30 Top Interview Questions and Answers (2024)

Ques 1

How lambda expression and functional interfaces are related?

Lambda expressions can only be applied to abstract method of functional interface.

For example:

Runnable has only one abstract method called run, so it can be used as below:

// Using lambda expression

Thread t1=new Thread(

()->System.out.println("In Run method")

);

Here we are using Thread constructor which takes Runnable as parameter. As you can see we did not specify any function name here, as Runnable has only one abstract method, java will implicitly create anonymous Runnable and execute run method.

It will be as good as below code.

Thread t1=new Thread(new Runnable() {

@Override

public void run() {

System.out.println("In Run method");

}

});

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Ques 2

Can you create your own functional interface?

Yes, you can create your own functional interface. Java can implicitly identify functional interface but you can also annotate it with @FunctionalInterface.

Example:

Create interface named "Readable" as below:

public interface Readable {

void read();

default void readBook()

{

System.out.println("Reading the book.");

}

Create main class named "MyFunctionalInteface":

public class MyFunctionalInteface {

public static void main(String[] args)

{

MyFunctionalInteface myFuncInterface = new MyFunctionalInteface();

myFuncInterface.readMyBook(() -> System.out.println("Reading my book"));

}

public void readMyBook(Readable p)

{

p.read();

}

When you run above program, you will get below output:

Reading my book

As you can see, since Readable has only one abstract method called read(), we were able to call it using lambda expression.

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Ques 3

What is method reference in java 8?

Method reference is used refer method of functional interface. It is nothing but compact way of lambda expression.You can simply replace lambda expression with method reference.
Syntax:
class::methodname

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Ques 4

What the issues with Old Date and time API before Java 8? Date and Time API differences between Java 8 and earlier Java version.

Issues with old Date and Time API:

Thread Safety: You might be already aware that java.util.Date is mutable and not thread safe. Even java.text.SimpleDateFormat is also not Thread-Safe. New Java 8 date and time APIs are thread safe.
Performance: Java 8 new APIs are better in performance than old Java APIs.
More Readable: Old APIs such Calendar and Date are poorly designed and hard to understand. Java 8 Date and Time APIs are easy to understand and comply with ISO standards.

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Ques 5

Do we have PermGen in Java 8? What is MetaSpace in Java 8?

Until Java 7, JVM used an area called PermGen to store classes. It got removed in Java 8 and replaced by MetaSpace.

Major advantage of MetaSpace over permgen:

PermGen was fixed in term of maximum size and can not grow dynamically but Metaspace can grow dynamically and do not have any size constraint.

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Ques 6

Difference between Intermediate and terminal operations in Stream.

Java 8 Stream supports both intermediate and terminal operation.

Intermediate operations are lazy in nature and do not get executed immediately. Terminal operations are not lazy, they are executed as soon as encountered.
Intermediate operation is memorized and is called when terminal operation is called.
All Intermediate operations return stream as it just transforms stream into another and terminal operation don't.

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Ques 7

Given the list of numbers, remove the duplicate elements from the list.

See the following code:

Integer[] arr=new Integer[]{1,2,3,4,3,2,4,2};

List<Integer> listWithDuplicates = Arrays.asList(arr);

Set<Integer> setWithoutDups = listWithDuplicates.stream().collect(Collectors.toSet());

setWithoutDups.forEach((i)->System.out.print(" "+i));

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Ques 8

Difference between Stream findFirst() and findAny().

findFirst will always return the first element from the stream whereas findAny is allowed to choose any element from the stream.
findFirst has deterministic behavior whereas findAny is nondeterministic behavior.

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Ques 9

What is consumer function interface?

It is a functional interface defined in java.util.function package. It contains an abstract accept() and a default andThen() method. It can be used as the assignment target for a lambda expression or method reference.

Consumer is single argument functional interface which does not return any value.

public static void main(String[] args) {

Consumer<String> consumerString = s->System.out.println(s);

consumerString.accept("John");

}

We have created consumer object which takes String object as input and print it. It is simple use of Consumer interface to print String.

Output:

John

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Ques 10

What is predicate function interface?

Predicate is single argument function which returns true or false. It has test method which returns boolean. Usually, it used to apply in a filter for a collection of objects.

public static void main(String[] args) {

Predicate<Integer> predicate = i -> i > 100;

boolean greaterCheck = predicate.test(200);

System.out.println("is 200 greater than 100: "+greaterCheck);

}

Output:

is 200 greater than 100: true

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Ques 11

What Is a Stream? How Does It Differ from a Collection?

In simple terms, a stream is an iterator whose role is to accept a set of actions to apply on each of the elements it contains.
The stream represents a sequence of objects from a source such as a collection, which supports aggregate operations. They were designed to make collection processing simple and concise. Contrary to the collections, the logic of iteration is implemented inside the stream, so we can use methods like map and flatMap for performing a declarative processing.
And yet another important distinction from collections is that streams are inherently lazily loaded and processed.
Another difference is that the Stream API is fluent and allows pipelining:

int sum = Arrays.stream(new int[]{1, 2, 3})
.filter(i -> i >= 2)
.map(i -> i * 3)
.sum();

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Ques 12

What Is the Difference Between Map and flatMap Stream Operation?

There is a difference in signature between map and flatMap. Generally speaking, a map operation wraps its return value inside its ordinal type while flatMap does not.
For example, in Optional, a map operation would return Optional<String> type while flatMap would return String type.
So after mapping, one needs to unwrap (read “flatten”) the object to retrieve the value whereas, after flat mapping, there is no such need as the object is already flattened. The same concept is applied to mapping and flat mapping in Stream.
Both map and flatMap are intermediate stream operations that receive a function and apply this function to all elements of a stream.
The difference is that for the map, this function returns a value, but for flatMap, this function returns a stream. The flatMap operation “flattens” the streams into one.

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Ques 13

What is Stream Pipelining in Java 8?

Stream pipelining is the concept of chaining operations together. This is done by splitting the operations that can happen on a stream into two categories: intermediate operations and terminal operations.
Each intermediate operation returns an instance of Stream itself when it runs, an arbitrary number of intermediate operations can, therefore, be set up to process data forming a processing pipeline.
There must then be a terminal operation which returns a final value and terminates the pipeline.

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Ques 14

What does the flatmap() function do? why you need it?

The flatmap function is an extension of the map function. Apart from transforming one object into another, it can also flatten it.

For example, if you have a list of the list but you want to combine all elements of lists into just one list. In this case, you can use flatMap() for flattening. At the same time, you can also transform an object like you do use map() function.

public static void main(String[] args) {

String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

//Stream<String[]>

Stream<String[]> temp = Arrays.stream(data);

//Stream<String>, GOOD!

Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));

Stream<String> stream = stringStream.filter(x -> "a".equals(x.toString()));

stream.forEach(System.out::println);

}

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Ques 15

What is the parallel Stream? How can you get a parallel stream from a List?

A parallel stream can parallel execute stream processing tasks. For example, if you have a parallel stream of 1 million orders and you are looking for orders worth more than 1 million, then you can use a filter to do that.

Unlike sequential Stream, the parallel Stream can launch multiple threads to search for those orders on the different part of Stream and then combine the result.

In short, the parallel Stream can paralyze execution.

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Experienced / Expert level questions & answers

How lambda expression and functional interfaces are related?

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Can you create your own functional interface?

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What is method reference in java 8?

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What the issues with Old Date and time API before Java 8? Date and Time API differences between Java 8 and earlier Java version.

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Do we have PermGen in Java 8? What is MetaSpace in Java 8?

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Difference between Intermediate and terminal operations in Stream.

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Given the list of numbers, remove the duplicate elements from the list.

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Difference between Stream findFirst() and findAny().

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What is consumer function interface?

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What is predicate function interface?

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What Is a Stream? How Does It Differ from a Collection?

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What Is the Difference Between Map and flatMap Stream Operation?

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What is Stream Pipelining in Java 8?

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What does the flatmap() function do? why you need it?

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What is the parallel Stream? How can you get a parallel stream from a List?

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