Top 25 Java 8 Interview Questions and Answers in 2024

Java is one of the leading programming models in the world. It has had several releases over the years owing to its prowess and wide application. Java 8 is one of its best releases, often considered revolutionary. It remains one of the biggest upgrades to the Java programming model.

You may be asked a few questions on Java 8 in your programming or software development interview, especially if it is a Java-related job. Since our main concern is to help you ace your upcoming interview, we will look at some common questions you should consider when preparing. Take a look at the following:

1. Define Java 8

Java 8 is one of the most popular releases of the JAVA programming language and model. It was released in March 2014 and goes by the code name spider. It packs several features, making it a huge upgrade to the previous versions. Java 8 comes with a new JavaScript engine and streaming APIs, functional programming support, and new date-time manipulations APIs, among many others, making it a revolutionary upgrade.

2. Java 8 Packs A Variety Of New Features. Mention Them

Being an upgrade, Java 8 has many new features. It has lambda expression, a function often shared or referred to as an object; stream API, which offers pipeline data processing; method references that use functions as parameters when invoking methods; functional interfaces, which are single abstract interfaces; date-time API, which is an updated time inspired API and a Nashorn, JavaScript engine that allows JavaScript execution in Java, thus replacing Rhino.

3. Mention The Pre-Defined Standard Functional Interfaces In Java 8

Previous Java versions had callable, comparable, runnable, and comparator interfaces. However, Java 8 j introduced several new functional interfaces such as predicate, supplier, and consumer. All in all, some of the standard pre-defined functional interfaces in Java are runnable, which executes class instances over another thread without arguments and no returned value; comparator, which sorts objects in a user-defined order; comparable, which sorts o jects in natural sort order and callable which like runnable executes class instances over another thread with no agents but returns a value or an exception.

4. Define A Lambda Expression

A lambda expression is an unnamed function. There is always a possibility that it may have or lack parameters. This expression can also be referred to as an anonymous function given that it independently lacks type information but gets executed on demand. Such an expression thus comes in handy in the extraction, filtration, and iteration of data from a dataset. Lambda expressions work with the single abstract method, given that they are similar to anonymous functions.

5. Mention The Parts Of A Lambda Expression

A basic lambda expression has three parts, which are both unique. The string name, also known as the list of arguments/params, is generally passed in round brackets. This list has no set parameters as it can have none or more. Declaration of parameter type is thus optional.

The second part is the arrow token, commonly known as the lambda arrow operator. It separates the parameters from the body and points the first part to the body in most cases. The third and last part is the body, which generally has expressions or statements.

6. What Do You Know About Java 8 Streams?

A stream expresses data processing queries in a declarative fashion. This abstraction represents data object sequences and operations series, making it a data pipeline. However, it does not relate to Java I/O streams and temporarily holds data. Java 8 Streams key interface accepts functional interfaces to permit the passage of lambdas. These streams also allow chaining or fluent interfaces. These abstractions have three main components: a single terminal operation for result production, a data source, and a collection of operations that process the data source.

7. Define The Intermediate And Terminal Operations In Java 8

Intermediate operations process stream elements, thus transforming one stream into another. They are generally lazy and hence cannot be executed until the invocation of a terminal operation. Everything follows a given set order in these operations, and all the operations are often chained in the processing pipeline. Intermediate operations also perform source elements iteration and are most lambda functions. On the other hand, terminal operations collect and stream data. They also kick-start the stream pipeline.

8. Mention The Most Popular Intermediate Operations In Java 8

There are seven common intermediate operations in Java 8. The special () operation deals with elements that haven’t been passed yet to the next page, while the limit (long max size) operation limits a given stream size to maxsize. The skip (long start) operation skips the initial elements till the beginning, while the peek(consumer) operation applies a consumer minus stream modification. The remaining two operations are quite bulky. The filter(predicate<T>) operation permits selective processing of different stream elements. Only elements that meet the supplied conditions by the predicate are returned. The map (Function<T, R>) operation transforms all the stream elements using the supplied mapper function, thus returning a new stream.

9. Mention The Different Types Of Terminal Operations

The four most common types of terminal operations are time collect (), reduce (), search/query operations, and iterative operations. The first one looks for single results of all the elements on a stream sequence and collects them; the second(reduce) produces one result from all the stream elements. These include count (), which returns the number of elements, max (), which returns the max element, and min (), which returns the min element. The search/query operations use a predicate as the input for a match condition, momentarily stopping stream processing. Lastly, iterative operations are handy in maintaining order in parallel streams and for operations with each stream element. The two responsible functions are for each ordered() and forEach.

10. Differentiate Collections From Streams

There are five main differences between collections and streams. However, it is worth noting that the former is the source for the latter. In collections, the data elements are held together by the data structure, while streams do not hold any data since they only process elements on demand. Collections can be processed infinitely, while streams are only transverse once. Also, whereas elements are easy to access, streams do not offer a direct means of specific elements access. Collections depend on external interactions while streams are internal. Lastly, a collection is a data store, while a stream in an API is used for data processing.

11. Java 8 Has A New Date And Time API. Can You Mention Its Features?

The new date and time API in Java 8 has several features confirming that Java 8 is an upgrade. It offers time zone support and fluent methods for creating objects and arithmetic purposes. Other features include immutable classes and thread-safe, influenced by common time packages; I18N addresses issues for earlier packages and packages based on the ISO-8601 calendar system.

12. Mention Some Of The Drawbacks Of The Old Date And Time Function Of Java

Java 8 revolutionized Java when it introduced a new date and time API. The older version (Java.util.Date) is not thread-safe, while Java 8’s API is thread-safe. The old date and time were also poorly designed and did not meet the ISO standards, which is different from the new Java 8 date and time API. Regarding performance, Java 8 is miles ahead of the old data and time function. Lastly, the old data and time regime did not have classes for LocalDate, Local time, and LocalDateTime.

13. Differentiate Stream API From Collection API

There are five main differences between these two APIS. Whereas Stream API was introduced in Java 8 standard edition version, collection API has been around for a while since the introduction of Java version 1.2. Stream API works without iterators and SPL iterators, while collection API has a forEach function that allows using iterators and SPL iterators for element iteration. The former can store infinite features while the latter only stores a countable number of elements. These two APIs further differ in functions since Stream APIs are used to compute data, and collection APIs store data. Lastly, you can only perform consumption and integration in a stream object once, while collection APIs allow multiple iterations and consumption on their collection objects.

14. Differentiate A Predicate From A Function

Predicates only accept single arguments as they are pre-defined functional interfaces. They differ in functions because the predicate has a Boolean return type, and a function has a similar return type as the object. A predicate is also written as predicate <T> since it accepts a single argument, whereas functions are written as Function<T, R> but can also accept a single argument. Lastly, a predicate is a functional interface that evaluates lambda expressions and can also be used as a method reference target while a function is a functional interface used to evaluate lambda expressions.

15. Why Do You Think We Need Stream API?

There are several reasons why stream APIs are important. This new feature was added in Java 8 as a special class used to process objects from a collection and other sources. The API allows parallel operations, offers faster processing, boosts performance, supports functional-style programming, and handles aggregate operations that demystify processing.

16. Differentiate Iterators And Spliterators

There are five main differences between iterators and SPL iterators. The former was introduced in Java 1.2, while the latter came with Java 8. An iterator serves collection APIs while a spliterator services Stream APIs. Common methods used for iterators are next () and Next() for element iteration while spliterator relies on try advance(). Also, iteration only happens in a sequential order while SPL iterators work in a parallel and sequential way. Lastly, you have to call the iterator () method on a collection of objects and the spliterator() method on stream objects.

17. Mention The Main Differences Between Internal And External Iteration

Internal iteration was introduced in Java 8, while external iteration came with the previous Java 7 version. As the names imply, the former iterates internally on the aggregated objects while the latter iterates externally. Internal iteration generally works with the functional programming style, while external iteration supports the OOPS programming style. It has a passive Internal iterator while its counterpart’s iterator is active. Lastly, an internal iterator requires less coding and is more error-free than external iterators.

18. Why Do You Think Java 8 Is Considered A Revolutionary Release Of Java?

Java 8 Packs several important features that other Java versions lack, which explains why it is generally known as Spider. It is a superior Java version, given that its ability is limitless. It can write parallel code, offers developers more usable codes, comes in handy in writing databases, including promotions, and has cleaner codes than the other versions. It is also more error-free and creates better-performing applications.

19. Can You Mention All The Guidelines One Should Adhere To When Dealing With Functional Interfaces?

Five main guidelines should be followed when it comes to functional interfaces. First, the interface must be defined with only one abstract method, which means that only one abstract can be defined. One should also use the @functionalinterface annotation when defining the interface. Fortunately, there is no limitation on the method used in defining numbers. Lastly, the Java lang object class method’s override is not usually considered an abstract method.

20. Mention The Roles Of The Map () And Filter () Methods In Java 8

Java 8 has several methods that contribute to its overall efficiency. The map () function is one of the most common. It performs map functional operations in Java and can transform one object into another through a function. You can easily convert a list of strings to a list of integers if you wish using the map () function. On the other hand, the filter () method filters elements that satisfy conditions set using the predicate function. Predicate functions take an object before returning a Boolean. Thanks to this method, you can easily obtain a list of even integers from a list of integers.

21. Define Functional Interfaces In Java 8

One of the biggest features that Java 8 came with was new functional interfaces. These interfaces represent functions, as implied by the name. They, therefore, have only one abstract method form defining. To annotate a functional interface, the @functional interface is often used. However, even after annotation, a compiler must confirm that the interface is indeed functional, i.e., if it truly has one abstract method. You can only pass a lambda expression to a functional interface if a method accepts it. Examples of such interfaces include callable, comparator, runnable and comparable.

22. There Are Two Types Of Interfaces: Supplier And Consumer. Can You Define Them

The supplier interface, as the name suggests, is an interface that returns an object. Other methods that similarly return objects are new () and factory methods. This interface normally has to get () as its functional method. The latter does not take arguments or return a T-type object, meaning that it can be called at any particular time if an object is needed. Lastly, it can also be used as the assignment target for a lambda expression or method reference, given that it is also a functional interface.

On the other hand, a consumer is a functional interface in Java 8. It accepts a single argument but returns no result. However, it is quite different as it operates through side effects. Its functional method is accepted (T, t) and can also be used as an assignment target for a lambda expression.

23. What Are Some Of The Pros Of Lambdas In Java 8?

The introduction of Lambda expressions by Java 8 came with its fair share of advantages. These expressions have codes that can be easily read without interpretation, giving junior developers an easy time. You can also know the interface type used by looking at each method’s calls. Lambdas further allow developers to discover the implemented methods by looking at each call. Lastly, a developer doesn’t have to be extremely experienced with code to read it.

24. Mention The Cons Of Lambdas And How They Can Be Managed

Even though the introduction of Lambda expressions came with its advantages, it also had some shortcomings. The main shortcomings are dependency management issues and trouble using without IDE. A lack of library sources causes dependency issues. Its best remedy is to navigate through the API documentation. For lambdas without IDEs, the Ctrl+ click features will come in handy as they bring you directly to the source codes you are targeting.

25. Can You Mention Some Of The Limitations Of Java 8

 Despite being a revolutionary release, Java 8 also has its fair share of shortcomings. It allows parallelism, which is a good and beneficial feature. However, parallel streams can slow down the code if used poorly. Lambda expressions also being out dependency management issues and are difficult to use without IDEs. Other cons are distracting default methods, operator overloading, primitives, and checked exceptions.


These are some of the most common Java 8 Interview questions you should anticipate. Make sure you prepare well ahead of your interview by reviewing all the technical aspects of this Java release. Stay confident and lively throughout the interview to increase your chances. We wish you all the best!

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