In this article, we will discuss some important and frequently asked Java 8 Interview Questions and Answers.

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1. What are new features were added in Java 8?

In Java 8, the following new features were added:

Lambda Expressions − lambda expression is a function that can be referenced and passed around as an object
Method References − Method references are the references that use a function as a parameter to request a method.
Optional − This class is to provide a type-level solution for representing optional values instead of using null references.
Functional Interface – An Interface that contains exactly one abstract method and implementation can be provided using a Lambda Expression
Default methods − give us the ability to add full implementations in interfaces besides abstract methods
Stream API − Stream API provides a functional approach to processing collections of objects.
Date and Time API − an improved, immutable JodaTime-inspired Date API
Nashorn, JavaScript Engine − Java-based engine for executing and evaluating JavaScript code

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

The below diagram shows all the Java 8 features and enhancements.

2. What is a Lambda Expression?

Lambda expression is simply a function without having any name. It can even be used as a parameter in a function. Lambda Expression facilitates functional programming and simplifies the development a lot.

The main use of Lambda expression is to provide an implementation for functional interfaces.

For example, Lambda expression provides an implementation for Printable functional interface

interface Printable {
    void print(String msg);
}

public class JLEExampleSingleParameter {

    public static void main(String[] args) {
         // without lambda expression
         Printable printable = new Printable() {
            @Override
            public void print(String msg) {
               System.out.println(msg);
            }
         };
         printable.print(" Print message to console....");
  
         // with lambda expression
         Printable withLambda = (msg) -> System.out.println(msg);
         withLambda.print(" Print message to console....");
     }
}

Output :

 Print message to console....
 Print message to console....

Example 2, Create a method that takes a lambda expression as a parameter:

interface StringFunction {
    String run(String str);
}

public class Main {
     public static void main(String[] args) {
       StringFunction exclaim = (s) -> s + "!";
       StringFunction ask = (s) -> s + "?";
       printFormatted("Hello", exclaim);
       printFormatted("Hello", ask);
    }
    public static void printFormatted(String str, StringFunction format) {
       String result = format.run(str);
       System.out.println(result);
     }
}

Example 3, Pass lambda expression as an argument to the constructor

    static Runnable runnableLambda = () -> {
        System.out.println("Runnable Task 1");
        System.out.println("Runnable Task 2");
    };
   //Pass lambda expression as argument
    new Thread(runnableLambda).start();

Read more in-detail about lambda expressions at Java 8 Lambda Expressions

3. Why use Lambda Expression?

Facilitates functional programming - Lambda Expression facilitates functional programming and simplifies the development a lot.
To provide the implementation of the Java 8 Functional Interface.
Reduced Lines of Code - One of the clear benefits of using lambda expression is that the amount of code is reduced, we have already seen that how easily we can create instances of a functional interface using lambda expression rather than using an anonymous class.
Passing Behaviors into methods - Lambda Expressions enable you to encapsulate a single unit of behavior and pass it to other code. For example, to other methods or constructors.

Read more in detail about lambda expressions at Java 8 Lambda Expressions.

4. Explain Lambda Expression Syntax

Java Lambda Expression Syntax:

(argument-list) -> {body}

Java lambda expression consists of three components.

Argument-list: It can be empty or non-empty as well.
Arrow-token: It is used to link arguments-list and body of expression.
Body: It contains expressions and statements for the lambda expression.

For example, Consider we have a functional interface:

interface Addable{  
    int add(int a,int b);  
}

Let's implement the above Addable functional interface using a lambda expression:

        Addable withLambdaD = (int a,int b) -> (a+b);  
        System.out.println(withLambdaD.add(100,200));

Read more in detail about lambda expressions at Java 8 Lambda Expressions.

5. What is a functional interface?

An Interface that contains exactly one abstract method is known as a functional interface. It can have any number of default, static methods but can contain only one abstract method. It can also declare the methods of the object class.

Functional Interface is also known as Single Abstract Method Interfaces or SAM Interfaces. A functional interface can extend another interface only when it does not have any abstract method.

Java 8 provides predefined functional interfaces to deal with functional programming by using lambda and method references.

For example:

interface Printable {
    void print(String msg);
}

public class JLEExampleSingleParameter {

    public static void main(String[] args) {  
         // with lambda expression
         Printable withLambda = (msg) -> System.out.println(msg);
         withLambda.print(" Print message to console....");
     }
}

Output :

 Print message to console....

6. Is it possible to define our own Functional Interface? What is @FunctionalInterface? What are the rules to define a Functional Interface?

Yes, it is possible to define our own Functional Interfaces. We use Java 8 provides the @FunctionalInterface annotation to mark an interface as a Functional Interface.

We need to follow these rules to define a Functional Interface:

Define an interface with one and only one abstract method.
We cannot define more than one abstract method.
Use @FunctionalInterface annotation in the interface definition.
We can define any number of other methods like default methods, static methods.

The below example illustrates defining our own Functional Interface:

Let's create a Sayable interface annotated with @FunctionalInterface annotation.

@FunctionalInterface  
interface Sayable{  
    void say(String msg);   // abstract method   
}

Let's demonstrate a custom functional interface via the main() method.

public class FunctionalInterfacesExample {

    public static void main(String[] args) {

        Sayable sayable = (msg) -> {
            System.out.println(msg);
        };
        sayable.say("Say something ..");
    }
}

7. Name some of the functional interfaces in the standard library

In Java 8, there are a lot of functional interfaces are introduced in the java.util.function package and 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 argument and returns a result
Predicate – it takes one argument and returns a boolean
BiFunction – it takes two arguments and returns a result
BiConsumer - it takes two (reference type) input arguments and returns no 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
Runnable: use to execute the instances of a class over another thread with no arguments and no return value.
Callable: use to execute the instances of a class over another thread with no arguments and it either returns a value or throws an exception.
Comparator: use to sort different objects in a user-defined order
Comparable: use to sort objects in the natural sort order

For more on functional interfaces, see the article at Java 8 Functional Interfaces with Examples.

8. What is a method reference?

Method reference is used to refer method of the functional interface. It is a compact and easy form of a lambda expression. Each time when you are using a lambda expression to just referring a method, you can replace your lambda expression with a method reference.

Below are a few examples of method reference:

(o) -> o.toString();

can become:

Object::toString();

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:

String::new;

Static method reference:

String::valueOf;

Bound instance method reference:

str::toString;

Unbound instance method reference:

String::toString;

You can read a detailed description of method references with full examples at Java 8 Method References.

9. What are different kinds of Method References?

There are four kinds of method references:

Reference to a static method. For example:

ContainingClass::staticMethodName

Reference to an instance method of a particular object. For example:

containingObject::instanceMethodName

Reference to an instance method of an arbitrary object of a particular type. For example:

ContainingType::methodName

Reference to a constructor. for example:

ClassName::new

You can read a detailed description of method references with full examples at Java 8 Method References.

10. What is a Stream? How to create Streams in Java?

The stream represents a sequence of objects from a source such as a collection, which supports aggregate operations.

You can use Stream to filter, collect, print, and convert from one data structure to another, etc.

Stream does not store elements. It simply conveys elements from a source such as a data structure, an array, or an I/O channel, through a pipeline of computational operations.

Stream is functional in nature. Operations performed on a stream do not modify its source. For example, filtering a Stream obtained from a collection produces a new Stream without the filtered elements, rather than removing elements from the source collection.

Here are the examples to create a Stream using different sources.

Creating Empty Stream

The empty() method should be used in case of the creation of an empty stream:

Stream<String> stream = Stream.empty();
stream.forEach(System.out::println);

Creating Stream from From Collections

A stream can be created of any type of Collection (Collection, List, Set):

        Collection<String> collection = Arrays.asList("JAVA", "J2EE", "Spring", "Hibernate");
        Stream<String> stream2 = collection.stream();
        stream2.forEach(System.out::println);

        List<String> list = Arrays.asList("JAVA", "J2EE", "Spring", "Hibernate");
        Stream<String> stream3 = list.stream();
        stream3.forEach(System.out::println);

        Set<String> set = new HashSet<>(list);
        Stream<String> stream4 = set.stream();
        stream4.forEach(System.out::println);

Creating Stream object from Arrays

Array can be a source of a Stream or Array can be created from the existing array or of a part of an array:

        // Array can also be a source of a Stream
        Stream<String> streamOfArray = Stream.of("a", "b", "c");
        streamOfArray.forEach(System.out::println);

Creating Stream object from String using chars() method

We can also use String as a source for creating a stream with the help of the chars() method of the String class.

IntStream streamOfChars = "abc".chars();

Read more about streams at Java 8 Stream APIs with Examples.

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

Collection API	Stream API
It’s available since Java 1.2	It is introduced in Java SE 8
It is used to store Data (A set of Objects).	It is used to compute data (Computation on a set of Objects).
We can use both Spliterator and Iterator to iterate elements.	We can’t use Spliterator or Iterator to iterate elements. We can use forEach to perform an action for each element of this stream.
It is used to store an unlimited number of elements.	Stream API is used to process the elements of a Collection.
Typically, it uses the External Iteration concept to iterate Elements such as Iterator.	Stream API uses internal iteration to iterate Elements, using the forEach method.
Collection Object is constructed Eagerly.	Stream Object is constructed Lazily.
We add elements to the Collection object only after it is computed completely.	We can add elements to Stream Object without any prior computation. That means Stream objects are computed on-demand.

12. What is Optional in Java 8?

Optional is a container object which is used to contain not-null objects. An optional object is used to represent null with an absent value.

The Optional class has various utility methods to facilitate code to handle values as 'available' or 'not available' instead of checking null values.

The purpose of the Optional class is to provide a type-level solution for representing optional values instead of using null references.

Read more about Optional class with examples at Java 8 Optional Class with Examples.

13. What are the Advantages of Java 8 Optional?

Null checks are not required.
No more NullPointerException at run-time.
We can develop clean and neat APIs.
No more Boilerplate code

Read more about Optional class with examples at Java 8 Optional Class with Examples.

14. 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 new functionality to an interface while maintaining backward compatibility with classes that are already implementing the interface:

public interface Vehicle {
    String getBrand();

    String speedUp();

    String slowDown();

    default String turnAlarmOn() {
        return "Turning the vehice alarm on.";
    }

    default String turnAlarmOff() {
        return "Turning the vehicle alarm off.";
    }
}

Usually, when a new abstract method is added to an interface, all implementing classes will break until they implement the new abstract method. In Java 8, this problem has been solved by the use of 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 introduces 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.

Read more about Default Methods with examples at Java 8 Static and Default Methods in Interface.

15. What is a Static Method? Why do we need Static methods in Java 8 Interfaces?

A Static Method is a Utility method or Helper method, which is associated with a class (or interface). It is not associated with any object.
We need Static Methods because of the following reasons:

We can keep Helper or Utility methods specific to an interface in the same interface rather than in a separate Utility class.
We do not need separate Utility Classes like Collections, Arrays, etc to keep Utility methods.
Clear separation of Responsibilities. That is we do not need one Utility class to keep all Utility methods of Collection API like Collections etc.
Easy to extend the API.
Easy to Maintain the API.

16. How will you call a default method of an interface in a class?

Using the super keyword along with the interface name.

interface Vehicle {
   default void print() {
      System.out.println("I am a vehicle!");
   }
}
class Car implements Vehicle {
   public void print() {
      Vehicle.super.print();                  
   }
}

17. How will you call a static method of an interface in a class?

Using the name of the interface.

interface Vehicle {
   static void blowHorn() {
      System.out.println("Blowing horn!!!");
   }
}
class Car implements Vehicle {
   public void print() {
      Vehicle.blowHorn();                  
   }
}

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Java 8 Interview Questions and Answers