35+ Java 8 Interview Questions and Answers (2024)

Be it a techie or any other professional, interviews always kick the adrenaline out of nervousness. However, to stand out from the crowd, it’s important to polish your skills and bet on your abilities. Moreover, having hands on the frequently asked Java 8 interview questions and answers can be your savvier.

Therefore, dig deep into these java 8 interview questions for experienced and beginners to ace your upcoming interviews.

Java 8 Interview Questions for Freshers and Experience

1) Java 8 falls in which programming paradigm?

Java 8 falls in the following programming paradigm:

  • Object or class-oriented programming language.
  • Functional programming language.
  • Specified procedural programming language.
  • Logical programming language.

2) Why should one use the Optional class?

Optional class is a newly introduced container class that is useful in demonstrating the existing or non-existing optional values. Its major advantage is to avoid null & void checks and facilitates methods useful in checking the existence of a value for a particular variable.


Java 8 Interview Questions

3) Name a few recent features introduced in Java 8.

Below are the most recent features which is introduced in Java 8:

  • Lambda Expressions: It is a Java function that you can share or refer to as an object.
  • Method interference: It uses function as a criterion to implement a method.
  • Functional Interference: Every functional interference is associated with a single abstract method which is known as the functional method.
  • Default Method: It is useful in implementing methods in the interfaces that help enable the ‘interface evolution’ potential.
  • Date Time API: It is an improved yet inspired version of java time APIs to deal with the drawbacks of the last version.
  • Stream API is referred to as the abstract layer, which helps to pipeline the processing data.
  • Optional: The wrapper class is useful in checking the null values and processing the further data.
  • JavaScript and Nashorn Engine: It is the improved version of the JavaScript Engine, which is useful in enabling its functionality in Java, replacing Rhino.

4) Specify the advantages of using java 8

Here are some benefits of using Java 8:

  • It helps in creating applications much faster and in an easier way.
  • It provides a very stable ambient for the developers.
  • Concise, reusable, and easy-to-comprehend codes.
  • Improved and effective support.
  • Easy to port across various operating systems.
  • Minimum boilerplate codes.

5) What is a collection, and how is it different from a stream?

A collection is an in-memory database that records all the values according to the current data structure. So, before adding you add it to the collection, it’s important to compute each of them. Whereas a stream is a visually fixed data structure where we can compute the elements according to our needs.


6) What is the Lambada expression?

Lambada expression is a kind of function that has no name and is sometimes referred to as the anonymous function because it doesn’t have its own type of information. You can use it as per your need, and it is very useful in repeating, filtering, and extracting any data from the collection.

Example:

import java. util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class LambdaExpressionExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Dave");
        List<String> filteredNames = names.stream()
                .filter(name -> name.startsWith("C"))
                .map(name -> name.toUpperCase())
                .collect(Collectors.toList());
        System.out.println(filteredNames);
    }
}

Output:

Charlie

7) Define stream pipelining

Stream pipelining is a method of grouping different operations altogether. You can achieve this by categorizing its stream operations into two parts. Intermediate and terminal operations.

Whenever an intermediate operation is performed, it bounces back an instance to the stream pipeline. Therefore, any user can set up an irrational number of intermediate operations to streamline data and ultimately form a processing pipeline.

Moreover, to terminate the pipelines, it’s necessary to have a terminal operation at the end of the process.


8) What are predicate and consumer in Java 8?

A predicate is a functional interface that typically receives arguments and retrieves a Boolean value. You can use it to apply filter for a collection of objects.

On the other hand, the consumer is referred to as an in-build functional interface found in Java.util.function package. You can use it to consume any object, and it takes the input value and gives out nothing.


9) Why is the peek () method used in Java 8?

The peek() method helps support debugging, where one wants to notice the elements as they tend to flow from a specific point in a pipeline. It is a representation of our observation of how each element passes.

Syntax: public Object peek()


10) What makes you the ideal candidate for the Java developer role?

Here are some important qualities that can make you an ideal candidate for the Java developer role:

  • You should have the ability to write clear, easy to comprehend, and efficient codes.
  • A deep understanding of the patterns and concepts of “OOP or object-oriented programming.”
  • A good knowledge of various architectural and design patterns.
  • Familiarity with Java concurrency patterns.
  • Experience and knowledge of working with various web application frameworks such as Spark and Play.
  • Fundamental knowledge of Java Virtual Machine or JVM, its disadvantages, workaround, etc.
  • A good experience in using embedded and external databases.

11) Explain the ‘Functional Interface’.

An interface with only one abstract is called a functional interface. It is also known as the Single Abstract Method or SAM. However, the annotation @FunctionalInterface is optional and can be used even if it is not there. It extends to the other interface if it doesn’t have any abstract.


12) Why are Static methods needed in Java 8?

Static method reference is referred to as a utility or helper method, which is linked with an interface. It is also a class that is not linked to any object.

Here are reasons for using the Static method in Java:

  • Useful for maintaining and extending the API.
  • Entails a clear and concise share of responsibilities, and you don’t need to have one utility class to have the utility collections.
  • Useful for keeping all the interfaces in a common interface rather than specific interfaces.

13) What are the advantages of Nashorn?

Nashorn is the most advanced and new JavaScript processing engine introduced in Java 8. Earlier, it was based on Mozilla Rhino. It provides enhanced compliance with ECMA JavaScript. It is faster than its previous versions.


14) How are Collection API and Stream API different?

Here are some important differences between Collection API and Stream API

Collection API Stream API
Collection API was first used in Java 1.2 Stream API was first introduced in Java SE 8.
You can use it to store unlimited data. You can use it to compute unlimited data.
Collection API allows you to use Iterator and Spilterator for iterating the data. Stream API allows you to use Iterator and Spilterator for iterating the data.
Collection API is easy to access. In-stream API, there is no direct way to reach specific elements.
It also gives ease to process it according to your needs. Stream API can be processed just once.

15) Describe PermGenSpace and MetaSpace.

PermGen or Permanent Generation Space is a unique type of heap space used for keeping track of stored metadata which is now removed in Java 8.

With this new version, the storage of the metadata will be done in a similar memory known as “MetaSpace.” It is not a transferable Java Heap memory and is an improved version of PermGen in aspects of auto-tuning, garbage collection, etc.


16) How is Functional Interface created in Java 8?

You can employ lambda expression to implement the abstract method of the functional interface in Java 8. Below is a coding example of the same:

Example

import java.util.function.Consumer;
public class FunctionalInterfaceExample {
    public static void main(String[] args) {
        Consumer<String> printer = System.out::println;
        printer.accept("Hello, world!");
    }
}

Output:

Hello, world!

17) What are the differences and similarities between Function and Predicate in Java 8?

Here are the differences between Function and Predicate in Java 8:

  • Function is useful in transformation, whereas predicate can be used to test any condition.
  • Function can receive an argument and give out a result, whereas predicate can also receive an argument but gives out only the Boolean value.
  • Function is denoted by <T, R>, and Predicate is denoted by only <R>, where T is the function type, and R is the result.

Below are the similarities between Function and Predicate:

  • Both function and predicate are functional interfaces.
  • Both function and predicate are single argument functions.

18) Explain the difference between skip(long) and limit(long) using an example.

The skip (long) is an intermediate operation that retrieves the leftover elements after eliminating the initial n elements of a specific stream.

Example:

import java.util.Arrays;
import java.util.List;
public class SkipExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Dave", "Eve");
        // Create a stream from the names list
        names.stream()
                // Skip the first two elements of the stream
                .skip(2)
                // Print the remaining elements to the console
                .forEach(System.out::println);
    }
}

Output:

Charlie
Dave
Eve

The limit (long) retrieves the stream of elements of a particular size

Example:

import java.util.Arrays;
import java.util.List;
public class LimitExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Dave", "Eve");
        // Create a stream from the names list
        names.stream()
                // Limit the stream to the first three elements
                .limit(3)
                // Print the elements to the console
                .forEach(System.out::println);
    }
}

Output:

AliceBob
Charlie

19) Which situation is most suitable for using stream API in Java 8?

You can use the stream API in Java 8. There’s a need for the below operations:

  • You can carry-out parallel processing.
  • For executing lazily operations
  • To perform database operations
  • Use for internal iterations.
  • For writing functional-style programming
  • You can use for using pipeline operations.

20) What is the function of Type Inference?

Type Inference assists the compiler in identifying or recognizing the types of arguments by just having an overview of the corresponding declaration and method invocation.


21) Which is the easiest way to identify and eliminate duplicate elements from a list in Java 8?

The easiest way to identify and remove the duplicate elements is by using the stream operation and carrying out a collection, followed by the use of the Collections.toSet() method.


22) What are the guidelines for Functional Interface?

Below are the guidelines for a functional interface which is amongst the most asked Java 8 programming questions:

  • It should have only one abstract method.
  • You can have as many as a number of default methods as it is not abstract, and implementation is given earlier by the same.
  • An abstract method, the main element of public methods from Java.lang, is known in a functional interface. Object is still regarded as a useful interface.

Note: The explanation for this is that each implementation class for this interface can have an implementation for this abstract method that is either defined by the implementation class itself or taken from a superclass.


23) Differentiate between spliterator and Iterator.

Here are some differences between spliterator and iterator:

Spliterator Iterator
It was introduced in Java 1.2. It was introduced in Java 1.8.
It only iterates all the elements separately. It transverse all the elements separately as well as in bulk.
It acts as an iterator for a complete collection of APIs. It acts as an iterator for both stream and collection API.
It will not support parallel programming. It does support parallel programming.

24) How can you call a default method of an interface in a class?

The default keyword, which is present at the start of the method signature, is used to designate a method declaration in an interface as a default method. You can skip the public modifier because every method declaration in an interface, including default methods, is automatically public.


25) Define the default method. When can you use it?

Default method is present in the interface, and it includes an implementation. This method is helpful in adding new functionality to a given interface whilst keeping the backward compatibility along with its classes that are already implemented in an interface.


26) How are Streams created in Java, and is it amongst the most asked java 8 stream interview questions?

Streams in Java describe the sequence of objects from a specific source, like a collection, that will support aggregate operations. You can utilize it to collect, filter, print, and change one data structure to other.

Here are the examples of how to create streams in Java:

import java.util.stream.Stream;
public class StreamExample {
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("Alice", "Bob", "Charlie", "Dave", "Eve");
        stream.forEach(System.out::println);
    }
}

Output:

Alice
Bob
Charlie
Dave
Eve

27) What are Map and FlatMap stream operations?

Map and FlatMap are regarded as the stream and intermediate stream operations which accepts a function. It also helps in applying the given function to the rest of the elements.


28) Why do we need to alter the Java programming language frequently?

We have to alter the Java programming language frequently due to the below reasons:

  • You can use the features of Functional programming.
  • To utilize the latest technology of Multi-Core CPUs effectively.

29) Mention a few functional interfaces used in Java 8.

Below are a few functional interfaces used in Java 8:

  • BinaryOperator
  • Consumer
  • Function
  • Supplier
  • UnaryOperator
  • BiFunction
  • Predicate

30) What are the main API classes for time and date?

Listed below are the the main API classes for time and date that are used in Java 8:

  • LocalTime
  • LocalDateTime
  • LocalDate

31) Give the basic structure/syntax of Lambda expression.

The basic structure/syntax of Lambda expression is:

(arugument - list) - > {body}

Here,

  • Argument – list: You can keep it filled or empty.
  • Arrow token: It is useful in linking body expressions and arguments – list.
  • Body: It consists of statements and expressions for the lambda expression.

32) What are the main characteristics of Stream?

Below are the main characteristics of Stream:

  • Pipelining
  • Source
  • Aggregate Operations
  • Sequence of Elements
  • Automatic Iterations

33) What are the most common Intermediate operations?

Some of the most common types of intermediate expressions are:

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

34) What are the most common types of Terminal operations?

Below are the most common types of terminal operations:

  • Count
  • Reduce
  • Min
  • AllMatch
  • Max
  • AnyMatch
  • To Array

35) What is method reference in Java 8?

Method reference in Java 8 is a way of referring to the functional interface. It is known as a much easier and more efficient form of lambda expression. When you use lambda expressions to refer a method, you can replace it with the method reference.


36) How are Internal and External iterations different?

Below are the differences between internal and external iteration:

Internal Iteration External Iteration
It is also known as the passive or implicit iterator. It is also known as the active or explicit iterator.
It is carried internally on various objects. It is carried externally on various objects.
It is utilized in functional programming. It is utilized in object-oriented programming.
It accounts for very less errors. However, coding is a must for it. It accounts for more errors. However, coding is not necessary.

37) Has type inference been used in previous versions of Java?

Java introduced type interference in Java 5 but with very limited access. But over the years, the possibilities of type interference have increased, and it is at its best in Java 8.


38) Write down a Java 8 program that can find a Stream’s minimum and maximum number.

import java.util.Arrays;
import java.util.stream.IntStream;
public class MinMaxExample {
    public static void main(String[] args) {
        int[] numbers = {9, 3, 8, 1, 5, 7, 2, 6, 4};
        int min = IntStream.of(numbers).min().getAsInt();
        int max = IntStream.of(numbers).max().getAsInt();
        System.out.println("Minimum number: " + min);
        System.out.println("Maximum number: " + max);
    }
}

Output:

Minimum number: 1
Maximum number: 9

39) How does one join multiple Strings through StringJoiner Class?

StringJoiner helps in joining one or more String, and it can be done through a specific joiner or delimiter. For instance, one can join a number of strings by separating them with a comma (,).

For creating a CSV string, one can generate a full path for a particular directory in Linux which can be done by joining the String with the help of the forward slash “/.”


40) What is the best alternative to lambda expressions in Java 8?

In general, we utilize lambda expressions to create anonymous methods that help get desired output. However, there are instances where lambda expressions fail to work and use an existing method only.

So, to avoid this, you can use method reference that can help in getting the desired output. The method reference is depicted using the: (double colon) symbol.