Interview Questions and Answers for 'Java' | Search Interview Question

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Q301. what is directive ?

Ans. A compiler directive is an instruction that tells the JVM how compilation should occur. A directive provides method-context precision in controlling the compilation process.

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Q302. can we use null as keyword ?

Core Java

Ans. null is a literal similar to true and false in Java. These are not keywords because these are the values of something. As null is the value of a reference variable, true is the value of a boolean variable.

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Q303. Sort a List of Objects using comparable and comparator

Core Java

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Q304. What are the drawbacks of Reactive extensions ?

ReactiveX

Ans. Reactive extensions have a steep learning curve which makes it confusing for beginner and intermediate developers.

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Q305. Difference between concatMap and flatMap ?

RxJava

Ans. Flatmap converts stream(s) into a single streams without maintaining a order where as concatmap does the same along with maintaining the order

flatmap example -[[1,2,3],[4,5,6],[7,8,9]] -> [1,4,7,2,5,8,3,6,9]

concatmap example -[[1,2,3],[4,5,6],[7,8,9]] -> [1,2,3,4,5,6,7,8,9]

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Q306. What is the difference between subscribe and blockingSubscribe method ?

RxJava

Ans. blockingSubscribe will block the current thread and wait till the chain completes

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Q307. What is the difference between subject and observable ?

RxJava

Ans. Subject has state, it keeps a list of observers. On the other hand, an Observable is really just a function that sets up observation. While Subjects are Observables, Subjects also implement an Observer interface.

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Q308. What is the difference between Observer and Observable ?

RxJava

Ans. Observable is emiting data and Observer is subscribing to catch the data

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Q309. Which of the Core Java functions / packages / concept do you believe have become irrelevant in today's market ?

Core Java

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Q310. What is the difference between round and scale method of BigDecimal ?

Core Java

Ans. Round method would round it to integer places irrespective of if it's decimal places or not whereas scale would only round the decimal places.

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Q311. Have you ever faced problems with rounding decimal places and what were the reasons ?

Core Java

Ans. 1. The problem with double (x*100)/100 doesn't return exact x but few fractions lesser than x and then if you are using floor rounding , it makes a big difference

2. Rounding only after getting a result vs rounding each outcome of 2 operand make difference

3. Usage of inappropriate Rounding mode and Rounding scale.

4. Results with double and BigDecimal

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Q312. Math.round method rounds the decimal to an integer by stripping off the decimal places. How can we use the same method to keep only 2 decimal places

For example -

Math.round(12.3456) will return 12. What should we do to get 12.34 without using any other class or method.

Core Java

Ans. We can multiply the value by 100 and then use Math.round on that and then divide the result by 100

For example -

(Math.round(12.3456 * 100)) / 100

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Q313. Find out the max and min number from given array

Core Java

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Q314. Make "I Know Java" as "I Java Know" without using any inbuilt functions.

Core Java

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Q315. Find Fibonacci first n numbers using recursion ?

Core Java

Ans. public int fibonacci(int n) {
if(n == 0)
return 0;
else
if(n == 1)
return 1;
else
return fibonacci(n - 1) fibonacci(n - 2);
}

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Q316. Find unique values between two arrays using single for loop ?

Core Java

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Q317. Find Intersection/union between two arrays.

Core Java

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Q318. Can we change return type of Overridden method ?

Core Java

Ans. Yes, we can change return type of overridden method in child class but child's return type should be sub-type of parent's return type.

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Q319. What is garbage collection ?

Core Java

Ans. The garbage collection is a facility wherein a program runs on the Java Virtual Machine which gets rid of objects, which are not being used by a Java application anymore. It is a form of automatic memory management and recollection.

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Q320. When to use List, Set and Map in Java

Core Java

Ans. I. If we want a Collection that does not store duplicate values, then we use a Set based collection.

II. If we want to frequently access elements operations based on an index value then we use a List based collection. E.g. ArrayList

III. If we want to maintain the insertion order of elements in a collection then we use a List based collection.

IV. For fast search operation based on a key, value pair, we use a HashMap based collection.

V. If we want to maintain the elements in a sorted order, then we use a TreeSet based collection.

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Q321. Get all numbers from a collection that are both perfect square and palindome

Core Java

Ans. public static void main(String[] args){
      List<Integer> listOfIntegers = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11);
      List<Integer> perfectAndPalindromeIntegers = new ArrayList<Integer>();

      for(Integer i: listOfIntegers){
         if(isPalindrome(i) && isPerfect(i)){
            perfectAndPalindromeIntegers.add(i);
         }
      }

      System.out.println(perfectAndPalindromeIntegers);
   }

   private static Boolean isPalindrome(Integer i){
      String str = String.valueOf(i);

      char[] charArray = str.toCharArray();

      int x = 0;
      int y = str.length()-1;

      while(x < y){
         if(charArray[x] != charArray[y]){
            return false;
         }
      }

      return true;
   }

   private static Boolean isPerfect(Integer i){
      for(int x=2;x<3;x++){
         if(x*x == i){
            return true;
         }
      }

      return false;
   }

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Q322. Explain Java collections

Core Java

Ans. Collection is a container which is used to store and manipulate the group of objects

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Q323. Write a program to sort the Array of Strings

Core Java

Ans. String[] s = { "sort", "string", "array" };
Collections.sort(Arrays.asList(s));
System.out.println(Arrays.toString(s)); // [array, sort, string]

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Q324. Write a program that identifies numbers in a list that are palindrome and perfect square ?

Core Java

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Q325. Can you explain filter and collectors in java 8 ?

Core Java

Ans. Filters are used to filter object in stream on certain criteria.
Collector is used to collect outcome of the stream. Collector is a terminal operation.
List numbers = List.of(1,2,3,4,5,6,7);
List evenNumbers = numbers.stream().filter(n -> n%2==0).collect(Collectors.toList());
The above code will filter out odd numbers and return the list of even number.

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Q326. Can you explain filter and collectors in java 8 ?

Core Java

Ans. Filters are used to filter object in stream on certail criteria. And collector is used to collect outcome of the stream. Collector is a terminal operation.
List numbers = List.of(1,2,3,4,5,6,7);
List evenNumbers = numbers.stream().filter(n -> n%2==0).collect(Collectors.toList());
The above code will filter out odd numbers and return the list of even number.

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Q327. Why do we need to close the Scanner ?

Core Java

Ans. For releasing input stream reference (System.in), you should close scanner.

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Q328. What is the difference between instantiation and initialization ?

Core Java

Ans. Instantiation is the process of reserving memory for the object

For ex -

List list = new ArrayList();

Initialization is the process of setting the default value for the object or default object construction.

List list = new ArrayList(set);

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Q329. Describe inheritance and polymorphism and their importance for OOP.

Core Java

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Q330. Design a class that find all files with SSN.

Core Java

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