Top 20 java 8 interview Questions and Answer (Most Asked)

We have provided the Most Important Java 8 Interview Questions & their Answers with Code Examples & Explanation in this tutorial. Top 20 Java 8 interview Questions and Answer

The questions in this tutorial are all specific to Java 8. With the introduction of new versions, Java has evolved a lot (over time). Java has new features with each version. This tutorial will cover all these important features.

Top 20 Java 8 interview Questions and Answer

In any Java interview that requires advanced skills, you will be asked these questions. Having a solid understanding of these concepts is essential if you are going to take any standard Java Certification Exams like Oracle Certified Associate (OCA). Top 20 java 8 interview Questions 

Most Frequently Asked Java 8 Interview Questions

Q #1) List down the new features introduced in Java 8?

Java 8 introduces the following new features:

  • Expressions in Lambda
  • References to Methods
  • This is an optional class
  • Interaction with the system
  • Methods by default
  • JavaScript Engine Nashorn
  • API for streaming
  • API Date

Q #2) What are Functional Interfaces?

Functional Interfaces have only one abstract method. These interfaces are implemented using the Lambda Expression, which means that to use the Lambda Expression, you need to create a new functional interface or you can use the predefined functional interface provided by Java 8.

“@FunctionalInterface” is the annotation used for creating a new Functional Interface.

Q #3) What is an optional class?

The optional class is a wrapper class introduced in Java 8 used to avoid NullPointerExceptions. The final class can be found in java. util package. NullPointerExceptions occur when Null checks are not performed.

Q #4) What are the default methods?

Default methods are methods of an Interface that have a body. As the name implies, these methods use the default keywords. The use of these default methods is for “Backward Compatibility”, which means that if the JDK modifies any Interface (without default methods), the classes that implement this Interface will break.

If you add the default method in an Interface, you can provide the default implementation. The implementing classes will not be affected.


public interface questions{


default void print() {





Q #5) What are the main characteristics of the Lambda Function?

The main characteristics of the Lambda Function are as follows:

  • Lambda Expressions can be passed as parameters to other methods.
  • It is possible for a method to exist independently of a class.
  • The compiler can retrieve the parameter type from the value of the parameter, so there is no need to declare it.
  • If we are using multiple parameters, then we can use parentheses, but if we are using a single parameter, there is no need for parentheses.
  • In the case of a single statement, curly braces are not needed.

Q #6) What was wrong with the old date and time?

Following is a list of the drawbacks of the old date and time:

  • The Java. util. The date class is mutable and is not thread-safe, but the new Java 8 Date and Time API is.
  • Java 8’s Date and Time API meets ISO standards, but the old data and time were poorly designed.
  • There are several API classes for dates like LocalDate, LocalTime, LocalDateTime, etc.
  • Comparing the performance of the two, Java 8 is faster than the old date and time system.

Q #7) What is the difference between the Collection API and Stream API?

The following table shows the difference between the Stream API and the Collection API:

Stream API Collection API
Java 8 Standard Edition introduced this feature. Java version 1.2 introduced it
The Iterator and Spliterator are not needed. We can use the Iterator and Spliterators with forEach to iterate over the elements and perform an action on each one.
It is possible to store an infinite number of features. It is possible to store a countable number of elements.
It is not possible to consume and iterate elements from a Stream object more than once. The Collection object can be consumed and iterated multiple times.
The function computes data. Data is stored in it.

Q #8) How can you create a Functional Interface?

Java can identify Functional Interfaces, but you can also define them using an annotation


Once you have defined the functional interface, you can only have one abstract method. There is only one abstract method, so you can write multiple static methods and default methods.

The following is an example of a functional interface written for the multiplication of two numbers.

@FunctionalInterface // annotation for functional interface

interface FuncInterface {


public int multiply(int a, int b);


public class Java8 {


public static void main(String args[]) {

FuncInterface Total = (a, b) -> a * b;

// simple operation of multiplication of ‘a’ and ‘b’

System.out.println(“Result: “+Total.multiply(30, 60));




Q #9) What is a SAM Interface?

There is only one abstract method that can be defined by a functional interface in Java 8. These interfaces specify only one abstract method, so they are sometimes referred to as SAM interfaces. “Single Abstract Method” stands for “Single Abstract Method”.

Q #10) What is Method Reference?

In Java 8, a new feature called Method Reference was introduced. The term refers to the method of functional interface. When referencing a method, it can replace the Lambda expression.

If the Lambda Expression looks like this:

num -> System.out.println(num)

Therefore, the corresponding Method Reference would be,


where “::” is an operator that distinguishes the class name from the method name.

Q #11) Explain the following Syntax

String:: Value of Expression

This is a static method reference to the ValueOf method of the String class. A static method reference to the print ln method of the out object of the System class is System.out::println.

The function returns the string representation of the argument passed. Arguments can be Characters, Integers, Booleans, and so on.

Q #14) What is a Stream API? Why do we require the Stream API?

Stream API is a new feature in Java 8. An instance of this class is used to process objects from a source, such as a Collection.

The Stream API is required because,

  • The processing is simplified because it supports aggregate operations.
  • Functional-style programming is supported.
  • It performs faster processing. Therefore, it is apt for better performance.
  • Parallel operations are possible.

Q #15) What is the difference between Stream’s findFirst() and findAny()?

Finding the first element of a stream is achieved with the findFirst() method. Finding any element of a stream is accomplished with finding().

The findFirst() method is predestinarian, whereas the finding() method is nondeterministic. Deterministic programming implies that the output depends on the input.

Q #16) What is Nashorn in Java 8?

Nashorn is a Java-based engine for executing and evaluating JavaScript code in Java 8.

Q #17) What is the Difference Between Map and flatMap Stream Operation?

Map Stream operation produces one output value per input value, whereas flatMap Stream operation produces zero or more output values per input value.

Q #18) What is MetaSpace in Java 8?

Java 8 introduced a new feature for storing classes. Java 8 stores its classes in a class database called MetaSpace. MetaSpace replaces PermGen.

Up until Java 7, PermGen was used to store classes by Java Virtual Machine. Java 8 replaced PermGen with MetaSpace because MetaSpace has no size restrictions and can grow dynamically.

Q #19) What is JJS?

JJS is a command-line tool used to execute JavaScript code. Java 8 introduces JJS, which is a JavaScript engine.

Q #20) What is ChronoUnits in Java 8?

ChronoUnits is an enum that replaces the integer values that were used in the old API to represent the month, day, etc.

Q #21) Explain StringJoiner Class in Java 8? How can we achieve joining multiple Strings using StringJoiner Class?

In Java 8, a new class was introduced in the package java. util named StringJoiner. With this class, we can join multiple strings separated by delimiters and add prefixes and suffixes to them.

Below is a program that demonstrates how to join multiple Strings using the StringJoiner class. There is a comma between the two strings here, rather than a space. With the help of the add() method, we have joined five different strings. Printed the String Joiner.

Why lambda expression is to be used?

Lambda expressions are mainly used for inline implementation of functional interfaces, that is, interfaces with a single method. In the above example, various types of lambda expressions were used to define the operation method of the MathOperation interface. We have defined the implementation of sayMessage for the GreetingService.

Java now has a very simple but powerful functional programming capability with lambda expressions.


We have discussed the new features introduced in Java 8 in this article. We have discussed all of the major Java 8 interview questions and their answers in detail.

Having studied this tutorial, you must have gained an understanding of the new APIs for date-time manipulation, the new features of Java 8, and the new Streaming APIs, along with the appropriate programming examples as per the tutorial. When you are applying for the more challenging Java positions, you will be asked about these new concepts or features during the interview process.

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Top 20 Java 8 interview Questions

Top 20 java 8 interview Questions
Top 20 java 8 interview Questions

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