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1. Introduction

In this tutorial, we're going to explore some of the JDK8-related questions that might pop up during an interview.

Java 8 is a platform release packed with new language features and library classes. Most of these new features are geared towards achieving cleaner and more compact code, while some add new functionality that has never before been supported in Java.

Further reading:

Memory Management in Java Interview Questions (+Answers)

A set of popular Memory Management-related interview questions and of course answers.

Java Collections Interview Questions

A set of practical Collections-related Java interview questions

2. Java 8 General Knowledge

Q1. What New Features Were Added in Java 8?

Java 8 ships with several new features, but the most significant are the following:

  • Lambda Expressions − a new language feature allowing us to treat actions as objects
  • Method References − enable us to define Lambda Expressions by referring to methods directly using their names
  • Optional − special wrapper class used for expressing optionality
  • Functional Interface – an interface with maximum one abstract method; implementation can be provided using a Lambda Expression
  • Default methods − give us the ability to add full implementations in interfaces besides abstract methods
  • Nashorn, JavaScript Engine − Java-based engine for executing and evaluating JavaScript code
  • Stream API − a special iterator class that allows us to process collections of objects in a functional manner
  • Date API − an improved, immutable JodaTime-inspired Date API

Along with these new features, lots of feature enhancements are done under the hood at both the compiler and JVM level.

3. Method References

Q1. What Is a Method Reference?

A method reference is a Java 8 construct that can be used for referencing a method without invoking it. It's used for treating methods as Lambda Expressions. They only work as syntactic sugar to reduce the verbosity of some lambdas. This way the following code:

(o) -> o.toString();

Can become:


A method reference can be identified by a double colon separating a class or object name, and the name of the method. It has different variations, such as constructor reference:


Static method reference:


Bound instance method reference:


Unbound instance method reference:


We can read a detailed description of method references with full examples by following this link and this one.

Q2. What Is the Meaning of String::Valueof Expression?

It's a static method reference to the valueOf method of the String class.

4. Optional

Q1. What Is Optional? How Can It Be Used?

Optional is a new class in Java 8 that encapsulates an optional value, i.e. a value that is either there or not. It's a wrapper around an object, and we can think of it as a container of zero or one element.

Optional has a special Optional.empty() value instead of wrapped null. Thus it can be used instead of a nullable value to get rid of NullPointerException in many cases.

We can read a dedicated article about Optional here.

The main purpose of Optional, as designed by its creators, is to be a return type of methods that previously would return null. Such methods would require us to write boilerplate code to check the return value, and we could sometimes forget to do a defensive check. In Java 8, an Optional return type explicitly requires us to handle null or non-null wrapped values differently.

For instance, the Stream.min() method calculates the minimum value in a stream of values. But what if the stream is empty? If it wasn't for Optional, the method would return null or throw an exception.

However, it returns an Optional value, which may be Optional.empty() (the second case). This allows us to easily handle such cases:

int min1 = int[]{1, 2, 3, 4, 5})
assertEquals(1, min1);

int min2 = int[]{})
assertEquals(0, min2);

It's worth noting that Optional is not a general purpose class like Option in Scala. It's not recommended that we use it as a field value in entity classes, which is clearly indicated by it not implementing the Serializable interface.

5. Functional Interfaces

Q1. Describe Some of the Functional Interfaces in the Standard Library

There are a lot of functional interfaces in the java.util.function package. The more common ones include, but are not limited to:

  • Function – it takes one argument and returns a result
  • Consumer – it takes one argument and returns no result (represents a side effect)
  • Supplier – it takes no arguments and returns a result
  • Predicate – it takes one argument and returns a boolean
  • BiFunction – it takes two arguments and returns a result
  • BinaryOperator – it is similar to a BiFunction, taking two arguments and returning a result. The two arguments and the result are all of the same types.
  • UnaryOperator – it is similar to a Function, taking a single argument and returning a result of the same type

For more on functional interfaces, see the article “Functional Interfaces in Java 8.”

Q2. What Is a Functional Interface? What Are the Rules of Defining a Functional Interface?

A functional interface is an interface with one single abstract method (default methods do not count), no more, no less.

Where an instance of such an interface is required, a Lambda Expression can be used instead. More formally put: Functional interfaces provide target types for lambda expressions and method references.

The arguments and return type of such an expression directly match those of the single abstract method.

For instance, the Runnable interface is a functional interface, so instead of:

Thread thread = new Thread(new Runnable() {
    public void run() {
        System.out.println("Hello World!");

We could simply do:

Thread thread = new Thread(() -> System.out.println("Hello World!"));

Functional interfaces are usually annotated with the @FunctionalInterface annotation, which is informative and doesn't affect the semantics.

6. Default Method

Q1. What Is a Default Method and When Do We Use It?

A default method is a method with an implementation, which can be found in an interface.

We can use a default method to add a new functionality to an interface, while maintaining backward compatibility with classes that are already implementing the interface:

public interface Vehicle {
    public void move();
    default void hoot() {

Usually when we add a new abstract method to an interface, all implementing classes will break until they implement the new abstract method. In Java 8, this problem was solved by using the default method.

For example, the Collection interface does not have a forEach method declaration. Thus adding such a method would simply break the whole collections API.

Java 8 introduced the default method so that the Collection interface can have a default implementation of the forEach method without requiring the classes implementing this interface to implement the same.

Q2. Will the Following Code Compile?

public interface Function2<T, U, V> {
    public V apply(T t, U u);

    default void count() {
        // increment counter

Yes, the code will compile because it follows the functional interface specification of defining only a single abstract method. The second method, count, is a default method that does not increase the abstract method count.

7. Lambda Expressions

Q1. What Is a Lambda Expression and What Is It Used for?

In very simple terms, a lambda expression is a function that we can reference and pass around as an object.

Moreover, lambda expressions introduce functional style processing in Java, and facilitate the writing of compact and easy-to-read code.

As a result, lambda expressions are a natural replacement for anonymous classes such as method arguments. One of their main uses is to define inline implementations of functional interfaces.

Q2. Explain the Syntax and Characteristics of a Lambda Expression

A lambda expression consists of two parts, the parameter part and the expressions part separated by a forward arrow:

params -> expressions

Any lambda expression has the following characteristics:

  • Optional type declaration – when declaring the parameters on the left-hand side of the lambda, we don't need to declare their types as the compiler can infer them from their values. So int param -> … and param ->… are all valid
  • Optional parentheses – when only a single parameter is declared, we don't need to place it in parentheses. This means param -> … and (param) -> … are all valid, but when more than one parameter is declared, parentheses are required
  • Optional curly braces – when the expressions part only has a single statement, there is no need for curly braces. This means that param – > statement and param – > {statement;} are all valid, but curly braces are required when there is more than one statement
  • Optional return statement – when the expression returns a value and it is wrapped inside curly braces, then we don't need a return statement. That means (a, b) – > {return a+b;} and (a, b) – > {a+b;} are both valid

To read more about Lambda expressions, follow this link and this one.

8. Nashorn Javascript

Q1. What Is Nashorn in Java8?

Nashorn is the new Javascript processing engine for the Java platform that shipped with Java 8. Until JDK 7, the Java platform used Mozilla Rhino for the same purpose, as a Javascript processing engine.

Nashorn provides better compliance with the ECMA normalized JavaScript specification and better runtime performance than its predecessor.

Q2. What Is JJS?

In Java 8, jjs is the new executable or command line tool we use to execute Javascript code at the console.

9. Streams

Q1. What Is a Stream? How Does It Differ From a Collection?

In simple terms, a stream is an iterator whose role is to accept a set of actions to apply on each of the elements it contains.

The stream represents a sequence of objects from a source such as a collection, which supports aggregate operations. They were designed to make collection processing simple and concise. Contrary to the collections, the logic of iteration is implemented inside the stream, so we can use methods like map and flatMap for performing a declarative processing.

Additionally, the Stream API is fluent and allows pipelining:

int sum = int[]{1, 2, 3})
  .filter(i -> i >= 2)
  .map(i -> i * 3)

Another important distinction from collections is that streams are inherently lazily loaded and processed.

Q2. What Is the Difference Between Intermediate and Terminal Operations?

We combine stream operations into pipelines to process streams. All operations are either intermediate or terminal.

Intermediate operations are those operations that return Stream itself, allowing for further operations on a stream.

These operations are always lazy, i.e. they do not process the stream at the call site. An intermediate operation can only process data when there is a terminal operation. Some of the intermediate operations are filter, map and flatMap.

In contrast, terminal operations terminate the pipeline and initiate stream processing. The stream is passed through all intermediate operations during terminal operation call. Terminal operations include forEach, reduce, Collect and sum.

To drive this point home, let's look at an example with side effects:

public static void main(String[] args) {
    System.out.println("Stream without terminal operation"); int[] { 1, 2, 3 }).map(i -> {
        System.out.println("doubling " + i);
        return i * 2;
    System.out.println("Stream with terminal operation"); int[] { 1, 2, 3 }).map(i -> {
            System.out.println("doubling " + i);
            return i * 2;

The output will be as follows:

Stream without terminal operation
Stream with terminal operation
doubling 1
doubling 2
doubling 3

As we can see, the intermediate operations are only triggered when a terminal operation exists.

Q3. What Is the Difference Between Map and flatMap Stream Operation?

There is a difference in signature between map and flatMap. Generally speaking, a map operation wraps its return value inside its ordinal type, while flatMap does not.

For example, in Optional, a map operation would return Optional<String> type, while flatMap would return String type.

So after mapping, we need to unwrap (read “flatten”) the object to retrieve the value, whereas after flat mapping, there is no such need as the object is already flattened. We apply the same concept to mapping and flat mapping in Stream.

Both map and flatMap are intermediate stream operations that receive a function and apply this function to all the elements of a stream.

The difference is that for the map, this function returns a value, but for flatMap, this function returns a stream. The flatMap operation “flattens” the streams into one.

Here's an example where we take a map of users' names and lists of phones and “flatten” it down to a list of phones of all the users using flatMap:

Map<String, List<String>> people = new HashMap<>();
people.put("John", Arrays.asList("555-1123", "555-3389"));
people.put("Mary", Arrays.asList("555-2243", "555-5264"));
people.put("Steve", Arrays.asList("555-6654", "555-3242"));

List<String> phones = people.values().stream()

Q4. What Is Stream Pipelining in Java 8?

Stream pipelining is the concept of chaining operations together. We do this by splitting the operations that can happen on a stream into two categories: intermediate operations and terminal operations.

Each intermediate operation returns an instance of Stream itself when it runs. Therefore, we can set up an arbitrary number of intermediate operations to process data, forming a processing pipeline.

There must then be a terminal operation which returns a final value and terminates the pipeline.

10. Java 8 Date and Time API

Q1. Tell Us About the New Date and Time API in Java 8

A long-standing problem for Java developers has been the inadequate support for the date and time manipulations required by ordinary developers.

The existing classes such as java.util.Date and SimpleDateFormatter aren’t thread-safe, leading to potential concurrency issues for users.

Poor API design is also a reality in the old Java Data API. Here's just a quick example: years in java.util.Date start at 1900, months start at 1, and days start at 0, which is not very intuitive.

These issues and several others have led to the popularity of third-party date and time libraries, such as Joda-Time.

In order to address these problems and provide better support in JDK, a new date and time API, which is free of these problems, has been designed for Java SE 8 under the package java.time.

11. Conclusion

In this article, we explored several important technical interview questions with a bias on Java 8. This is by no means an exhaustive list, but it contains questions that we think are most likely to appear in each new feature of Java 8.

Even if we're just starting out, ignorance of Java 8 isn't a good way to go into an interview, especially when Java appears strongly on a resume. It is therefore important that we take some time to understand the answers to these questions and possibly do more research.

Good luck in the interview.

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