Java Interview Questions

Here’s a list of 100 Java interview questions and answers that cover a range of topics, from basic to advanced concepts:

Basic Java Questions

1. What is Java?
   • Java is a high-level, class-based, object-oriented programming language designed to have as few implementation dependencies as possible.
2. What are the main features of Java?
   • Platform-independent, object-oriented, secure, robust, portable, multithreaded, and distributed.
3. What is the JVM?
   • JVM (Java Virtual Machine) is a virtual machine that enables computers to run Java programs.
4. What is the JDK?
   • JDK (Java Development Kit) is a software development kit used to develop Java applications.
5. What is the JRE?
   • JRE (Java Runtime Environment) is a set of software tools used for running Java applications.
6. What is a class in Java?
   • A class is a blueprint for objects; it defines a datatype by bundling data and methods.
7. What is an object in Java?
   • An object is an instance of a class, containing properties (fields) and behaviors (methods).
8. What is inheritance in Java?
   • Inheritance is a mechanism in Java where one class can inherit fields and methods from another.
9. What is polymorphism in Java?
   • Polymorphism allows objects to be treated as instances of their parent class, enabling a single method to operate on different types of objects.
10. What is encapsulation in Java?
    • Encapsulation is the wrapping of data (variables) and code (methods) into a single unit, such as a class, and restricting access to some of the object’s components.
11. What is abstraction in Java?
    • Abstraction is the concept of hiding the complex implementation details and showing only the essential features of an object.
12. What is an interface in Java?
    • An interface is a reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types.
13. What is the difference between an interface and an abstract class?
    • An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods (until Java 8, which introduced default and static methods).
14. 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.
15. What are the types of constructors in Java?
    • Default constructor and parameterized constructor.
16. What is method overloading in Java?
    • Method overloading occurs when two or more methods in the same class have the same name but different parameters.
17. What is method overriding in Java?
    • Method overriding occurs when a subclass provides a specific implementation for a method that is already defined in its superclass.
18. What is the this keyword in Java?
    • this refers to the current instance of the class.
19. What is the super keyword in Java?
    • super refers to the parent class objects and is used to call superclass methods and constructors.
20. What is a static method in Java?
    • A static method belongs to the class rather than instances of the class and can be called without creating an object of the class.

Java Data Types and Variables

21. What are the primitive data types in Java?
    • byte, short, int, long, float, double, char, and boolean.
22. What is the difference between int and Integer in Java?
    • int is a primitive data type, while Integer is a wrapper class.
23. What is typecasting in Java?
    • Typecasting is converting one data type into another. For example, converting a float to an int.
24. What is autoboxing and unboxing in Java?
    • Autoboxing is the automatic conversion of a primitive type to its corresponding wrapper class. Unboxing is the reverse process.
25. What is the difference between == and equals() in Java?
    • == checks for reference equality, while equals() checks for value equality.
26. What is the final keyword in Java?
    • The final keyword is used to restrict the user. It can be used with variables, methods, and classes to prevent re-assignment, overriding, and inheritance, respectively.
27. What is the static keyword in Java?
    • The static keyword is used to indicate that a member belongs to the class rather than instances of the class.
28. What are local variables in Java?
    • Local variables are declared within a method and can only be accessed within that method.
29. What are instance variables in Java?
    • Instance variables are declared inside a class but outside any method and are associated with an instance of the class.
30. What are class variables in Java?
    • Class variables are variables declared with the static keyword in a class but outside any method.

Java Control Flow and Loops

31. What are the control flow statements in Java?
    • if, if-else, switch, for, while, and do-while.
32. What is the difference between while and do-while loops in Java?
    • while checks the condition before executing the loop body, whereas do-while checks the condition after executing the loop body.
33. What is the break statement in Java?
    • break is used to exit a loop or switch statement immediately.
34. What is the continue statement in Java?
    • continue skips the current iteration of a loop and proceeds with the next iteration.
35. What is the return statement in Java?
    • return exits from the current method and optionally returns a value.
36. What is the switch statement in Java?
    • The switch statement allows a variable to be tested for equality against a list of values, with each value having its code block.
37. What is a nested loop in Java?
    • A loop inside another loop is called a nested loop.
38. How does the for-each loop work in Java?
    • The for-each loop iterates over elements of an array or a collection without using an index.
39. What is an infinite loop in Java?
    • An infinite loop runs indefinitely because the condition for terminating the loop is never met.
40. What is the enhanced for loop in Java?
    • The enhanced for loop (or for-each loop) is used to iterate over elements of arrays or collections.

Java OOP Concepts

41. What is object-oriented programming (OOP)?
    • OOP is a programming paradigm based on the concept of objects, which contain data and methods to manipulate that data.
42. What is the difference between a class and an object in Java?
    • A class is a blueprint for objects, while an object is an instance of a class.
43. What is inheritance in Java?
    • Inheritance allows one class to inherit the properties and methods of another class.
44. What is multiple inheritance, and does Java support it?
    • Multiple inheritance is when a class inherits from more than one class. Java does not support multiple inheritance with classes to avoid ambiguity but supports it through interfaces.
45. What is the difference between super() and this() in Java?
    • super() is used to call the superclass constructor, while this() is used to call another constructor within the same class.
46. What is method overriding in Java?
    • Method overriding occurs when a subclass provides a specific implementation of a method already defined in its superclass.
47. What is method overloading in Java?
    • Method overloading is when multiple methods have the same name but different parameters in the same class.
48. What is encapsulation in Java?
    • Encapsulation is the concept of wrapping data and code into a single unit and restricting access to the object’s components.
49. What is abstraction in Java?
    • Abstraction is the concept of hiding the internal implementation details and showing only the essential features.
50. What is a constructor in Java?
    • A constructor is a special method used to initialize objects when an instance of a class is created.

Java Strings and Arrays

51. What is a string in Java?
    • A string is a sequence of characters treated as a single data type in Java.
52. What is the difference between String, StringBuilder, and StringBuffer?
    • String is immutable, while StringBuilder and StringBuffer are mutable. StringBuffer is thread-safe, whereas StringBuilder is not.
53. What is the String pool in Java?
    • The String pool is a special storage area in the heap memory where Java stores string literals.
54. How do you compare two strings in Java?
    • Using equals() for value comparison and == for reference comparison.
55. What is an array in Java?
    • An array is a collection of elements of the same data type stored in contiguous memory locations.
56. How do you declare an array in Java?
    • Example: int[] arr = new int[10];
57. What is the difference between a one-dimensional and a two-dimensional array?
    • A one-dimensional array is a list of elements, while a two-dimensional array is an array of arrays, representing a table of elements.
58. How do you find the length of an array in Java?
    • Using the length attribute, e.g., arr.length.
59. What is the difference between an array and a list in Java?
    • An array has a fixed size, while a list (ArrayList) can grow or shrink dynamically.
60. What is a jagged array in Java?
    • A jagged array is an array of arrays where each sub-array can have different lengths.

Java Collections Framework

61. What is the Java Collections Framework?
    • The Java Collections Framework provides a set of interfaces and classes to store and manipulate groups of objects.
62. What are the main interfaces of the Java Collections Framework?
    • Collection, List, Set, Map, Queue, and Deque.
63. What is the difference between ArrayList and LinkedList in Java?
    • ArrayList uses a dynamic array, is better for random access, and has slower insertion/removal. LinkedList uses a doubly linked list, is better for insertion/removal, and has slower random access.
64. What is the difference between HashSet and TreeSet in Java?
    • HashSet does not maintain order, while TreeSet stores elements in sorted order.
65. What is a Map in Java?
    • A Map is a collection that maps keys to values, with no duplicate keys allowed.
66. What is the difference between HashMap and TreeMap in Java?
    • HashMap is unordered, while TreeMap maintains elements in natural or custom order.
67. What is a Queue in Java?
    • A Queue is a collection that holds elements prior to processing, typically following FIFO (First-In-First-Out) order.
68. What is the difference between ArrayList and Vector in Java?
    • ArrayList is not synchronized, while Vector is synchronized.
69. What is an iterator in Java?
    • An iterator is an object that allows you to traverse a collection, typically using hasNext() and next() methods.
70. What is the difference between Iterator and ListIterator?
    • Iterator allows traversing a collection in one direction, while ListIterator allows traversing in both directions.

Java Exception Handling

71. What is an exception in Java?
    • An exception is an event that occurs during the execution of a program, disrupting the normal flow of instructions.
72. What are the types of exceptions in Java?
    • Checked exceptions, unchecked exceptions, and errors.
73. What is the difference between throw and throws in Java?
    • throw is used to explicitly throw an exception, while throws is used in method signatures to declare an exception that may be thrown by the method.
74. What is the difference between checked and unchecked exceptions in Java?
    • Checked exceptions are checked at compile-time, while unchecked exceptions occur at runtime.
75. What is a try-catch block in Java?
    • A try-catch block is used to handle exceptions, with try containing code that might throw an exception and catch handling the exception.
76. What is a finally block in Java?
    • A finally block contains code that is always executed, whether or not an exception is thrown.
77. What is the difference between final, finally, and finalize()?
    • final is a keyword for constant variables or non-overridable methods. finally is a block that executes after try-catch. finalize() is a method called by the garbage collector before object destruction.
78. What is custom exception in Java?
    • Custom exceptions are user-defined exceptions that extend Exception or RuntimeException classes.
79. What is the throws keyword in Java?
    • throws is used in a method signature to declare exceptions that the method might throw.
80. What is a throwable in Java?
    • Throwable is the superclass of all errors and exceptions in Java.

Advanced Java Concepts

81. What is multithreading in Java?
    • Multithreading is a process of executing multiple threads simultaneously to achieve parallelism.
82. What is the difference between Thread and Runnable in Java?
    • Thread is a class, while Runnable is an interface. Runnable is preferred for implementing threads because it allows the class to extend other classes.
83. What is synchronization in Java?
    • Synchronization is the process of controlling access to shared resources in a multithreaded environment.
84. What is a volatile keyword in Java?
    • volatile ensures that the value of a variable is always read from the main memory, providing visibility guarantees in a multithreaded environment.
85. What is the difference between synchronized method and synchronized block?
    • A synchronized method locks the entire method, while a synchronized block only locks the specified object.
86. What is the transient keyword in Java?
    • The transient keyword is used to indicate that a field should not be serialized.
87. What is the Serializable interface in Java?
    • Serializable is a marker interface used to enable serialization, allowing objects to be converted into a byte stream.
88. What is a Lambda expression in Java?
    • A Lambda expression is a short block of code that takes in parameters and returns a value, providing a clear and concise way to represent one method interface using an expression.
89. What is the Stream API in Java?
    • The Stream API is used to process collections of objects, enabling functional-style operations on streams of elements, such as map-reduce transformations.
90. What is the Optional class in Java?
    • Optional is a container object used to contain not-null objects and avoid null pointer exceptions.

Java Design Patterns

91. What is a design pattern in Java?
    • A design pattern is a general reusable solution to a commonly occurring problem in software design.
92. What is the Singleton pattern in Java?
    • The Singleton pattern ensures that a class has only one instance and provides a global point of access to it.
93. What is the Factory pattern in Java?
    • The Factory pattern is used to create objects without exposing the instantiation logic to the client.
94. What is the Observer pattern in Java?
    • The Observer pattern is used when one object (subject) is changed, all its dependents (observers) are notified and updated automatically.
95. What is the Builder pattern in Java?
    • The Builder pattern is used to construct complex objects step by step.
96. What is the Adapter pattern in Java?
    • The Adapter pattern allows incompatible interfaces to work together by acting as a bridge.
97. What is the Decorator pattern in Java?
    • The Decorator pattern allows behavior to be added to individual objects, dynamically.
98. What is the Prototype pattern in Java?
    • The Prototype pattern is used to create duplicate objects while keeping performance in mind.
99. What is the Command pattern in Java?
    • The Command pattern is used to encapsulate a request as an object, thereby allowing users to parameterize clients with different requests, queue or log requests, and support undoable operations.
100. What is the MVC pattern in Java? – MVC (Model-View-Controller) is a pattern used to separate the application logic (model) from the user interface (view) and the control flow (controller).

This comprehensive list of Java interview questions should help you prepare effectively for various interview scenarios, covering the fundamentals, OOP concepts, advanced Java, and design patterns.