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Interview Questions and Answers for 'Arraylist' - 23 question(s) found - Order By Newest

Frequently asked.
 Q1. If you are given a choice to use either ArrayList and LinkedList, Which one would you use and Why ?Core Java
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Ans. ArrayList are implemented in memory as arrays and hence allows fast retrieval through indices but are costly if new elements are to be inserted in between other elements. LinkedList allows for constant-time insertions or removals using iterators, but only sequential access of elements
1. Retrieval - If Elements are to be retrieved sequentially only, Linked List is preferred.
2. Insertion - If new Elements are to be inserted in between other elements , Linked List is preferred.
3. Search - Binary Search and other optimized way of searching is not possible on Linked List.
4. Sorting - Initial sorting could be pain but lateral addition of elements in a sorted list is good with linked list.
5. Adding Elements - If sufficiently large elements needs to be added very frequently ,Linked List is preferable as elements don't need consecutive memory location.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q2. What is the difference between ArrayList and LinkedList ?Core Java
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Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences -

1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList.

2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList.

3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references.

4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion.

5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster.

  Sample Code for ArrayList

  Sample Code for LinkedList

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  Q3. Difference between Vector and ArrayList ?Core Java
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Ans. Vectors are synchronized whereas Array lists are not.

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 Q4. Will this code give error if i try to add two heterogeneous elements in the arraylist. ? and Why ?

List list1 = new ArrayList<>();
list1.add(5);
list1.add("5");
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Ans. If we don't declare the list to be of specific type, it treats it as list of objects.

int 1 is auto boxed to Integer and "1" is String and hence both are objects.

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 Q5. Difference between ArrayList and LinkedList ?
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Ans. LinkedList and ArrayList are two different implementations of the List interface. LinkedList implements it with a doubly-linked list. ArrayList implements it with a dynamically resizing array.

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 Q6. What is CopyOnWriteArrayList ?
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Ans. Its a type of ArrayList in which all Write operations , i.e add and set are performed by creating a new copy. This array never changes during the lifetime of the iterator, so it never throws ConcurrentModificationException

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 Q7. What are the advantages and disadvantages of CopyOnWriteArrayList ?Core Java
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Ans. This collections class has been implemented in such a manner that it can never throw ConcurrentModificationException. As it performs update and write operations by creating a new copy of ArrayList, It's slower compared to ArrayList.

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 Q8. What will the following code result ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get(null));
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Ans. Compile time error at last line as the get method expect argument of type native int.

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 Q9. What will the following code result ? Will it compile ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get((Integer)null));
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Ans. Yes but the last line will throw NullPointerException upon execution.

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 Q10. Can we add more elements to an array list that has been marked as final ?

Ans. Yes, the array list can hold more elements. Final only puts the restriction that the array list reference cannot hold any other array list.

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 Q11. Does an ArrayList allow elements of different types ? If not, Why the following code works List list1 = new ArrayList<>(); list1.add(1); list1.add("1");Core Java2016-07-29 11:44:07

Ans. With Java 7 or Later. If you don't declare the list to be of specific type , it treats it as list of objects. int 1 is auto boxed to Integer and "1" is String and hence both are objects.

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 Q12. Explain what happens when insertion is performed in case of ArrayList and LinkedList.Data Structure2017-10-26 15:52:53

Ans. Array List works on Array and when we add an element in middle of the list, Array List need to update the index of all subsequent elements. I the capacity is full, it even may need to move the whole list to a new memory location . Linked List works on Double linked list algorithm and all it has to do is to adjust the address of the previous and next elements.

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 Q13. Difference between Array and ArrayList ?Core Java2016-12-15 14:13:12

Ans. http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html

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 Q14. How can we get an array out of ArrayList ?Core Java2017-05-07 16:42:41

Ans. We can get an array out of ArrayList by using toArray() method of an array list.

String a[] = arrayList.toArray();

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 Q15. What is the trade off between using Collections.synchronizedList() on an ArrayList and a CopyOnWriteArrayList?Core Java2017-05-07 19:31:54

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 Q16. why aren't arrays better than array list ?Core Java2017-05-31 11:42:53

 This question was recently asked at 'Wells Fargo'.This question is still unanswered. Can you please provide an answer.


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 Q17. Why iterators of an array list are fail fast ?Core Java2017-06-08 07:30:13

Ans. Because it's access isn't synchronized and hence access / modification by multiple threads may lead to inconsistent state.

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 Q18. What is meant by "Vector is synchronized and ArrayList isn't " ?Core Java2017-06-08 07:32:02

Ans. It means that only 1 thread can access have access to Vector at a time and no parallel access is allowed whereas Array List allows parallel access by multiple threads.

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 Q19. What problem we could have with ArrayList which aren't possible with Vectors ?Core Java2017-06-08 07:34:24

Ans. ArrayLists aren't synchronized and hence doesn't allow synchronized access. As multiple threads can access an arraylist in parallel, it may result in an inconsistent state.

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 Q20. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?Core Java2017-06-09 08:19:17

Ans. We can copy the elements to a Set and then find the difference of count between ArrayList and Set. As Set don't allow duplicates , they will be removed in the set.

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 Q21. What does the following initialization mean ?

ArrayList<LinkedList> traversalPaths = new ArrayList<LinkedList>();

What could be the use of such a collection.
Core Java2017-07-23 08:33:28

Ans. Initialize an ArrayList that will hold LinkedLists i.e every element of the arraylist will be a linked list.

Such collection could be used in algorithms that require first random access and then sequential traversal. For example - Storing traversal paths for a graph wherein we can start from any vertex. Implementing dictionary with each arraylist element holding staring with character and then linked list holding duplicate words.

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 Q22. Which of the two - Arrays or LinkedList - is a better data structure for implementing Queue ? and Why ?Data Structure2017-08-28 19:34:12

 This question is still unanswered. Can you please provide an answer.


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 Q23. What will be the output of exceuting main method ?

public static void main(String[] args){
      List list = new ArrayList();
      list.add(1);
      list.add(2);
      list.add(3);
      System.out.println(list);
   }
Core Java
a. 1,2,3
b. Order cannot be determined
c. compilation error
d. 3,2,1

Ans.a. 1,2,3



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