1. What is Java?

Answer: Java is a high-level, object-oriented programming language that is designed to run on any platform.

2.What are the features of Java?

Answer: Features of Java are: Object-Oriented, Platform Independent, Secured, Robust, High Performance, Multi-Threaded, Distributed.

3.What is JVM?

Answer: JVM stands for Java Virtual Machine. It is an abstract machine that provides a runtime environment for Java bytecode to execute.

4.What is JIT compiler?

Answer: JIT stands for Just-In-Time compiler. It is a type of compiler that compiles bytecode at runtime and optimizes the code for better performance.

5.What is a class?

Answer: A class is a blueprint or template for creating objects in Java.

6.What is an object?

Answer: An object is an instance of a class that has its own state and behavior.

7.What is Inheritance?

Answer: Inheritance is a mechanism in Java by which one class can acquire the properties of another class.

8.What is Polymorphism?

Answer: Polymorphism is the ability of an object to take on many forms.

9.What is Abstraction?

Answer: Abstraction is a mechanism that allows you to hide implementation details and only show the necessary details to the user.

10.What is Encapsulation?

Answer: Encapsulation is the mechanism of wrapping data and code together in a single unit called a class.

11.What is a constructor?

Answer: A constructor is a special method in Java that is used to initialize objects.

12.What is the difference between a constructor and a method?

Answer: A constructor is a special method that is used to initialize objects, whereas a method is a general-purpose block of code that can be called by an object to perform some action.

13.What is a static method?

Answer: A static method is a method that belongs to the class rather than an instance of the class.

14.What is a static variable?

Answer: A static variable is a variable that belongs to the class rather than an instance of the class.

15.What is an interface?

Answer: An interface is a collection of abstract methods and constants that can be implemented by a class.

16.What is an abstract class?

Answer: An abstract class is a class that cannot be instantiated but can be inherited by other classes.

17.What is a package?

Answer: A package is a group of related classes and interfaces that are used to organize code.

18.What is a jar file?

Answer: A jar file is a compressed file format that is used to distribute Java classes and related resources.

19.What is a thread?

Answer: A thread is a lightweight process that allows multiple tasks to be executed concurrently within a single program.

20.What is synchronization?

Answer: Synchronization is the mechanism of coordinating access to shared resources in a multi-threaded environment.

21.What is a deadlock?

Answer: A deadlock is a situation in which two or more threads are blocked waiting for each other to release resources.

22.What is serialization?

Answer: Serialization is the process of converting an object into a stream of bytes for storage or transmission.

23.What is deserialization?

Answer: Deserialization is the process of converting a stream of bytes into an object.

24.What is an exception?

Answer: An exception is an event that occurs during the execution of a program that disrupts the normal flow of the program.

25.What is the difference between checked and unchecked exceptions?

Answer: Checked exceptions are checked at compile time, whereas unchecked exceptions are not checked at compile time.

26.What is a try-catch block?

Answer: A try-catch block is used to handle exceptions in Java.

27.What is the difference between a checked and an unchecked exception?

Answer: Checked exceptions are checked by the compiler at compile time and must be handled by the programmer using a try-catch block or by declaring them in the method signature using the throws keyword. Unchecked exceptions, on the other hand, are not checked by the compiler and can occur at runtime. They do not need to be explicitly handled, but it is recommended to handle them for better program reliability.

28.What is a finally block?

Answer: A finally block is a block of code that is executed after a try-catch block, regardless of whether an exception is thrown or not.

29.What is the difference between final, finally and finalize in Java?

Answer: Final is a keyword that can be used to make a variable, method, or class constant and unmodifiable. Finally is a keyword used to execute a block of code after a try-catch block, whether an exception is thrown or not. Finalize is a method that is called by the garbage collector to perform any necessary cleanup operations on an object before it is destroyed.

30.What is a string in Java?

Answer: A string is a sequence of characters in Java that is used to represent text.

31.What is a StringBuilder in Java?

Answer: StringBuilder is a class in Java that is used to create and manipulate strings.

32.What is an ArrayList in Java?

Answer: An ArrayList is a class in Java that is used to create dynamic arrays.

33.What is the difference between an ArrayList and an array in Java?

Answer: An array in Java is a fixed-length data structure that can hold elements of the same data type. An ArrayList is a dynamic data structure that can grow or shrink in size and can hold elements of different data types.

34.What is a hash table in Java?

Answer: A hash table in Java is a data structure that stores key-value pairs in a way that allows fast retrieval of values based on the key.

35.What is a HashSet in Java?

Answer: A HashSet is a class in Java that is used to create a collection of unique elements.

36.What is a HashMap in Java?

Answer: A HashMap is a class in Java that is used to store key-value pairs in a way that allows fast retrieval of values based on the key.

37.What is a TreeMap in Java?

Answer: A TreeMap is a class in Java that is used to store key-value pairs in a way that allows fast retrieval of values based on the key, and also maintains the keys in sorted order.

38.What is the difference between a HashSet and a TreeSet in Java?

Answer: A HashSet is an unordered collection of unique elements, whereas a TreeSet is an ordered collection of unique elements that are sorted based on their natural ordering or using a Comparator.

39.What is a lambda expression in Java?

Answer: A lambda expression is a new feature introduced in Java 8 that allows the creation of anonymous functions.

40.What is the functional interface in Java?

Answer: A functional interface is an interface that contains only one abstract method and can be used as the target for a lambda expression or method reference.

41.What is a method reference in Java?

Answer: A method reference is a shorthand notation for referring to a method that can be used as the target for a lambda expression.

42.What is the difference between a lambda expression and a method reference in Java?

Answer: A lambda expression is an anonymous function that can be used as the target for a functional interface. A method reference is a shorthand notation for referring to a method that can be used as the target for a lambda expression.

43.What is a stream in Java?

Answer: A stream is a sequence of elements that can be processed in a functional way.

44.What is the difference between a stream and a collection in Java?

Answer: A collection is a data structure that stores a group of objects or elements, whereas a stream is a sequence of elements that can be processed in a functional way.

45.What is the difference between a parallel stream and a sequential stream in Java?

Answer: A sequential stream processes elements one at a time, whereas a parallel stream processes elements concurrently using multiple threads.

46.What is a functional programming in Java?

Answer: Functional programming is a programming paradigm that emphasizes the use of functions and immutable data structures.

47.What is an interface in Java?

Answer: An interface is a collection of abstract methods and constants that can be implemented by classes.

48.What is an abstract class in Java?

Answer: An abstract class is a class that cannot be instantiated and can contain abstract methods, which are declared but not implemented in the abstract class.

49.What is the difference between an abstract class and an interface in Java?

Answer: An abstract class can have both implemented and abstract methods, whereas an interface can only have abstract methods. A class can extend only one abstract class, but can implement multiple interfaces.

50.What is the purpose of the keyword "this" in Java?

Answer: The keyword "this" is a reference to the current object in Java.

51.What is the purpose of the keyword "super" in Java?

Answer: The keyword "super" is a reference to the superclass of the current class in Java.

52.What is the difference between an instance variable and a class variable in Java?

Answer: An instance variable is a variable that is unique to each instance of a class, whereas a class variable is a variable that is shared by all instances of a class.

53.What is a constructor in Java?

Answer: A constructor is a special method in Java that is used to initialize objects of a class.

54.What is method overloading in Java?

Answer: Method overloading is a feature in Java that allows a class to have multiple methods with the same name, but different parameters.

55.What is method overriding in Java?

Answer: Method overriding is a feature in Java that allows a subclass to provide its own implementation of a method that is already defined in its superclass.

56.What is polymorphism in Java?

Answer: Polymorphism is a feature in Java that allows objects of different classes to be treated as if they are of the same class.

57.What is inheritance in Java?

Answer: Inheritance is a mechanism in Java that allows a subclass to inherit the properties and methods of its superclass.

58.What is encapsulation in Java?

Answer: Encapsulation is a mechanism in Java that hides the implementation details of a class from the outside world and provides access to its properties and methods only through a public interface.

59.What is abstraction in Java?

Answer: Abstraction is a mechanism in Java that allows the creation of abstract classes and interfaces, which provide a high-level view of a system or program without exposing its implementation details.

60.What is a package in Java?

Answer: A package is a namespace that is used to organize classes and interfaces in Java.

61.What is the difference between a private and a protected access modifier in Java?

Answer: Private members are only accessible within the same class, whereas protected members are accessible within the same class and its subclasses.

62.What is a static method in Java?

Answer: A static method is a method that belongs to a class, rather than an instance of the class, and can be called using the class name.

63.What is a static variable in Java?

Answer: A static variable is a variable that belongs to a class, rather than an instance of the class, and can be accessed using the class name.

64.What is a final class?

 Answer: A final class is a class that cannot be subclassed.

65.What is a final method in Java?

Answer: A final method is a method that cannot be overridden by subclasses.

66.What is a final variable in Java?

Answer: A final variable is a variable whose value cannot be changed once it has been assigned.

67.What is the difference between final, finally, and finalize in Java?

Answer: Final is used to declare a constant or to make a class or method unmodifiable. Finally is used to specify a block of code that will always be executed, regardless of whether an exception is thrown or not. Finalize is a method that is called by the garbage collector to perform cleanup operations on an object before it is destroyed.

68.What is a try-catch block in Java?

Answer: A try-catch block is used to handle exceptions in Java. The try block contains the code that might throw an exception, and the catch block contains the code that handles the exception if it is thrown.

69.What is an exception in Java?

Answer: An exception is an event that occurs during the execution of a program that disrupts the normal flow of the program.

70.What is a checked exception in Java?

Answer: A checked exception is an exception that must be caught or declared in the method signature.

71.What is an unchecked exception in Java?

Answer: An unchecked exception is an exception that does not need to be caught or declared in the method signature.

72.What is a runtime exception in Java?

Answer: A runtime exception is an unchecked exception that is thrown at runtime.

73.What is the difference between a checked exception and an unchecked exception in Java?

Answer: A checked exception must be caught or declared in the method signature, whereas an unchecked exception does not need to be caught or declared.

74.What is the difference between a runtime exception and an error in Java?

Answer: A runtime exception is an unchecked exception that is thrown by the JVM at runtime, whereas an error is a serious problem that cannot be handled by the program and usually results in termination of the program.

75.What is a stack trace in Java?

Answer: A stack trace is a list of method calls that shows the execution path of a program at the time an exception was thrown.

76.What is the purpose of the "finally" block in a try-catch-finally statement?

Answer: The "finally" block is used to specify a block of code that will always be executed, regardless of whether an exception is thrown or not.

77.What is the purpose of the "throws" keyword in a method signature in Java?

Answer: The "throws" keyword is used to declare that a method may throw a particular type of exception.

78.What is the purpose of the "throw" keyword in Java?

Answer: The "throw" keyword is used to throw an exception.

79.What is the purpose of the "assert" keyword in Java?

Answer: The "assert" keyword is used to test a condition and throw an AssertionError if the condition is false.

80.What is the purpose of the "enum" keyword in Java?

Answer: The "enum" keyword is used to define an enumerated type, which is a fixed set of values.

81.What is the difference between a method and a constructor in Java?

Answer: A method is a block of code that can be called by name, whereas a constructor is a special method that is used to initialize objects of a class.

82.What is the difference between a while loop and a do-while loop in Java?

Answer: A while loop checks the condition at the beginning of the loop, whereas a do-while loop checks the condition at the end of the loop.

83.What is the difference between a break statement and a continue statement in Java?

Answer: A break statement is used to exit a loop or switch statement, whereas a continue statement is used to skip the current iteration of a loop and continue with the next iteration.

84.What is the purpose of the "this" keyword in Java?

Answer: The "this" keyword is used to refer to the current object instance.

85.What is the purpose of the "super" keyword in Java?

Answer: The "super" keyword is used to refer to the parent class of the current object instance.

86.What is a static method in Java?

Answer: A static method is a method that is associated with the class rather than with any particular object instance.

87.What is a static variable in Java?

Answer: A static variable is a variable that is associated with the class rather than with any particular object instance.

88.What is a static block in Java?

Answer: A static block is a block of code that is executed when the class is loaded.

89.What is the difference between a static method and an instance method in Java?

Answer: A static method is associated with the class rather than with any particular object instance, whereas an instance method is associated with a particular object instance.

90.What is the difference between a static variable and an instance variable in Java?

Answer: A static variable is associated with the class rather than with any particular object instance, whereas an instance variable is associated with a particular object instance.

91.What is a package in Java?

Answer: A package is a collection of related classes and interfaces.

92.What is the purpose of the "import" statement in Java?

Answer: The "import" statement is used to import classes from other packages into the current file.

93.What is the purpose of the "interface" keyword in Java?

Answer: The "interface" keyword is used to define an interface, which is a collection of abstract methods and constants.

94.What is the purpose of the "implements" keyword in Java?

Answer: The "implements" keyword is used to indicate that a class implements an interface.

95.What is the purpose of the "extends" keyword in Java?

Answer: The "extends" keyword is used to indicate that a class extends another class.

96.What is a singleton in Java?

Answer: A singleton is a class that can have only one instance, and that instance is globally accessible.

97.What is a synchronized block in Java?

Answer: A synchronized block is a block of code that is executed by only one thread at a time.

98.What is the difference between a StringBuilder and a StringBuffer in Java?

Answer: A StringBuilder is not thread-safe, whereas a StringBuffer is thread-safe.

99.What is the purpose of the "transient" keyword in Java?

Answer: The "transient" keyword is used to indicate that a variable should not be serialized.

100.What is the purpose of the "volatile" keyword in Java?

Answer: The "volatile" keyword is used to indicate that a variable is shared by multiple threads and should not be cached by any one thread.