Top 75+ Java Interview Questions

Java is a popular programming language used for developing a wide range of applications, including web applications, mobile apps, desktop applications, and enterprise-level software solutions. Java is known for its reliability, security, and platform independence, which makes it a popular choice for software development.

In Java interviews, you can expect questions on various topics such as the basics of Java, object-oriented programming concepts, Java collections, exception handling, multithreading, Java frameworks, and design patterns. The questions may range from simple to complex, and the interviewer may ask you to solve coding problems or explain your thought process for solving a particular problem. One can take Java courses online and enhance their programming skills and get ready for interviews and lucrative job opportunities

It’s important to have a good understanding of Java concepts, syntax, and best practices, as well as hands-on experience in coding Java programs. Being familiar with commonly used Java tools, frameworks, and libraries will also be helpful. It is recommended to familiarize yourself with the basics of Java to get a better understanding of the following questions.

Let’s look at some of the various categories of questions that an interviewer may ask you related to Java.

Java Interview Questions for Freshers

Here are the Java interview questions for freshers:

1. What is Java?

Java is a high-level, object-oriented programming language that is platform-independent. The syntax of java is based on C and C++ languages.

2. What are the features of Java?

Some features of Java include:

• Object-oriented programming
• Platform-independence
• Memory management
• Exception handling
• Multithreading
• Security
• Networking
• Garbage collection

3. What is the difference between JDK, JRE, and JVM?

JDK– (Java Development Kit) is a software development environment that includes the Java compiler, Java Runtime Environment (JRE), and other development tools. 

JRE -(Java Runtime Environment) is a runtime environment that allows Java programs to run on a machine. 

JVM -(Java Virtual Machine) is a virtual machine that provides a runtime environment for Java programs.

4. What are the data types in Java?

Java has two categories of data types: primitive and non-primitive. Primitive data types include byte, short, int, long, float, double, char, and boolean. Non-primitive data types include Strings, arrays, and classes.

5. What is a variable in Java?

A variable is a container that holds a value in Java. Variables are declared using a data type and a name and can be assigned a value using an assignment operator.

6. What is an object in Java?

An object is an instance of a class in Java. Objects have properties (attributes) and behaviors (methods).

7. What is a constructor in Java?

A constructor is a special method that is used to create an object in Java. Constructors have the same name as the class and are called when an object is created using the new keyword.

8. What is inheritance in Java?

Inheritance is a mechanism in Java that allows one class to inherit the properties and methods of another class. The class that is being inherited from is called the superclass or parent class, and the class that is inherited is called the subclass or child class.

9. What is polymorphism in Java?

Polymorphism is a feature in Java that allows objects to take on multiple forms. In Java, polymorphism is achieved through method overriding and method overloading.

10. What is encapsulation in Java?

Encapsulation is a mechanism in Java that hides the implementation details of a class from the outside world and exposes a public interface for interacting with the class. Encapsulation is achieved through the use of access modifiers and getter/setter methods.

11. What is an interface in Java?

An interface is a collection of abstract methods that define a contract for a class to implement. Interfaces are used to achieve abstraction and decoupling in Java.

12. What is an abstract class in Java?

An abstract class is a class that cannot be instantiated and is meant to be extended by other classes. Abstract classes can contain abstract methods, which are meant to be implemented by the subclasses.

13. What is a package in Java?

A package is a namespace that organizes classes and interfaces in Java. Packages are used to avoid naming conflicts and to create a hierarchical structure for classes and interfaces.

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

An array is a fixed-size collection of elements of the same data type, while an ArrayList is a dynamic-size collection of elements of any data type. Arrays can be multidimensional, while ArrayLists are always one-dimensional.

Java Interview Questions for Experienced Candidates

Here are the Java interview questions for experienced professionals:

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

The final keyword is used to declare a variable or method that cannot be changed or overridden. The final keyword is used in a try-catch block to execute a block of code regardless of whether an exception is thrown or not. The finalize() method is called by the garbage collector before an object is removed from memory.

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

A checked exception is a type of exception that must be declared in a method’s signature using the throws keyword or handled using a try-catch block.

Examples of checked exceptions include IOException and SQLException. An unchecked exception, such as NullPointerException or IllegalArgumentException, does not need to be declared or handled explicitly.

17. What is a JVM and how does it work?

The JVM (Java Virtual Machine) is an abstract machine that provides a runtime environment for Java programs. It interprets compiled Java code and executes it on the host machine. The JVM provides features such as memory management, garbage collection, security, and class loading.

18. What are some differences between the Comparable and Comparator interfaces in Java?

• The Comparable interface is used to define a natural ordering for a class, while the Comparator interface allows you to define a custom order for a class.
•  The Comparable interface is implemented by the class itself, while the Comparator interface is implemented by a separate class.

19. What are annotations in Java?

Annotations are a way of adding metadata to Java code. They provide additional information about a class, method, field, or parameter. Annotations are used for documentation, code analysis, and code generation.

20. What are generics in Java?

Generics allow you to create classes, interfaces, and methods that can work with different data types. They provide type safety and eliminate the need for casting. Generics are implemented using parameterized types and type variables.

21. What is the difference between a stack and a queue in Java?

A stack is a last-in, first-out (LIFO) data structure, while a queue is a first-in, first-out (FIFO) data structure. In Java, a Stack is implemented using the Stack class, while a queue is implemented using the LinkedList class or the PriorityQueue class.

22. What is the use of the synchronized keyword in Java?

The synchronized keyword is used to make a method or a block of code thread-safe. It ensures that only one thread can access the synchronized code at a time, preventing concurrent access and race conditions.

23. What is the purpose of the Java NIO package?

The Java NIO (New Input/Output) package provides a non-blocking I/O API for Java programs. It allows for efficient I/O operations on large amounts of data by using channels and buffers instead of streams.

24. What is a lambda expression in Java?

A lambda expression is a concise way of implementing functional interfaces in Java. It allows you to define a method without a name that can be passed as an argument to another method. Lambda expressions make it easier to write and use functional interfaces, which are a key feature of Java 8 and later versions.

25. What use do the contracts for hashCode() and equals() serve?

Think about the HashMap object’s scenario. By distinctly identifying the key-value combination in the map, we can infer that the Key of the HashMap employs the hashCode() and equals() methods to retrieve the index or the value of a given key. Two keys might yield the same outcome for these methods if they are not implemented properly, which could lead to inaccurate results and the updating of the value of the erroneous key when the method is updated. Therefore, it is crucial to correctly implement the equals method and hashCode. If we adhere to the hashCode-equals contract, this can be done correctly. The agreement specifies that:

If two objects are equal, then the hashCode method should produce the same result for both objects. To ensure this, we have to override the hashCode() method whenever we override the equals() method.

Core Java Interview Questions

Here are the Core Java interview questions:

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

 A constructor is a special method that is used to initialize objects, while a method is a regular function that is used to perform a specific task.

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

An interface is a collection of abstract methods that can be implemented by any class, while an abstract class is a class that cannot be instantiated and can contain both abstract and concrete methods.

28. What is the difference between ArrayList and LinkedList in Java?

ArrayList is an implementation of a resizable array, while LinkedList is an implementation of a doubly-linked list. ArrayList provides constant-time access to its elements, while LinkedList provides constant-time insertion and deletion of elements.

29. What is the purpose of the “static” keyword in Java?

The “static” keyword is used to denote a variable or method that belongs to the class rather than an instance of the class. Static variables and methods can be accessed without creating an instance of the class.

30. What is the difference between a private and a protected method in Java?

A private method can only be accessed within the same class, while a protected method can be accessed by any subclass of the class that defines the method.

31. What is the purpose of the “final” keyword in Java?

The “final” keyword is used to denote a variable or method that cannot be modified once it has been initialized.

32. What is the purpose of the “this” keyword in Java?

The “this” keyword is used to refer to the current object instance. It helps to eliminate the confusion between class attributes and parameters with the same name.

33. What is the purpose of the “super” keyword in Java?

The “super” keyword is used to refer to the superclass of the current class. It can be used to call a superclass constructor or method.

Java Collections Interview Questions

Here is the collection of Java interview questions:

34. What are the types of Collections in Java?

The types of Collections in Java are:

• List
• Set
• Queue
• Deque
• Map

35. What is the difference between List and a Set in Java?

A List is an ordered collection of elements with duplicates allowed while a Set is an unordered collection of unique elements.

36. What is the difference between HashSet and TreeSet?

HashSet is an unordered Set that does not allow duplicates, while TreeSet is a sorted Set that does not allow duplicates.

37. What is the difference between ArrayList and LinkedList in Java?

ArrayList is implemented as a resizable array, while LinkedList is implemented as a doubly linked list. ArrayList provides faster element access, while LinkedList provides faster element insertion and deletion.

38. What is the difference between HashMap and TreeMap in Java?

HashMap is an unordered Map that does not allow duplicates and allows one null key, while TreeMap is a sorted Map that does not allow duplicates and does not allow null keys.

39. What is the difference between Iterator and ListIterator in Java?

Iterator is used to iterate over a collection in a forward direction only, while ListIterator is used to iterate over a List in both forward and backward directions.

40. How does the compareTo() method work in Java?

The compareTo() method is used to compare two objects based on a natural ordering. It returns a negative integer, zero, or a positive integer as the first object is less than, equal to, or greater than the second object.

41. What is the difference between fail-fast and fail-safe iterators in Java?

Fail-fast iterators fail as soon as they detect that the collection has been modified while the iterator is still iterating, while fail-safe iterators make a copy of the collection before iterating and iterate over the copy instead.

42. What is the difference between Hashtable and HashMap in Java?

Hashtable is a synchronized Map that does not allow null keys or values, while HashMap is an unsynchronized Map that allows one null key and any number of null values.

43. What is the purpose of the Comparable and Comparator interfaces in Java?

The Comparable interface is used to define a natural ordering for a class, while the Comparator interface is used to define multiple sorting orders for a class.

Java OOPs Interview Questions

Here are the OOPS Java interview questions:

44. What is the OOP concept in Java?

Object-oriented programming (OOP) is a programming paradigm that focuses on the concept of objects, which can contain data and code to manipulate that data. 

Java is an object-oriented programming language, which means that it supports OOP concepts such as inheritance, encapsulation, polymorphism, and abstraction.

45. What is inheritance in Java?

Inheritance is a feature in Java that allows a class to inherit properties and methods from another class. The class that inherits the properties and methods is called the subclass or derived class, and the class that provides the properties and methods is called the superclass or base class.

46. What is encapsulation in Java?

Encapsulation is the concept of binding data and methods that operate on that data into a single unit, called a class. Encapsulation is important because it protects the data from being accessed or modified by code outside of the class, which can help improve the robustness and maintainability of the code.

47. What is an abstraction in Java?

Abstraction is the process of hiding implementation details while showing only the necessary information to the user. In Java, abstraction is achieved through abstract classes and interfaces. Abstract classes are classes that cannot be instantiated but can be extended, while interfaces are a collection of abstract methods that can be implemented by any class.

48. What is the difference between abstract classes and interfaces in Java?

Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods. Abstract classes can also have instance variables, constructors, and static methods, while interfaces cannot. A class can implement multiple interfaces but can only extend one abstract class.

49. What is a constructor in Java?

A constructor is a special method that is used to initialize objects in Java. Constructors have the same name as the class and are called automatically when an object is created. Constructors can be used to set default values for instance variables and to perform other initialization tasks.

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

The final keyword is used to make a variable, method, or class constant and unmodifiable. The finally keyword is used in a try-catch-finally block to execute a block of code regardless of whether an exception is thrown or not. The finalize() method is called by the garbage collector before an object is destroyed and can be used to perform final cleanup tasks.

51. What is the difference between static and non-static methods in Java?

Static methods are associated with the class, rather than with an instance of the class. Non-static methods, on the other hand, are associated with an instance of the class. Static methods can be called directly on the class, while non-static methods can only be called on an instance of the class.

52. What is a package in Java?

A package is a way of organizing related classes and interfaces into a single unit, called a package. Packages help to avoid naming conflicts between classes and also make it easier to manage and maintain large Java applications.

Advanced Java Interview Questions

Here are the advanced Java interview questions:

53. What is the difference between composition and inheritance in Java, and when would you use each?

Composition is when one class contains an instance of another class, while inheritance is when a subclass extends a superclass. Composition is generally preferred when you need greater flexibility and modularity, while inheritance is preferred when you need a more hierarchical structure and code reuse.

54. What is a lambda expression in Java, and how can you use it?

A lambda expression is a concise way to define an anonymous function, which can be used in place of a single-method interface or class. It is defined using the “->” operator and can take arguments and return a value. Lambda expressions can make code more concise and readable and can be used with functional interfaces in Java.

55. What is the difference between a class and an object in Java?

A class is a blueprint or template for creating objects, while an object is an instance of a class. Classes define the properties and behaviors that objects will have, while objects have specific values for those properties and can perform specific actions.

56. What is the difference between a synchronized method and a synchronized block in Java?

A synchronized method is a method that locks the entire object on which it is called, while a synchronized block allows you to lock a specific block of code within a method. Synchronized methods can be simpler to use, but can lead to performance issues in certain situations, while synchronized blocks provide more fine-grained control over locking.

57. What is a singleton pattern in Java, and how would you implement it?

A singleton pattern is a design pattern that ensures that only one instance of a class can be created. This can be useful in situations where you need to ensure that there is only one instance of a particular resource or service. In Java, you can implement a singleton pattern using a private constructor, a static instance variable, and a public static method to return the instance.

58. What is the difference between an abstract class and an interface in Java, and when would you use each?

Abstract classes and interfaces are both used for abstraction, but they have some key differences. Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods. Additionally, a class can implement multiple interfaces, but can only extend one abstract class. When deciding which to use, it depends on the situation.

59. What is a reflection in Java?

Reflection is a mechanism in Java that allows code to examine and manipulate the structure, behavior, and properties of classes at runtime. It allows you to inspect classes, methods, fields, and annotations at runtime, and even invoke methods dynamically.

Java String Interview Questions

Here are the string Java interview questions:

60.What is a Java string, and how is it different from other data types?

A Java string is a sequence of characters that represents text. It is different from other data types in that it is a class rather than a primitive data type. Strings are immutable, meaning that their value cannot be changed once they are created.

61. How can you create a Java string, and what are some ways to manipulate it?

You can create a Java string using double quotes or the String constructor, like this

String myString1 = “Hello, world!”; 

String myString2 = new String(“Hello, world!”);

Some ways to manipulate strings include concatenation, substring extraction, and case conversion.

62. What is the difference between the equals() method and the == operator for comparing Java strings?

The equals() method checks whether two strings have the same value, while the == operator checks whether two string variables refer to the same object in memory. You should use the equals() method for comparing the values of strings and the == operator for checking whether two string variables refer to the same object in memory.

63. What is a StringBuilder in Java, and how can you use it?

A StringBuilder is a mutable sequence of characters that can be used to efficiently build strings. It provides methods for appending, inserting, and deleting characters, and can be used to create complex strings without creating a large number of intermediate string objects. 

To use a StringBuilder, you can create an instance of the class and call its methods, like this:

StringBuilder sb = new StringBuilder(“Hello”);

sb.append(“, world!”);

String result = sb.toString(); // result is “Hello, world!”

64. Describe Java’s String pool.

The Java heap memory’s String Pool, commonly referred to as SCP (String Constant Pool), is a dedicated storage area used to house distinct string objects. Each time a string object is generated, a check is made to see if there is an existing string object with the same string value in the String pool. If there is, the reference to the existing string object is returned. Otherwise, the string object is created without checking. If not, the reference will be returned and the new string object will be added to the string pool.

65. What is the difference between the String, StringBuffer, and StringBuilder classes in Java?

The String class is immutable, meaning that its value cannot be changed once it is created. The StringBuffer and StringBuilder classes are mutable and can be used to efficiently manipulate strings. StringBuffer is thread-safe, meaning that it can be safely accessed by multiple threads simultaneously, while StringBuilder is not thread-safe.

66. How can you convert a Java string to uppercase or lowercase?

You can use the toUpperCase() and toLowerCase() methods to convert a Java string to uppercase or lowercase, like this:

String myString = “Hello, world!”;

String uppercaseString = myString.toUpperCase();

String lowercaseString = myString.toLowerCase();

67. How can you remove whitespace from the beginning and end of a Java string?

You can use the trim() method to remove whitespace from the beginning and end of a Java string, like this:

String myString = ”  Hello, world!  “;

String trimmedString = myString.trim(); // trimmedString is “Hello, world!”

68. What is the difference between the indexOf() and lastIndexOf() methods for searching a Java string?

The indexOf() method searches for the first occurrence of a substring in a Java string, while the lastIndexOf() method searches for the last occurrence of a substring. Both methods return -1 if the substring is not found.

Java 8 Interview Questions 

Here are the Java 8 interview questions:

69. What are the new features introduced in Java 8?

Java 8 introduced several new features including Lambda expressions, Streams, Optional class, Default and static methods in interfaces, Date and Time API, and several other improvements.

70. What are Lambda expressions in Java 8?

Lambda expressions are a new feature in Java 8 that allows us to express a method or a function in a concise way without using anonymous inner classes. Lambda expressions are mainly used for implementing functional interfaces, which have a single abstract method.

71. What is a Stream in Java 8?

A Stream is a new feature in Java 8 that allows us to process collections of objects functionally. Streams provide a set of powerful methods that can be used for filtering, mapping, sorting, and reducing data.

72. What is the Optional class in Java 8?

The Optional class is a new feature in Java 8 that is used to represent a nullable value. It helps to avoid NullPointerExceptions by explicitly stating whether a value is present or not.

73. What are the Default methods in Java 8 interfaces?

Default methods are a new feature in Java 8 that allows interfaces to have methods with a default implementation. This feature was introduced to allow interfaces to evolve without breaking the existing implementations.

74. What is the Date and Time API in Java 8?

The Date and Time API is a new feature in Java 8 that provides a comprehensive set of classes for representing dates, times, and durations. It also provides methods for formatting and parsing dates and times.

75. What is the difference between a lambda expression and an anonymous class in Java 8?

The main difference between a lambda expression and an anonymous class is that a lambda expression is shorter and more concise than an anonymous class. Lambda expressions can only be used for implementing functional interfaces, which have a single abstract method, while anonymous classes can be used for implementing any interface.

76. What is the functional interface in Java 8?

A functional interface is an interface that has a single abstract method. It is used for implementing Lambda expressions and method references in Java 8.

77. What is the difference between a Consumer and a Supplier in Java 8?

A Consumer is an interface that has a single abstract method that takes an input and returns no output. It is used for performing operations on an object. A Supplier, on the other hand, is an interface that has a single abstract method that takes no input and returns an output. It is used for supplying an object.

78. What is the difference between a Predicate and a Function in Java 8?

A Predicate is an interface that has a single abstract method that takes an input and returns a boolean value. It is used for testing a condition.

A Function, on the other hand, is an interface that has a single abstract method that takes an input and returns an output. It is used for transforming an object.

Conclusion

As a result of Java’s widespread use as a programming language, you might be asked a variety of Java-related interview questions. You may improve your chances of acing your Java job interview and getting your dream job in software development by practicing for these questions and having a firm grasp of Java.