Top Java interview questions and answers
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1. What is the difference between JDK, JRE, and JVM?
- Answer:
- JDK (Java Development Kit): It’s a software development kit used to develop Java applications, which includes tools like the compiler (
javac), JavaDoc, and Java Debugger. It also includes JRE. - JRE (Java Runtime Environment): Provides libraries, Java Virtual Machine (JVM), and other components to run Java applications. It’s used by end-users who want to run Java programs.
- JVM (Java Virtual Machine): The heart of Java, it allows Java applications to run on any device or OS. It converts bytecode into machine code.
- JDK (Java Development Kit): It’s a software development kit used to develop Java applications, which includes tools like the compiler (
2. What is the difference between == and equals() in Java?
- Answer:
==is used to compare primitive types and checks for reference equality with objects, meaning it checks if two references point to the same object in memory.equals()is a method from theObjectclass and is used to compare the contents of two objects. For example,Stringclass overridesequals()to check if two strings have the same characters.
3. Explain method overloading and method overriding.
- Answer:
- Method Overloading: Occurs within the same class when two or more methods have the same name but different parameters (different type or number of parameters).
- Method Overriding: Happens in a subclass where a method in the superclass is redefined in the subclass to provide a specific implementation. It’s used to achieve runtime polymorphism.
4. What are final, finally, and finalize() in Java?
- Answer:
final: A keyword used with variables, methods, or classes. Afinalvariable cannot be changed once assigned, afinalmethod cannot be overridden, and afinalclass cannot be subclassed.finally: A block in exception handling that executes whether an exception occurs or not, typically used for cleanup code.finalize(): A method in theObjectclass that is called by the garbage collector before an object is destroyed. It’s generally not recommended to rely onfinalize()for resource management.
5. What is the difference between ArrayList and LinkedList?
- Answer:
- ArrayList: Internally uses a dynamic array, which provides fast random access. However, it’s slower for insertions and deletions (especially in the middle) as it requires shifting elements.
- LinkedList: Uses a doubly linked list, which provides better performance for insertions and deletions but is slower for random access, as each node needs to be traversed sequentially.
6. What are access modifiers in Java?
- Answer: Access modifiers define the visibility of classes, methods, and variables.
public: Accessible from any other class.protected: Accessible within the same package and subclasses.default(no keyword): Accessible within the same package only.private: Accessible only within the class it is defined.
7. Explain the concept of inheritance in Java.
- Answer: Inheritance allows a class (subclass) to inherit the properties and methods of another class (superclass). It promotes code reusability and establishes a parent-child relationship between classes. Java supports single inheritance (one class extends one class) but allows multiple inheritances through interfaces.
8. What is a static keyword in Java?
- Answer:
staticvariable: Shared among all instances of a class.staticmethod: Belongs to the class rather than any instance and can be called without creating an object of the class.staticmethods cannot access instance variables directly.staticblock: Executes when the class is loaded and is used for static initializations.
9. What is polymorphism, and how is it implemented in Java?
- Answer: Polymorphism allows one interface to be used for a general class of actions. Java implements polymorphism in two ways:
- Compile-time polymorphism: Achieved by method overloading.
- Runtime polymorphism: Achieved by method overriding, where the method that is called is determined at runtime based on the object type.
10. Explain the difference between String, StringBuilder, and StringBuffer.
- Answer:
String: Immutable; once created, it cannot be modified. Every change creates a newStringobject.StringBuilder: Mutable and not synchronized, making it faster for single-threaded applications.StringBuffer: Mutable and synchronized, making it thread-safe but slower thanStringBuilderin single-threaded scenarios.
11. What is an interface in Java? How is it different from an abstract class?
- Answer:
- Interface: Defines a contract that implementing classes must follow. All methods are abstract by default (before Java 8). A class can implement multiple interfaces.
- Abstract Class: Can have both abstract methods and concrete methods. It’s used when classes share common behavior but are not completely similar. A class can only extend one abstract class.
12. What is a lambda expression, and in which version of Java was it introduced?
- Answer: A lambda expression provides a clear and concise way to represent one method interface using an expression. It was introduced in Java 8 and enables functional programming by passing behavior as arguments.
13. Explain the concept of multithreading in Java.
- Answer: Multithreading allows multiple threads to run concurrently, providing efficient CPU usage. Java achieves multithreading using the
Threadclass andRunnableinterface. It’s commonly used in applications like games, animations, and large data processing.
14. What are exceptions in Java, and how is exception handling done?
- Answer: Exceptions are unwanted events that disrupt the program's normal flow. Exception handling in Java is done using
try,catch,finally, andthrow/throwskeywords. It ensures the program can handle errors gracefully without crashing.
15. What is a singleton class in Java?
- Answer: A singleton class ensures that only one instance of the class exists throughout the application. This is achieved by making the constructor private and providing a global access method, often with lazy instantiation.
16. How does garbage collection work in Java?
- Answer: Java’s garbage collector automatically manages memory by destroying unreferenced objects. The JVM runs a garbage collection process that identifies objects no longer in use and reclaims their memory.
17. What are hashCode() and equals() methods? Why are they important?
- Answer:
hashCode()returns an integer value for an object, used for faster access in collections likeHashMap.equals()checks if two objects are equal. Properly implementing both methods is essential to ensure correct behavior in hash-based collections.
18. Explain the difference between throw and throws in Java.
- Answer:
throw: Used to explicitly throw an exception within a method.throws: Declares that a method can throw specific exceptions, informing the calling method to handle them or declare further.
19. What are Java annotations?
- Answer: Annotations provide metadata to Java code. Common annotations include
@Override,@Deprecated, and@SuppressWarnings. Custom annotations can be created and used for various purposes, such as code generation or enforcing specific behaviors.
20. What is the purpose of the transient keyword in Java?
- Answer: The
transientkeyword is used in serialization to skip certain fields during the serialization process. Variables marked astransientare not saved in the serialized form of the object.
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