Java 8 Interview Questions

In this post, we are going to cover some of the most common Java 8 interview questions, along with simple explanations and examples.

Java 8 introduced a lot of new features that changed the way we write code. Understanding these features is important not only for interviews but also for improving your coding skills.

   Java 8 Interview Questions

1. What are the main features introduced in Java 8?

There are multiple features introduced in Java 8, some main features are Lambda expression which allows us to write an anonymous method with concise syntax, Stream API provides a way to process and filter the collections, Functional Interfaces contains only one abstract method, Optional Class used to avoid NullPointerException, and  Date and Time API introduced new set of classes to handle date and time in a more flexible way. 

2. What is a Lambda Expression in Java 8?

A Lambda Expression is an anonymous or nameless function that doesn't have a name, return type and access modifiers. It is a way to represent a method in a simplified form. It helps to write code that is more readable and concise. A lambda expression is used to provide the implementation of a functional interface.

Example:

// Traditional way
public void m1() {
   sop("hello"");
}

//Using Lambda Expression 
() ->{
	sop("hello"");
     }
//or we can also write like
() -> sop("hello");

3. What is the Stream API in Java 8?

The Stream API is used to process collections of objects in a functional style. It allows for operations like filtering, mapping, and reducing without modifying the original data. If we want to process the bulk data or objects of collections then Stream API is very useful. 

For Example: Fetch all objects from the collection of lists whose value Starts with "J".

List<String> names = Arrays.asList("John", "Paul", "George", "Ringo");

//To filter names starting with "J"
List<String> filteredNames = names.stream()
    .filter(name -> name.startsWith("J"))
    .collect(Collectors.toList());

System.out.println(filteredNames);

OUTPUT:

[John]

4. What is a Functional Interface?

A Functional Interface is an interface that contains exactly one abstract method, and lambda expression is used to provide the implementation of that abstract method.

@FunctionalInterface
interface Greeting {
    void sayHello();
}

// Implementing with lambda
Greeting greeting = () -> System.out.println("Hello!");
greeting.sayHello();

OUTPUT:

Hello!

5. What is the purpose of the Optional class in Java 8?

The Optional class is used to handle values that might be null. Instead of checking for null directly, you can use Optional to avoid NullPointerException and write safer code.

6. How is the Date and Time API different in Java 8?

Java 8 introduced a new Date and Time API that is more powerful and easier to work with compared to the old Date and Calendar classes. It includes classes like LocalDate, LocalTime, and LocalDateTime for representing dates and times in a more simpler way.

Example:

LocalDate date = LocalDate.now();
LocalTime time = LocalTime.now();
System.out.println(date); // Output: current date
System.out.println(time); // Output: current time

7. What is the purpose of the default keyword in interfaces in Java 8?

The default keyword allows you to create default methods in interfaces. These methods provide a default implementation that can be used by classes that implement the interface. This feature helps in adding new methods to interfaces without breaking the existing implementation of classes that already implement the interface.

8. What is the difference between map() and flatMap() in Java 8?

The map() method is used to transform each element in a stream into another object, usually of the same or related type. On the other hand, flatMap() is used when each element in a stream can be transformed into multiple objects or even another stream. flatMap() flattens the result, meaning it combines multiple streams into a single stream.

9. What are method references in Java 8?

Method references are a shorthand notation for calling a method directly from a lambda expression. They allow you to refer to methods of existing classes or objects without needing to write a full lambda expression. There are four types of method references: static method references, instance method references of a particular object, instance method references of an arbitrary object of a specific type, and constructor references.

We can write like class::methodName

10. How does the Collectors class in Java 8 work?

The Collectors class provides a set of static methods to collect the results of a stream into different forms like lists, sets, maps, or other collections. It is often used with the collect() method of streams to gather the results of stream processing. The Collectors class supports various operations such as grouping, partitioning, and joining, making it a powerful tool for data collection and manipulation in Java 8.

If you are preparing for Java interviews knowing these interview questions is crucial Because Java 8 introduced major features that changed how developers write code and Features, like lambda expressions and the Stream API, made Java more powerful and easier to use.

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