Originally known as Oak, Java emerged in early 1996 with its major version as Java 1 or JDK 1.0. James Gosling designed and developed Java first at Sun Microsystems. With the introduction of new versions, Java has changed a lot (over time). Each version of Java brings us new features. Currently, Oracle Corporation manages the Java open-source project.

Java 8, also known as JDK 8.0, is a significant release of Java in 2014. The codename for it is Spider. In this article, all the questions are specific to Java 8. Throughout this article, we will walk you through the Java 8 interview questions for freshers and experienced professionals, as well as the opportunity to apply.

Learn Java to develop several applications such as enterprise applications, network applications, desktop applications, web applications, games, android app, and more with the best Java tutorials for beginners in 2021.

Java 8 Interview Questions and Answers

This article includes a comprehensive list of frequently-asked Java 8 interview questions to help you ace your interview round. The interview questions in this article are both technical and are dependent on your learning and experience.

1. What new features have been added to Java 8?

Answer: Java 8 has the following new features:

• Lambda Expressions
• Method References
• Optional Class
• Functional Interface
• Default methods
• Nashorn, JavaScript Engine
• Stream API
• Date API

2. What makes you the ideal candidate for this Java Developer position?

Answer: In a Java 8 interview, this is a frequent question. Here, the interviewer wants to know how proficient you are with Java, and how you plan to use it for your career path. Answering this question in light of your interest in Java programming and how you plan to use it when you join the firm would be advantageous.

3. Have you ever worked in a production environment involving Java?

Answer: You will likely encounter this question if you are an experienced developer. Make sure to answer it honestly and to the best of your abilities regarding your experience with Java development in a production environment.

4. Have you ever worked in the same industry as us?

Answer: Depending on the type of company you have applied for, you will be asked this question. Many industries use Java 8 to meet their specific needs. By knowing what your job description and responsibilities will be, you'll be able to answer this question, along with mentioning any previous experience you have had in this field. When coming from another industry, be sure to explain how you plan to adopt and bring your best to the table.

5. What is a lambda expression?

Answer: Lambda expression is a type of function with no name. It may or may not contain results and parameters. As it does not have any identifiable information by itself, it is known as an anonymous function. Execution is on-demand. It helps iterate, filter, and extract data from a collection.

Lambda expression was added to Java 8 to allow users with an anonymous function they can call without using the function name but has a functional set of parameters with a lambda entity.

6. What are default methods?

Answer: Default methods are the methods of the Interface that have a body. As the name suggests, these methods use the default keywords. Default methods are used as a "backward compatibility" feature, which means if JDK modifies any Interface (without default method), then the classes which implement this Interface will break.

In contrast, if you add default methods to Interfaces then the default implementation will be available. It will not affect the implementing classes.

Syntax:

public interface questions{

        default void print() {

System.out.println("www.softwaretestinghelp.com");
             }
    }

7. What is an optional class?

Answer: Java 8 introduced a special wrapper class to prevent NullPointerExceptions, known as the Optional class. It is present under the package java.util. We encounter NullPointerExceptions when we fail to perform the NULL check.

8. What are Functional Interfaces?

Answer: An interface that has only one abstract method is called a functional interface. This functionality is implemented using Lambda Expressions, so to use them, you must create a new functional interface, or you can use the predefined functional interface provided by Java 8. To create a functional interface, one uses the annotation: @FunctionalInterface

9. What was wrong with the old-time and date?

Answer: Here are the disadvantages of the old date and time system:

• Date and Time API in Java 8 are thread-safe, whereas Java.util.Date is mutable and not thread-safe.
• Java 8 Date and Time API meets ISO standards, whereas the old date and time API failed to meet ISO standards.
• The new Java 8 introduces several API classes for dates, such as LocalDate, LocalTime, LocalDateTime, etc.
• On the subject of performance, Java 8 is faster than the old regime of date and time.

10. What are the main characteristics of the Lambda Function?

Answer: Lambda functions have the following characteristics:

• Lambda Expressions can be passed as parameters to other methods.
• Methods do not need to be a part of a class to exist.
• The type of a parameter does not need to be declared since the compiler can get it from its value.
• There is no need for parenthesis when using a single parameter, but we can use parentheses when using multiple parameters.
• There is no need to include curly braces if the body of the expression contains just one statement.

11. How will you get the current date and time using Java 8 Date and Time API?

Answer: Using Java 8's new API, the following program is written. For getting the current date and time, we used the LocalDate, LocalTime, and LocalDateTime APIs. Furthermore, we have retrieved the current date and time from the system clock in the first and second print statements. Using the third statement, we use the LocalDateTime API, which prints both the date and the time.

class Java8 {
    public static void main(String[] args) {
        System.out.println("Current Local Date: " + java.time.LocalDate.now());
        //Used LocalDate API to get the date
        System.out.println("Current Local Time: " + java.time.LocalTime.now());
        //Used LocalTime API to get the time
        System.out.println("Current Local Date and Time: " + java.time.LocalDateTime.now());
        //Used LocalDateTime API to get both date and time
    }
}

12. Explain the following Syntax.

String:: Valueof Expression

Answer: The static method refers to the ValueOf method of the String class. System.out::println is a static method reference to the println method of the out object in the System class.

Upon passing an argument, it returns the appropriate string representation. You can use Character, Integer, Boolean, and so on as arguments.

13. What is a Stream API? Why do we need the Stream API?

Answer: Java 8 introduced a new feature called Stream API. It is a special class used for processing objects from a source like Collection.

• By supporting aggregate functions, it simplifies the processing.
• Functional-style programming is supported.
• It performs faster processing. Therefore, it is suitable for better performance.
• Parallel operations are possible.

14. What is Method Reference?

Answer: Java 8 introduced a new feature called Method Reference. It refers to the method of functional interface. When referring to a method, it can be used to replace a lambda expression.

Syntax: class::methodName

15. What is a SAM Interface?

Answer: As of Java 8, FunctionalInterfaces have only one abstract method. As these Interfaces specify only one abstract method, they are sometimes referred to as SAM Interfaces. This interface is known as a Single Abstract Method (SAM).

16. How can you create a Functional Interface?

Answer: It is possible to define a functional interface in Java using the annotation

@FunctionalInterface

As soon as you define the functional interface, you can only have one abstract method. The fact that there is only one abstract method lets you write multiple static methods and default methods.

17. What is the difference between the Collection API and Stream API?

Answer: Below is a table explaining the difference between the Stream API and Collection API:

18. Is there anything wrong with the following code? Will it compile or give any specific error?

@FunctionalInterface
public interface Test<A, B, C> {
    public C apply(A a, B b);

    default void printString() {
        System.out.println("softwaretestinghelp");
    }
}

Answer: Yes. Since only one abstract method is defined in the functional interface specification, the code will compile. As for printString(), it is a standard method that does not count as an abstract method.

19. What is the difference between limit and skip?

Answer: The limit() method returns a Stream of the specified size. To give an example, if you specify limit(5), the number of output elements should be 5. On the other hand, the skip() method skips the element.

20. Do you have any certifications to boost your candidature for this Java Developer role?

Answer: A certification from a reputable organization will enhance your resume in addition to providing you with comprehensive knowledge of the technology you wish to master. You will have the opportunity to work on industry-level projects if you earn such a certification, which will assist you in proving to the interviewer that you have invested enough time and effort to master the technology. Your career growth in this technology will also benefit from your certification. The knowledge and learning you gain will give you a significant advantage during the interview, and the competition.

21. What is MetaSpace? How does it differ from PermGen?

PermGen: Before Java 8, classes' meta-data was stored in the PremGen (Permanent-Generation) memory type. PermGen has a fixed size and cannot be dynamically resized. It was a contiguous Java Heap Memory.

MetaSpace: In Java 8, classes' MetaData is stored in native memory called 'MetaSpace'. There are no contiguous heaps in this memory, and therefore it can be grown dynamically, overcoming the size constraints. As a result, garbage collection, automatic tuning, and de-allocating of metadata are improved.

22. Can a functional interface extend/inherit another interface?

Answer: It is not allowed for a functional interface to extend another interface with abstract methods since this would void the rule for one abstract method per functional interface. For example:

interface Parent { 
public int parentMethod(); 
} 
@FunctionalInterface // This cannot be FunctionalInterface 
interface Child extends Parent { 
public int childMethod(); 
// It will also extend the abstract method of the Parent Interface 
// Hence it will have more than one abstract method 
// And will give a compiler error 
}

In addition, it can extend other interfaces without abstract methods and only have the default, static, another class is overridden, and normal methods. As an example:

interface Parent { 
public void parentMethod(){ 
System.out.println("Hello"); 
} 
} 
@FunctionalInterface 
interface Child extends Parent { 
public int childMethod(); 
}

23. What are the various categories of pre-defined function interfaces?

Answer: The various categories of pre-defined function interfaces are:

• Function: Converts arguments into returnable values.

• Predicate: Performs a test and returns a Boolean value.

• Consumer: Accepts arguments but does not return any values.

• Supplier: Do not accept any arguments, just returns a value.

• Operator: Perform a reduction operation with the same input types as input.

24. What are some standard Java pre-defined functional interfaces?

Answer: Runnable, Callable, Comparator and Comparable are some of the most famous pre-defined functional interfaces from previous Java versions. Meanwhile, Java 8 offers functional interfaces such as Supplier, Consumer, Predicate, etc. These and other predefined functional interfaces are described in the java.util.function documentation.

• Runnable: executes the instances of a class over another thread with no arguments and no return value.
• Callable: used to execute instances of a class over another thread without arguments, and it either returns a value or throws an exception.
• Comparator: sorts objects in a user-defined order.
• Comparable: sort objects according to their natural order.

25. How does a lambda expression relate to a functional interface?

Answer: Due to the similarity between lambda expressions and anonymous functions, they can be used only with the single abstract method of Functional Interface. The signature of an abstract method of a functional interface allows one to infer the return type, type, and several arguments.

26. What are the features of a lambda expression?

Answer: Two significant characteristics of the lambda expressions are listed below:

• You can pass lambda expressions to another method as parameters.
• Without belonging to any class, lambda expressions can be standalone.

27. What is a type interface?

Answer: Even earlier versions of Java provide a type interface. During compilation, the compiler looks at the method invocation and the corresponding declaration to determine the type of argument. This process is called type interface.

28. What are some of the examples of terminal operations in Java 8?

Answer: Java 8 offers the following examples of terminal operations:

• AnyMatch
• AllMatch
• Count
• Max
• Min
• Reduce
• ToArray

29. Can you give examples of intermediate operations in Java 8?

Answer: The following examples are widely used in intermediate operations:

• Distinct()
• Filter(Predicate)
• Limit(long n)
• Map(Function)
• skip(long n)

30. What are the similarities between map and flatMap stream operations in Java 8?

Answer: Map and flatMap are both intermediate stream operations that accept a function as input and use the function as input for various activities in the stream.

31. In Java 8, what is a Spliterator?

Answer: Spliterator is a new Java 8 iterator interface. Throughout the runtime environment, it handles API-related operations smoothly.

32. Can JavaScript code be executed from the Java 8 codebase?

Answer: With the use of ScriptEngineManager, JavaScript code is executed seamlessly through the codebase. Java 8 interprets code with this.

33. How is a Base64 decoder created in Java 8?

Answer: A Base64.Decoder can be returned using the getDecoder() method in the Base64 class. Decoding takes place using the Base64 encoding scheme.

34. Which class implements the encoder used for encoding byte data in Java 8?

Answer: Java 8 implements a static class Base64.Encoder for encoding byte data using the Base64 encoding scheme.

35. How is a Base64 encoder that encodes URLs created in Java 8?

Answer: URLs are encoded using the getUrlEncoder() method of the Base64 class.

36. What is a supplier in Java 8?

Answer: A supplier is a mere functional interface in Java 8 that does not take in any argument. Lambda expressions use it as an assignment target. The following is an example of how a supplier is used:

import java.util.function.Supplier;
public class Java8SupplierExample {
public static void main(String[] args) {
Supplier<String> supplier= ()-> "HelloLearners";
System.out.println(supplier.get());
}
}

37. Differentiate between findFirst() and findAny() in the Stream API of Java 8.

Answer:

38. When is an ideal situation to use the Stream API in Java 8?

Answer: You can use the Stream API in Java 8 if your Java project entails any of the following operations:

• Execute operations lazily.
• Perform parallel processing.
• Use pipeline operations
• Use internal iteration
• Perform database operations
• Write functional-style programming

39. What are the common types of functional interfaces in the standard library?

Answer: The standard library contains several functional interface types, including the following:

• BiFuction
• BinaryOperator
• UnaryOperator
• Predicate
• Consumer
• Supplier

40. What is the easiest way to find and remove duplicate elements from a list using Java 8?

Answer: By applying stream operations and later performing a collection using the Collections.toSet() method, duplicate elements can be identified and removed easily. It will remove all redundant elements from the list.

41. What is the syntax of a predicate interface in Java 8?

Answer: In Java, a predicate is a functional interface that takes in an object and returns a boolean value. The syntax of a predicate function is as follows:

public boolean test(T object){

return boolean;

}

42. What is a consumer in Java 8?

Answer: In Java 8, a consumer is a single-argument functional interface just like a predicate. Consumers, however, don't return values and can be used as lambda expressions. Here is an example of using the consumer interface to print a string:

import java.util.function.Consumer;
public class Java8ConsumerExample {
public static void main(String[] args) {
Consumer<String> consumerString=s->System.out.println(s);
consumerString.accept("HelloWorld");
}
}

43. Define Nashorn in Java 8

Answer: Nashorn is a JavaScript processing engine included with Java 8. As a result, it offers better compatibility with ECMA (European Computer Manufacturers Association) JavaScript specifications and improves performance over the older versions.

44. What are the features of the new Date and Time API in Java 8?

Answer:

• Timezone support
• Immutable classes and Thread-safe.
• Addresses I18N issue for earlier APIs.
• All packages are based on the ISO-8601 calendar system.
• Fluent methods for object creation and arithmetic
• Influenced by popular joda-time package.

45. What is the use of JJS in Java 8?

Answer: JJS is a Java 8 command-line tool for executing JavaScript code in the console. The following are examples of CLI commands:

JAVA>jjs
jjs> print("Hello, Java 8 - I am the new JJS!")
Hello, Java 8 - I am the new JJS!
jjs> quit()
>>

46. What is the stateful intermediate operation? Give some examples of stateful intermediate operations.

Answer: In order to complete some of the intermediate operations, some state has to be maintained, and such intermediate operations are called stateful intermediate operations. This type of parallel execution is complex. For example: sorted(), distinct(), limit(), skip() etc.

Data elements are not sent to further steps in the pipeline until all data is sorted for sorted() and stream data elements are stored in temporary data structures.

47. Write a program to print 5 random numbers in sorted order using forEach in Java 8?

Answer: In Java 8, the following program generates five random numbers using forEach. Depending on how many random numbers you want to generate, you can set the limit variable to any number. All you need to add here is the sorted() method.

import java.util.Random;

class Java8 {

    public static void main(String[] args) {

        Random random = new Random();
        random.ints().limit(5).sorted().forEach(System.out::println);
        /* sorted() method is used to sort the output after
         terminal operation forEach
         */

    }
}

48. What is ChronoUnits in Java 8?

Answer: ChronoUnits is an enum that replaces the integer values used by the old API to represent the month, day, etc.

49. What are the sources of data objects a Stream can process?

Answer: Streams can process the following data:

• A collection of an Array.
• An input device or an I/O channel.
• A stream generator function or a static factory.
• A reactive source (e.g., comments in social media or tweets/re-tweets)

50. What is the difference between Intermediate and Terminal Operations in Stream?

Answer: There are two types of Stream operations: Intermediate and Terminal. An intermediate operation returns the Stream so that other operations can be performed on it. In intermediate operations, the Stream is not processed at the call site, hence the name lazy.

When a Terminal operation is carried out, these operations (Intermediate Operations) process data. Examples: map and filter.

On the other hand, Terminal Operations start Stream processing. All Intermediate operations are performed on the Stream during this call. Examples: sum, Collect, and forEach.

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