Top 51 Java Interview Questions And Answers 2024

1.What do you understand by Java?

2. Compare between Java and Python.

3. Outline the major Java features.

4. What do you mean by an object?

5. Distinguish between StringBuffer and StringBuilder in Java programming.

6. Differentiate between JDK, JRE, and JVM.

7. Define inheritance.

8. Explain method overloading.

9. Compare overloading with overriding.

10. Explain the creation of a thread-safe singleton in Java using double-checked locking.

11. What is a class in Java?

12. Differentiate between an ArrayList and a Vector.

13. Mention the difference between Iterator and Enumeration.

14. Explain the difference between the inner class and the subclass.

15. Can we execute any code, even before the main method? Explain.

16. How can we restrict inheritance for a class?

17. Java doesn’t support multiple inheritance. Why?

18. Are constructors inherited? Can a subclass call the parent’s class constructor?

19. Define JSON.

20. What are the advantages of JSON over XML?

21. What is the difference between Java and C++?

22. What is JIT Compiler?

23. What is Classloader?

24. Is an empty .java file name a valid source file name in java?

25. What is the difference between Object-oriented and object-based programming language?

26. How many types of constructors are there in the Java programming language?

27. What is the purpose of a Default constructor in Java?

28. What is the use of a copy constructor in Java?

29. Why is the main method static in Java?

30. Can we declare the static variables and methods in an abstract class?

31. What is Aggregation?

32. What is composition?

33. What is the difference between aggregation and composition?

34. What is object cloning?

35. Can we overload the main method in Java?

36. Explain public static void main(String argos[]) in Java?

37. What are various exception handling keywords in Java?

38. What is the NullPointerException?

39. What is the difference between a constructor and a destructor?

40. Can a Java interface have static methods?

41. How to manually throw an exception in Java programming?

42. Differentiate between ‘==’ and equals()?

43. What are JAR files?

44. What is a WAR file?

45. Differentiate between this() and super() in Java.

46. Name the methods of an object class.

47. Define content negotiation.

48. Can we import the same package/class twice? Will the JVM load the package twice at runtime?

49. Define an abstract class.

50. Describe annotations.

51. Java doesn’t use pointers. Why?

Java Interview Questions And Answers

1. Java:
   Java is a high-level, object-oriented, and platform-independent programming language. It was developed by Sun Microsystems (now owned by Oracle) and released in 1995. One of its key features is the “Write Once, Run Anywhere” (WORA) philosophy, which means that Java code can run on any device that supports Java without modification.
2. Java vs. Python:

• Syntax:
  • Java has a statically-typed and verbose syntax, requiring explicit declaration of data types and more lines of code.
  • Python is dynamically typed and has a concise syntax, allowing for more expressiveness with fewer lines of code.
• Performance:
  • Java tends to have better performance due to its static typing and compilation process.
  • Python is generally slower as it is an interpreted language, but performance-critical modules can be implemented in compiled languages.
• Platform Independence:
  • Java is platform-independent, thanks to the Java Virtual Machine (JVM).
  • Python is also platform-independent, but it requires an interpreter on each platform.
• Community and Ecosystem:
  • Both languages have large and active communities with extensive libraries and frameworks.
  • Python is known for its simplicity and readability, attracting a broad range of users.

3.

a. Object-oriented:

Java follows the principles of object-oriented programming, emphasizing the creation and manipulation of objects, encapsulation, inheritance, and polymorphism.

b.Portable:

Java’s bytecode, generated by the compiler, is platform-independent, allowing Java programs to run on any device with a compatible Java Virtual Machine (JVM).

c.Platform-independent:

The “write once, run anywhere” principle is a key feature of Java, enabling developers to create applications that can run on various platforms without modification.

d.Robust:

Java’s design includes features like strong type checking, automatic garbage collection, and exception handling, contributing to the language’s robustness and stability.

e.Interpreted:

.While it’s true that Java code is initially compiled into bytecode, the bytecode is later interpreted by the JVM at runtime. However, Java also employs Just-In-Time (JIT) compilation, which translates bytecode into native machine code for improved performance during execution. So, it combines elements of both interpretation and compilation.

4. Object:
An object is an instance of a class in object-oriented programming. It represents a real-world entity and encapsulates data and behavior. Objects are instances of classes, and they interact with each other through methods.

5. StringBuffer vs. StringBuilder:

• Both StringBuffer and StringBuilder are classes in Java used for manipulating strings.
• StringBuffer:
  • Thread-safe (synchronized), making it safer for use in a multi-threaded environment.
  • Slower than StringBuilder due to synchronization.
• StringBuilder:
  • Not thread-safe, making it more efficient in a single-threaded environment.
  • Faster than StringBuffer because it is not synchronized.

6. JDK, JRE, and JVM:

• JDK (Java Development Kit):
  • It is a software development kit that includes tools for developing, debugging, and monitoring Java applications.
  • It contains JRE, an interpreter/loader (Java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc), and other tools.
• JRE (Java Runtime Environment):
  • It provides the runtime environment for Java applications to run.
  • It includes the JVM, core libraries, and other components required for running Java applications but not for developing them.
• JVM (Java Virtual Machine):
  • It is an abstract machine that provides a runtime environment in which Java bytecode can be executed.
  • JVM is responsible for the execution of Java programs and translates Java bytecode into native machine code.

7.Inheritance:
Inheritance is a fundamental concept in object-oriented programming (OOP) where a class (subclass/derived class) inherits properties and behaviors from another class (superclass/base class). This allows for code reuse and the creation of a hierarchy of classes.

8. Method Overloading:
Method overloading occurs when multiple methods in the same class have the same name but different parameters (either different types or a different number of parameters). The compiler determines which method to call based on the number and types of arguments passed.

9.Overloading vs. Overriding:

• Overloading:
  • Involves multiple methods in the same class with the same name but different parameters.
  • Decided at compile time (compile-time polymorphism).
• Overriding:
  • Involves providing a specific implementation for a method in a subclass that is already defined in its superclass.
  • Decided at runtime (runtime polymorphism).

10. Thread-Safe Singleton with Double-Checked Locking:

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // Private constructor to prevent instantiation.
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

• The volatile keyword ensures that the instance variable is not cached, helping to prevent multiple threads from creating separate instances.
• The double-checked locking ensures that only one thread enters the synchronized block to create the instance, improving performance by avoiding unnecessary synchronization once the instance is created.