods.
- **Abstraction**: Hiding implementation details.
- **Inheritance**: Allows one class to inherit fields and methods from another.
- **Polymorphism**: One name, many forms (method overloading and overriding).
22. **What is encapsulation in Java?**
- Explain the use of access modifiers (`private`, `protected`, `public`) to protect object state.
23. **What is polymorphism in Java?**
- Explain compile-time (method overloading) and runtime (method overriding) polymorphism.
24. **What is the difference between composition and inheritance?**
- **Inheritance**: "Is-a" relationship.
- **Composition**: "Has-a" relationship.
### Advanced Java
25. **What is the difference between `StringBuilder` and `StringBuffer`?**
- **StringBuilder**: Non-synchronized, faster.
- **StringBuffer**: Synchronized, thread-safe but slower.
26. **What is the `transient` keyword in Java?**
- It prevents serialization of a variable when the object is serialized.
27. **What are lambda expressions in Java?**
- Introduced in Java 8, used to simplify code by enabling functional programming features.
28. **What is the `Stream API` in Java?**
- Used for processing sequences of elements (like collections) in a declarative way (introduced in Java 8).
29. **What is the `Optional` class in Java?**
- A container object that may or may not contain a non-null value, used to avoid `NullPointerException`.
### Coding Problems
30. **Write a program to find the second largest number in an array.**
- You may be asked to implement code for this, so practice array manipulation.
31. **Write a program to reverse a linked list.**
- Focus on both iterative and recursive methods.
32. **Write a program to check if a string is a palindrome.**
- You might be asked to implement this for strings or numbers.
33. **How do you implement a singleton class in Java?**
- Understand how to ensure that only one instance of a class is created (use of `private` constructor, `static` method, and `synchronized` block for thread-safety).
These questions cover both fundamental and advanced topics in Java, giving you a solid foundation for technical interviews. Practicing these concepts and explaining them clearly will help you succeed in your interview.
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