Java Interview @ Polaris

Hi friends,

In this post, I'm sharing interview questions asked in Polaris.

Question 1:

public class Quiz23{

    public static void main(String[] args){

        int x = 0;

        int[] nums = {1,2,3,5};

        for(int i : nums){

            switch(i){

                case 1:

                    x += i;

                case 5:

                    x += i;

                default:

                    x += i;

                case 2:

                    x  += i;

            }

            System.out.println(x);

        }

    }

}

What will be the output of above code?

Answer:

27

Explanation:

For case i = 1:  All the cases will run.

For case i = 2: only case 2 will run.

For case i = 3: Default and case 2 will run.

For case i = 5: case , default and case 2 will run.

Question 2:

What is the difference between Map and FlatMap?

Answer:

Map: Transforms the elements into something else. It accepts a function to apply to each element and returns a new Stream of values returned by the passed function.

It takes function object as the parameter e.g.: Function, ToIntFunction, ToIntDoubleFunction, ToLongFunction.

FlatMap: Combination of Map and Flat operations.So, we first apply map operation on each element and then flattens the result.

So, if function used by Map is returning a single value, then map is ok. But, if function used by Map operation is returning a stream of list or stream of stream, then we need to use flatmap to get stream of values.

For example:

If we have a stream of String containing {"12", "34"} and a method getPermutations() which returns a list of permutations of given string.

When we apply getPermutation() into each string of Stream using map , we get something like [["12","21"], ["34","43"]], but if we use flatMap, we get a stream of strings e.g.: ["12","21","34","43"].

Another example:

List evens = Arrays.asList(2,4,6);

List odds = Arras.asList(3,5,7);

List primes = Arrays.asList(2,3,5,7,11);

List numbers = Stream.of(evens, odds, primes).flatMap(list -> list.stream()).collect(Collectors.toList());

System.out.println("flattened list : "+numbers);

Output: flattened list : [2,4,6,3,5,7,2,3,5,7,11]

 

Question 3:

What is the difference between passing int array and String array to Stream.of()?

Answer:

Stream.of(int[]) gives Stream<int[]>

Stream.of(String[]) gives Stream<String>

So, when using Stream.of() with int[] , we get Stream<int[]> and then for getting ints from Stream, we use flatMapToInt(i -> Arrays.Stream(i)) to get IntStream and then we can either use map() or forEach().

e.g.:

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

Stream<int[]> streamArr = Stream.of(arr);

IntStream intStream = streamArr.flatMapToInt(i -> Arrays.Stream(i));

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

Question 4:

What is a Boxed Stream?

Answer:

If we want to convert stream of objects to collection:

List<String> strings = Stream.of("how", "to", "do", "in", "java").collect(Collectors.toList());

The same process doesn't work on streams of primitives, however.

//Compilation Error:

IntStream.of(1,2,3,4,5).collect(Collectors.toList());

To convert a stream of primitives, we must first box the elements in their wrapper class and then collect them. This type of stream is called boxed stream.

Example of IntStream to List of Integers:

List<Integer> ints = IntStream.of(1,2,3,4,5).boxed().collect(Collectors.toList());

System.out.println(ints);

Output: [1,2,3,4,5]

 

Question 5:

List Spring Core and Stereotype annotations.

Answer:

Spring Core Annotations:

• @Qualifier
• @Autowired
• @Configuration
• @ComponentScan
• @Required
• @Bean
• @Lazy
• @Value

Spring framework Stereotype annotations:

• @Component
• @Controller
• @Service
• @Repository

Question 6:

What is difference  between @Controller and @RestController?

Answer:

@Controller creates a Map of model object and finds a view for that.

@RestController simply returns the object and object data is directly written into HTTP response as JSON or XML.

@RestController = @Controller + @ResponseBody

@RestController was added in  Spring 4.

Question 7:

Why Spring introduced @RestController when this job could be done by using @Controller and @ResponseBody?

Answer:

The functioning or output of @Controller and @ResponseBody is the default behavior of RESTFUL web services, that's why Spring introduced @RestController which combined the behavior of @Controller and @ResponseBody.  

Question 8:

Tell something about Swagger tools which you have used.

Answer:

Swagger Editor:  Can edit OpenAPI specifications.

Swagger UI : A collection of HTML/CSS/Javascript assets that dynamically generate beautiful documentation.

Swagger Codegen: Allows generation of API client libraries.

Swagger Parser: Library for parsing OpenAPI definitions from java.

Swagger Core: Java related libraries for creating, consuming and working with OpenAPI definitions.

Swagger Inspector: API testing tool

SwaggerHub : Built for teams working with OpenAPI.

That's all for this post.

Thanks for reading!!