Java 8 Interview Questions for Freshers

1. What are the main features introduced in Java 8?

The few important features of Java 8 are:

Lambda Expressions: A functional programming feature that allows you to treat functionality as a method argument or pass blocks of code around.

Stream API: For working with sequences of elements, especially for filtering, mapping, and reducing collections.

Functional Interfaces: An interface with a single abstract method, like Runnable or Comparator.

Default Methods: Methods in interfaces that provide default implementations, allowing interface evolution without breaking existing code.

2. What is Polymorphism?

Polymorphism is the ability of an object to take many forms. It allows performing the same action in many different ways. Polymorphism in Java has two types: Compile time polymorphism (static binding) and Runtime polymorphism (dynamic binding).

3. What is a functional interface, and can you give an example?

A functional interface is an interface that has exactly one abstract method. It can have multiple default or static methods. Functional interfaces are used primarily with lambda expressions. The @FunctionalInterface annotation is optional but helps in preventing accidental addition of other abstract methods.

4. What is runtime polymorphism or dynamic method dispatch?

Runtime polymorphism or Dynamic Method Dispatch is a process in which a call to an overridden method is resolved at runtime rather than compile-time. It is called as through the reference variable of a superclass.

5. What is abstraction in Java?

Data Abstraction is the property by virtue of which only the essential details are displayed to the user. We use abstract class and interface to achieve abstraction.

6. What do you mean by an interface in Java?

An interface in the Java programming language is an abstract type that is used to specify a behavior that classes must implement. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface. Method bodies exist only for default methods and static methods.

7.What is the purpose of Optional in Java 8?

The Optional class is used to represent a value that may or may not be present. It is a container object which may contain a value or may be empty (null-safe). It helps avoid NullPointerException by enforcing null checks in a more structured way.

8. What is the difference between abstract classes and interfaces?

An abstract class allows creating functionality that subclasses can implement or override. An interface only allows defining functionality, not implementing it and whereas a class can extend only one abstract class, it can take advantage of multiple interfaces.

9. What is inheritance in Java?

Java Inheritance is a mechanism in which one class acquires the property of another class. When an “Is-A” relationship exists between two classes, we use Inheritance. The parent class is called a super class and the inherited class is called a subclass.

10. What are the different types of inheritance in Java?

Single Inheritance
Multiple Inheritance
Multilevel Inheritance
Hierarchical Inheritance
Hybrid Inheritance

11. What are streams in Java 8, and how are they different from collections?

A stream is a sequence of elements that supports functional-style operations for processing collections of data.

Key differences:

Streams are lazily evaluated: Operations on streams are evaluated only when a terminal operation is invoked (like collect, forEach).
Streams can be parallelized: Stream operations can run in parallel, making them more efficient for large datasets.
Streams are not reusable: Once a stream is consumed (with a terminal operation), it cannot be reused.

12. What is method overloading and method overriding?

Method Overloading is used to increase the readability of the program
Method Overriding Provides a specific implementation of the method already in the parent class
Method Overloading is performed within the same class
Method Overriding involves multiple classes

13. Can you override a private or static method in Java?

No, we cannot override private or static methods methods declared as private can never be overridden, they are in-fact bounded during compile time. Private methods in Java are not visible to any other class which limits their scope to the class in which they are declared.

14. What is multiple inheritance? Is it supported by Java?

Multiple Inheritance is a feature of object oriented concept, where a class can inherit properties of more than one parent class. The problem occurs when there exist methods with same signature in both the super classes and subclass.

15. What is encapsulation in Java?

Encapsulation is defined as the wrapping up of data under a single unit. It is the mechanism that binds together code and the data it manipulates. It is the process of hiding information details and protecting data and behavior of the object.

16. What is the difference between map and flatMap in Java 8?

Both map and flatMap are used in the Stream API, but they serve different purposes:

map(): It is used to transform each element of the stream into another form (1-to-1 mapping). The result is a Stream of elements.
flatMap(): It is used to flatten and transform a stream of collections into a single stream (1-to-many mapping). The result is a flattened Stream.

17. What is composition in Java?

Composition is a restricted form of Aggregation in which two entities are highly dependent on each other. It represents part-of relationship. In composition, both entities are dependent on each other. When there is a composition between two entities, the composed object cannot exist without the other entity.

18. What is a copy constructor in Java?

Copy constructor is a special type of constructor that creates an object using another object of the same Java class. It returns a duplicate copy of an existing object of the class. We can assign a value to the final field but the same cannot be done while using the clone() method.

19. What is a servlet?

A servlet is a Java programming language class that is used to extend the capabilities of servers that host applications accessed by means of a request-response programming model.

20. What is the life-cycle of a servlet?

Servlet life cycle can be defined as the entire process from its creation till the destruction. The servlet is initialized by calling the init () method. The servlet calls service () method to process a client’s request.

21. What are default methods in interfaces, and why are they introduced in Java 8?

Default methods are methods in interfaces that have an implementation. They were introduced to allow new functionality to be added to interfaces without breaking existing implementations.

Before Java 8, adding a new method to an interface would require all implementing classes to define that method, potentially breaking large amounts of existing code. Default methods solve this problem by providing a method with a default implementation, so classes implementing the interface can inherit the behavior without overriding it unless necessary.

22. How do cookies work in Servlets?

In cookies technique, we add cookies with a response from the servlet. So the cookie is stored in the cache of the browser. After that if a request is sent by the user, a cookie is added with the request by default. Thus, we recognize the user as the old user.

23. Explain JDK, JRE and JVM?

JDK stands for Java Development Kit is a software development kit whereas JRE expand as Java Runtime Environment is a software bundle that allows Java program to run, whereas JVM(Java Virtual Machine) is an environment for executing byte code.

24. Explain public static void main(String args[]) in Java

Public static void main (String[] args) Java main method is the entry point of any java program. Its syntax is always public static void main (String[] args) and can change the name of String array argument.

25. What are method references in Java 8?

Method references are a shorthand notation of a lambda expression to call a method directly. Instead of writing a lambda expression that calls a method, you can use a method reference. They are used to improve code readability and reduce boilerplate.

26. Why is Java not 100% Object-oriented?

Java is not a pure Object oriented language, but so called a “Hybrid” language. It supports primitive data type such as int ,char, float .All these are not object but you can argue we have wrapper class for each of the data type such Integer(int) , Float(float) etc . But as java supports primitive data type is the one main reason why it is not 100 % oops.

27. What are wrapper classes in Java?

A Wrapper class is a class whose object wraps or contains primitive data types. When we create an object to a wrapper class, it contains a field and in this field, we can store primitive data types.

28. State the different types of method references

There are four types of method references:

Reference to a static method: ClassName::staticMethod

Reference to an instance method of a particular object: object::instanceMethod

Reference to an instance method of an arbitrary object of a particular type: ClassName::instanceMethod

Reference to a constructor: ClassName::new

29. What are constructors in Java?

Constructor is a block of code similar to the method. It is called when an instance of the class is created. It is a special type of method which is used to initialize the object. Every time an object is created using the new () keyword, at least one constructor is called.

30. What is a singleton class in Java and how can we make a class singleton?

Singleton class is a class that can have only one object (an instance of the class) at a time. After the first time, if we try to instantiate the Singleton class, the new variable also points to the first instance created.

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

Both Predicate and Function are functional interfaces in Java 8, but they serve different purposes: Predicate: Represents a condition or predicate that takes one argument and returns a boolean value (true or false). It’s commonly used in filtering operations. Example:

Predicate<Integer> isEven = num -> num % 2 == 0; System.out.println(isEven.test(4)); // true Function<T, R>: Represents a function that accepts one argument and produces a result. It’s used for mapping or transforming data. Example: Function<String, Integer> stringLength = String::length; System.out.println(stringLength.apply(“Hello”)); // 5

32. What is the difference between an Array list and vector in Java?

Vector and ArrayList both use Array internally as data structure. They are dynamically resizable. But, ArrayList increases by half of its size when its size is increased. Therefore as per Java API the only main difference is, Vector’s methods are synchronized and ArrayList’s methods are not synchronized.

33. What are the differences between Heap and Stack Memory in Java?

The major difference between Stack memory and heap memory is that the stack is used to store the order of method execution and local variables while the heap memory stores the objects and it uses dynamic memory allocation and deallocation.

34. What is a package in Java? List down various advantages of packages.

Java package is used to categorize the classes and interfaces so that they can be easily maintained. It provides access protection and removes naming collisions.

35. Why are pointers not used in Java?

Java doesn’t have pointers because it doesn’t need them for general purpose OOP programming. Adding pointers to Java would undermine security and robustness and make the language more complex.

36. What is a JIT compiler in Java?

The JIT compiler aids in improving the performance of Java programs by compiling bytecode into native machine code at run time. It helps improve the performance of Java programs by compiling bytecode into native machine code at run time.

37. What is meant by Collections in Java?

Any group of individual objects which are represented as a single unit is known as the collection of the objects. In Java, a separate framework named the “Collection Framework” has been defined in JDK 1.2 which holds all the collection classes and interface in it.

38. What are all the Classes and Interfaces that are available in the collections?

Java Collection means a single unit of objects. The Java Collection framework provides many interfaces (Set, List, Queue, Deque) and classes (ArrayList, Vector, LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet).

39. What do you understand about Java virtual machines?

A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java byte code. The JVM reference implementation is developed by the OpenJDK project as open source code.

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

JDK is a software development kit whereas JRE is a software bundle that allows Java programs to run, whereas JVM is an environment for executing byte code. The full form of JDK is Java Development Kit, while the full form of JRE is Java Runtime Environment, while the full form of JVM is Java Virtual Machine.

41. How many types of memory areas are allocated by JVM?

The memory in the JVM is divided into five different parts namely:

Method area− the method area stores the class code − code of the variables and methods.
Heap − The Java objects are created in this area.
Java Stack− While running methods the results are stored in the stack memory.

42. What is the java platform?

Java platform is a collection of programs that help to develop and run programs written in the Java programming language. It includes an execution engine, a compiler, and a set of libraries. JAVA is a platform-independent language. It is not specific to any processor or operating system.

43. What is a classloader?

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes.

44. What is the default value of the local variables?

The local variables do not have any default values in Java. This means that they can be declared and assigned a value before the variables are used for the first time, otherwise, the compiler throws an error.

45. Can you explain the thread lifecycle in Java?

The life cycle of the thread in java is controlled by a Java virtual machine. The java thread states are as follows:

New
Runnable
Running
Non-Runnable (Blocked)
Terminated

NEW is a newly created thread that has not yet started the execution.

RUNNABLE is either running or ready for execution but it’s waiting for resource allocation.

BLOCKED is waiting to acquire a monitor lock to enter or re-enter a synchronized block/method.The thread is in running state if the thread scheduler has selected it.

A thread is in terminated or dead state when its run() method exits.

46. How do you make a thread stop in Java?

Modern ways to suspend/stop a thread are by using a boolean flag and Thread. interrupt () method. We can define a boolean variable which is used for stopping/killing threads saying ‘exit’ using a boolean flag. Whenever we want to stop a thread, the ‘exit’ variable will be set to true.

47. What purpose do the Volatile variables serve in Java?

Volatile keyword is used to modify the value of a variable by different threads and used to make classes thread safe. It means that multiple threads can use a method and instance of the classes at the same time without any problem.

48. What do you understand about OutOfMemoryError in Java?

OutOfMemoryError is a runtime error which occurs when the Java Virtual Machine is unable to allocate an object due to insufficient space in the Java heap. The Java Garbage Collector cannot free up the space required for a new object.

49. Explain public static void main(String args[])

public static void main(String[] args) Java main method is the entry point of any java program. Its syntax is always public static void main(String[] args) . You can only change the name of the String array argument, for example you can change args to myStringArgs.

50. Different Data types in Java.

Primitive data types – includes byte , short , int , long , float , double , boolean and char. Non-primitive data types – such as String, Arrays and Classes.

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Java 8 Interview Questions for Freshers