Java 8 interview questions and answers

Java is one of the top programming languages in the world today. Every developer knows that Java keeps getting new features and updates at regular intervals. Java 8 SE is among the latest releases in the programming language. It was released in March 2014 and was designed to overcome the restrictions and drawbacks of all previous versions while adding new characteristics. Its primary objective was to implement functional programming, create a new search engine for JavaScript, deploy a new API for streaming and manipulate the date-time using new APIs.

If you are applying for a job as a developer or programmer working with Java, you need to understand all the changes that come with Java 8 SE to answer all the important Java 8 interview questions. With the competition getting more cutthroat every day, you need to understand how to stay ahead of your peers and other job applicants. Preparing for an interview beforehand is a good way to do that.

Related: Popular Java Interview Questions and Answers that you should check out

Java 8 Interview Questions and Answers for Fresher Level: 

Q1. Who created Java?

Java was created by Oracle Corporation. The latest Java 8 SE version comes with new features like a new JavaScript search engine, functional programming and more. 

Q2. What is a lambda expression?

It is an anonymous function that can accept various input parameter sets and provide different types of output. 

Q3. Why are lambda expressions called blocks of code?

A lambda expression doesn’t have a name and can exist with or without results or parameters. Thus, it qualifies as a block of code. 

Q4. What are the different components of a lambda expression?

A lambda expression has three parts:

  • Parameter list

  • Lambda Body Expression

  • Lambda Arrow Operator

Q5. How are a parameter list and lambda arrow operator different from each other?

A lambda expression can exist with zero or multiple parameters at the same time. A lambda arrow operator helps separate each parameter from the body and list using the ‘ ->’ icon.

Q6. What is the difference between the SAM interface and the functional interface?

There are no differences between the SAM interface and the functional interface. The Single Abstract Method interface or SAM interface is one type of functional interface according to the definitions in the Java 8 SE API.

Q7. Explain the guidelines needed for a functional interface.

  • The interface must be defined using only one abstract method.

  • Don’t define more than one abstract.

  • Use the @functionalInterface annotation while defining the interface.

  • Overriding the Java.lang.object class method is not considered an abstract method.

  • You can use any method to define a number. 

Q8. What is a functional interface?

A functional interface can be defined as an interface with a single abstract method only. Implementing these interfaces requires using a Lambda expression. In other words, developers must create new functional interfaces to use a lambda expression or use the predefined Java 8 functional interface. The @FunctionalInterface annotation is used to create a new functional interface. 

Q9. What is an optional class?

An optional class is a special wrapper class that came with the Java 8 SE version. It helps avoid NullPointerExceptions. The final class is given under the java.util package. A NullPointerException occurs when developers don’t perform Null checks. 

Q10. What are the types of default methods?

A default method refers to any interface method with a body. As the name says, default methods use all default keywords. They are used for backward compatibility, which means that if JDK modifies an interface without using a default method, the class implementing this interface is going to break. Meanwhile, if you add a default method to any interface, you can provide and execute a default implementation without affecting any implementation class. 

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Java 8 Questions for Intermediate Level:

Q1. What is Type Interference?

Type Interference helps a compiler to determine an argument’s type by understanding every method invocation and the corresponding declaration. 

Q2. What are some Java 8 Date-Time APIs?

The core date-time API classes are:

  • LocalDate

  • LocalTime

  • LocalDateTime

Q3. Why does an interface require default methods?

A default method allows a developer to add new functionalities to library interfaces and ensures binary compatibility with code that was previously written for interfaces. 

Q4. What is the Java 8 StringJoiner class used for?

The Java 8 StringJoiner class creates a character sequence that is separated using a delimiter. This allows users to pass delimiters like commas and hyphens to create a string.

Q5. What are the more common functional interfaces you can find in a standard library?

  • Function: It takes an argument and gives back a result.

  • Consumer: It takes an argument, but gives no result back.

  • Supplier: It takes a not argument and gives a result.

  • Predicate: It takes an argument and gives back a boolean.

  • BiFunction: It takes two arguments and gives a result back.

  • BinaryOperator: This is similar to BiFunction, with the exception that the results that come back are all the same type.

  • UnaryOperator: It is like a function, except that it takes an argument and returns results of the same type.

Q6. What is a stream? How is it different from a collection?

A stream is an iterator with the primary function of accepting a set of actions that it applies to every element it contains. It represents an object sequence within a collection or any other source supporting aggregate operations. Unlike a collection, iteration logic implements within a stream. Streams are also inherently lazy processed and loaded.

Q7. What is a default method? When should you use it?

The default method requires implementation and is found within the interface. It adds several new functionalities to an interface and preserves backward compatibility with classes already implementing this interface. 

Q8. What is jjs in Java 8?

Jjs is a new command-line tool that is used for executing JavaScript code at the console. 

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Java 8 Interview Questions for 8 to 12 Years Experienced:

Q1. What do you know about Nashorn? What are its advantages?

Nashorn is a JavaScript processing engine. It is one of the newest features added to the Java 8 SE API. Before this, Java used Mozilla Rhino as its engine. But Nashorn provides improved compliance with JavaScript specifications that are ECMA-normalised. It also offers faster run-time and better performance than Rhino.

Q2. What is stream pipelining?

Stream pipelining refers to the process in which various operations are chained together. Pipelining helps achieve this by dividing operations into terminal operations and intermediate operations. When every intermediate operation is running, it gives back an instance from the stream. Therefore, a user can set up any number of intermediate data processing operations. This results in the formation of a processing pipeline. By the end of a stream pipelining process, you must have terminal operations that return the desired final value and terminate the pipeline. 

Q3. How can you get the largest number to exist on a list?

You can do so by using the code segment given below:

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

IntSummaryStatistics stats = integers.stream().mapToInt((x) −> x).summaryStatistics();

System.out.println("Lowest number in List : " + stats.getMin());

Q4. What are some of the standard predefined functional interfaces in Java?

Some of the most popular predefined functional interfaces in Java are Callable, Comparable, Comparator and Runnable. Java 8 has introduced new interfaces like Predicate, Consumer and Supplier. The java.util.function doc contains all the predefined functional interfaces and the descriptions newly introduced.

  • Runnable: It is used to execute instances in a class over another thread without a return value or argument. 

  • Callable: It is used for executing instances of a class over a thread with no argument. It will either throw up a value or an exception.

  • Comparator: It is used to sort various objects in an order defined by the user.

  • Comparable: It is used to sort objects within the natural sorting order. 

Q5. What are the most common intermediate operations?

  • Filter(Predicate<T>) - It allows selective processing of Stream elements and returns elements that satisfy the supplied predicate condition.

  • map(Funtion<T, R>) - It returns a new Stream and transforms every element by applying the supplied mapper function.

  • = sorted() - It sorts out the input elements and helps them reach the next stage.

  • distinct() - It only passes on elements to the next stage that haven’t been passed yet.

  • limit(long maxsize) - It limits the size of the stream to maxsize.

  • skip(long start) - It skips the initial elements till the start.

  • peek(Consumer) - It applies a consumer without any modification to the stream.

  • flatMap(mapper) - It transforms every element into a stream of its constituent elements and flattens all the streams into a single stream.

Q6. What is the most common type of terminal operation?

Terminal operations are divided into four categories.

  • collect(): This operation gathers a single result from every element in the stream sequence.

  • reduce(): This operation produces a single result from every element in the stream sequence.

    • count()

    • min()

    • max()

  • Search/query operations

    • anyMatch(), noneMatch(), allMatch() are all short-circuiting operations

    • This operation takes a predicate as an input for the match condition

    • It stops stream processing as and when the result is found.

  • Iterative operations

    • forEach() is used to operate on every Stream element and accepts a consumer.

    • forEachOrdered() helps in maintaining order in parallel streams.

Q7. What is a Spliterator?

Spliterator is a portmanteau that combines splittable and iterator. It is another new feature provided in Java 8 SE. Its function is primarily in the Stream API for iterating streams in sequential or parallel order through internal iteration.

Q8. What is the difference between function and predicate?

Both function and predicate are functional interfaces. Predicate<T> is a single-argument function that comes back as either true or false. Function <T,R> also represents a single argument function, but it returns an object instead of true or false. The T in this function represents the input while R denotes the result. 

These questions are not the only questions you will face in your interview. There is also no guarantee that you will face these exact questions. However, understanding these questions and the topics they are related to can teach you how to frame your answers and face various questions more confidently. Several recruiters recount how they have seen candidates get rejected even though they know the answer, just because they don’t frame it right. 

Java is one of the world’s leading programming languages, second-highest in both PYPL and TIOBE language rankings. It is used by global leaders like LinkedIn, Twitter, PayPal and Amazon, among others. There are more than 1,751,600 questions about Java you can find on StackOverflow, besides 123,775 Java public repositories on GitHub which are continuously growing. With Java 8 being one of the most stable Java versions, the number of opportunities you have in this domain is immense. If you are considering all-around development before your interview, consider talking to the experts at Koenig today.

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Armin Vans
Sheyans is a Android Certified Developer and certified professional with rich experience in Application Development, Programming, and Corporate training. He delivered training on CEH, CPENT, ECSA, Security+, CNDV2, CISSP, CISM, Android, iOS, Java, Objective C, Swift, and Xamarin Mobile Framework.

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