Top 50 java 8 Interview Questions

Java 8 interview questions give you good knowledge about Java 8 interview. After reading these question you can perform good in your interview and get job as java developer. Java initially evolved from Oak language and was invented in early 1996 using its main version as JDK 1.0 or Java 1. Sir James Gosling designed & developed the first Java at Sun Microsystems. JDK 8.0 or Java 8 was among the main releases of Java programming language in 2014. Codename Spider also knows it. Java language is an open-source project & is presently under Oracle Corporation.

Java 8 has new features, improvements & bug fixes which improves efficiency in developing & operating Java programs. This article has been made especially to have you acquainted with the kind nature of questions that you can encounter at an interview for Java 8 Language.

Java 8 Interview Questions


List the new features available in JAVA 8?

There are more features that were added to Java 8. The major features are stated below −

  • Method references – References function through their names, unlike invoking directly to them. Utilizing parameter functions.
  • Lambda expression – This adds the functional capability of Java processing.
  • Default method – The Interface to have automatic method implementation.
  • Stream API – New-stream API to ease pipeline processing.
  • New tools & compiler− New compiler utilities and tools are placed like ‘jdeps’ which figure out dependencies.
  • Date-Time API − Enhanced date-time API.
  • Nashorn, Engine for JavaScript – this is a Java-based engine that performs JavaScript code.
  • Optional – This emphasizes excellent practices which handle null values properly.

Along with the new features, more feature improvements are completed under-the-hood, at all compiler & JVM levels.

What’s the significance of Java 8?

  1. Compact, reusable & readable code
  2. Parallel execution & operations
  3. Fewer boilerplate code
  4. It can be ported via OS
  5. Has stable environment
  6. It’s high stable
  7. Sufficient support

Example of forEach() method in Iterable interface?

package com.journaldev.java8.foreach;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.function.Consumer;
import java.lang.Integer;
public class Java8ForEachExample {
    public static void main(String[] args) {
        //creating sample Collection
        List<Integer> myList = new ArrayList<Integer>();
        for(int i=0; i<10; i++) myList.add(i);
        //traversing using Iterator
        Iterator<Integer> it = myList.iterator();
            Integer i =;
            System.out.println("Iterator Value::"+i);
        //traversing through forEach method of Iterable with anonymous class
        myList.forEach(new Consumer<Integer>() {
            public void accept(Integer t) {
                System.out.println("forEach anonymous class Value::"+t);
        //traversing with Consumer interface implementation
        MyConsumer action = new MyConsumer();
//Consumer implementation that can be reused
class MyConsumer implements Consumer<Integer>{
    public void accept(Integer t) {
        System.out.println("Consumer impl Value::"+t);

What’s String: Value of Expression?

It is a static-method reference to the valueOf method of a String class.

Define Functional Interfaces?

Functional Interface refers to an interface that has just a single abstract method. Implementation of the interfaces is offered using a Lambda-Expression that means that using Lambda Expression, you require creating a new operational interface, or one can utilize the Java 8 predefined functional interface.

@FunctionalInterface” is the annotation utilized for making a new Functional Interface.

Java 8 Filter Example

List<String> strList = Arrays.asList("abc", "", "bcd", "", "defg", "jk"); long count = .filter(x -> x.isEmpty()) .count();

Why is Java’s new version require?

There are two major reasons:

Allows users to utilize new Functional Programming (FP) structures

Dramatic changes in hardware made the requirement for Java to utilize modern multi-core CPUs extra efficiently.

What’s Type Inference?

This assists the compiler in deciding argument kinds by looking at every method invocation & corresponding declaration.

What’s optional, & what’s it used?

This is an added container class that’s defined in Java.util package, & it’s used to signify optional values which either exist or doesn’t exist. The optional chief advantage is preventing null checks & none anymore “NullPointerException” outcomes at runtime.

What’s the method reference?

Method reference assists in pointing to methods through their names. Method reference describes using double colon (::) symbol. Method reference is used in pointing the following kinds of methods −

  • Instance methods
  • Static methods
  • Constructors utilizing new-operator (TreeSet::new)

What’s the function of the DoubleConsumer functional interface?

This represents operations that accept single-double valued arguments & don’t return results.

What are various sets of interfaces predefined functions? And what is the purpose of the BooleanSupplier functional interface?

  • Predicate: Performs a test & return some Boolean value.
  • Function: Transforms arguments at returnable value.
  • Consumer: Accepts an argument though doesn’t return a value.
  • Operator: Performs reduction kind of operation which accepts similar input kinds.
  • Supplier: Doesn’t accept an argument though returns a value.
  • BooleanSupplier functional interface represents a supplier of Boolean-valued results.

What’s the function of the BooleanSupplier functional interface?

It shows a dealer of Boolean-valued outcomes.

What’s Optional, and how can it be utilized?

Optional refers to a new class at Java 8 that encapsulates a possible value, i.e. the value that’s either there or it’s not. It is covered around an object, & we can contemplate it like a container that has zero or a single element.

Optional comes with a distinctive Optional.empty() value in its place of a wrapped void. Thus this can be utilized instead of a void value to eliminate NullPointerException in more cases.

Java 8 Map functional Example

List<String> G7 = Arrays.asList("USA", "Japan", "France", "Germany", "Italy", "U.K.","Canada"); String G7Countries = .map(x -> x.toUpperCase()) .collect(Collectors.joining(", "));

What’s a default method?

The default method is a method of Interface that consists of a body. Methods utilize default keywords. Using default methods like “Backward Compatibility” that suggests if JDK amends every Interface, then classes that implement the Interface break.

What are core API classes for date & time at Java 8?

There are three major core API classes for date & time at Java 8, as shown below:

  • LocalTime
  • LocalDate
  • LocalDateTime

What are Metaspace & PermGen at Java 8?

Virtual Machine for Java uses PermGen for class storage to Java version 7. It’s now succeeded by Metaspace.

Metaspace contains a huge benefit over PermGen, which makes former development dynamically minus any constraint, while PermGen contains a permanent maximum size.

What’s Nashorn in relation to Java 8?

This is a freshly introduced JavaScript that processes engine which comes bundled using Java 8. This offers tighter compliance with ECMA-JavaScript stipulations & contains runtime performance that beats Rhino, its antecedent.

What’s JJS concerning Java 8?

JJS it’s a common line tool that comes packaged using Java 8. It’s utilized to operate JavaScript code faultlessly utilizing just a console.

Java 8 forEach() Method In Iterable Interface

Example of the forEach() method.

public class Main {  
     public static void main(String[] args) {  
        List<String> subList = new ArrayList<String>();  
        System.out.println("------------Subject List--------------");  
        subList.forEach(sub -> System.out.println(sub));  

What’s the Java-8 StringJoiner Class utilized for?

Java-8 StringJoiner class makes characters sequence separated using a delimiter to allow clients to create a string through passing delimiters like hyphens & commas.

What’s a stream, & how does it vary from the collection?

A stream refers to an iterator whose role is to accept a set of actions & applies them to every element it comprises. A stream represents object structure from some collection or different source which supports aggregate operations. Different from collections, repetition logic implements in the Stream. Streams are integrally lazily loaded & processed, different from collections.

What’s a default method, & when is it used?

Default method includes an implementation, & it’s found in Interface. The method puts new functionalities to the Interface while conserving backward compatibility with classes that already use the Interface.

What’s SAM Interface?

Java 8 introduces the idea of FunctionalInterface, which can just have a single abstract method. Interfaces state has one abstract way, they’re also referred to as SAM Interfaces, “Single-Abstract-Method“.

What are standard Java predefined role interfaces?

Some common role interfaces from Java of the previous version are Callable, & Comparator. Others had Runnable & Comparable. Java 8 presents functional interfaces including Consumer, Supplier Predicate, etc.

  • Callable: Utilized to execute class instances over a different thread minus arguments & it returns a cost or throws an exception.
  • Runnable: Utilized to execute instances of class over a different thread minus arguments & no return value.
  • Comparable: utilized to sort objects at natural kind order
  • Comparator: Utilize to sort various objects in a user-defined order

What are the sources of information objects Stream can process?

Stream is capable of processing the following information:

  • An I/O-channel or input device
  • Collection of Array
  • Stream generator role or static factory
  • Reactive source

What are the characteristics of the new Time and Date API at Java 8?

  • Influenced by common joda-time package
  • Immutable classes & Thread-safe
  • Flowing methods for creation of object & arithmetic
  • Every package is based on the ISO-8601 calendar system
  • Timezone support
  • Addresses I18N problem for the previous APIs

Will the following code compile?

Yes. This code will compile the reason being it follows the functional interface requirement of stating just one abstract method. The other default method is printString(). It doesn’t count as an abstract method.

Java 8 Optional Class example

import java.util.Optional;   
public class Main{   
   public static void main(String[] args) {   
        String[] str = new String[10];   
        Optional<String>checkNull =  
        if (checkNull.isPresent()) {   
            String word = str[5].toLowerCase();   
         } else 
           System.out.println("string is null");   

What’s the difference that’s there in between Stream’s findFirst() & findAny()?

FindFirst() method is utilized in finding the 1st element from Stream, and the findAny() method is utilized to get an element from Stream.

FindFirst() in nature is predestinarianism whereas findAny() in nature is non-deterministic. At programming, Deterministic suggest the output is dependent on input/initial system state.

What’s ChronoUnits concerning Java 8?

This is the enum that’s introduced in replacing Integer values which are utilized in the old API for signifying the day, month, etc.

How is the Base64 decoder generated in Java 8?

getDecoder() method, that’s a part of Base64 class, is utilized in returning a Base64.Decoder. This decrypts by the use of Base64 scheme for encoding

When is the best situation to utilize Stream API concerning Java 8?

Stream API at Java 8 is effectively utilized if Java projects call for the following operations:

  • Execute processes lazily
  • Does parallel processing
  • Does database operations
  • Utilizes internal iteration
  • Compose functional-style programming
  • Utilize pipeline operations

What’s the use of the peek() method to Java 8?

peek() method is part of the stream class at Java 8. It’s utilized to see activities performed via a stream pipeline. Moreover, peeking is done on every step for print messages on code being executed on the console.

Peeking comes with a broad amount of use when effectiveness is a requirement, when doing stream processing or when debugging code using a lambda expression.


With this article, you will gain knowledge about the new features of Java 8. Java being a predominant programming language makes it the second rank at popularity at PYPL & TIOBE ranking of programming language.

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