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Q31. Which of the following - arrays or LinkedList allow elements to be accessed using index and how ?

Ans. Arrays allows elements to be accessed directly using the index.

As Array elements are stored in continuous memory locations it's very easy to find the memory address of any element using the formula as following

Memory Address of Array start or index 0 + ( Size of array element * Index )

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Q32. Explain what happens when insertion is performed in case of ArrayList and LinkedList.

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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Q33. Write an efficient program for printing k largest elements in an array. Elements in array can be in any order.

^{This question was recently asked at 'Amazon'.This question is still unanswered. Can you please provide an answer.}

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Q34. Which of the two - Arrays or LinkedList - is a better data structure for implementing Queue ? and Why ?

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Q35. How can we convert a character or a character array into a String ?

Ans. String has an argument constructor that take char array as argument and creates a string.

There is no constructor available with String that takes in a character and creates a String. We can use StringBuilder which has a char argument constructor.

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Q36. How can we convert String into Char array and vice versa ?

Ans. There is a method toCharArray() within String class that can be used to convert string to char array.

string.toCharArray();

String class has an argument constructor that takes a char array and create a string

new String(charArray);

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Q37. What will the following code print

int x[] = new int[5];
x[0] = 1;
x[1] = 2;

for(int count=0;count<=x.length;count++){
System.out.println(x[count]);
}

Ans. 1
2
0
0
0
ArrayIndexOutOfBoundException: 5

As array index starts with 0 and ends with the index of (size - 1), the index 5 is inaccessible for the array and hence will throw the exception.

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Q38. What will the following code print ?

int x[] = new int[5];
x[0] = 1;
x[1] = 2;

for(int count=0;count<x.length;count++){
System.out.println(x[count]);
}

Ans. 1
2
0
0
0

As arrays are not dynamically expanded , we have declared the array for size 10. As we have only initialized only 2 values , it will print rest as their default values.

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Q39. What does the following initialization mean ?

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

What could be the use of such a collection.

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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Q40. Write code to find second largest number in an array of integers.

Ans. int arr[]={1,3,5,6,4,8,9,2,10};
Arrays.sort();
System.out.println(arr[arr.length-1]);

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Q41. Why do we pass an array of strings to main method ?

Ans. Array of strings in the main method are the list of arguments or parameters which are sent to the application / program.

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Q42. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

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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Q43. What problem we could have with ArrayList which aren't possible with Vectors ?

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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Q44. What is meant by "Vector is synchronized and ArrayList isn't " ?

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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Q45. Why iterators of an array list are fail fast ?

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

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Q46. why aren't arrays better than array list ?

Ans. 1. Arrays don't provide any built in support for sorting searching like operation and we need to do it explicitly.

2. It can hold only homogeneous objects,

3. We have to reserve memory as no dynamic expansion allowed.

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Q47. What is the trade off between using Collections.synchronizedList() on an ArrayList and a CopyOnWriteArrayList?

Ans. Synchronized List locks the whole list to provide synchronization and thread safety during the read or write operation, while, CopyOnWriteArrayList doesn’t lock the whole list during these operations.
The CopyOnWriteArrayList class works according to its name i.e. copy-on-write which performs different actions for reading and write operations. For every write operation (add, set, remove, etc), it makes a new copy of the elements in the list. and for the read operations (get, iterator, listIterator, etc), it works on a different copy. So there is no additional overhead during a read operation and its read operation is faster than Collections.SynchronizedList(). Thus, COWAL is better for reading operation than Synchronized List.

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Q48. How can we get an array out of ArrayList ?

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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Q49. Is array an object in Java ? How can you prove that ?

Ans. Yes.

There are 2 ways this can be confirmed

1. Accessing object class methods using array reference.

2. Checking if the array is an instance of Object class

if (arrayRef instanceof Object)

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Q50. Write a Program to merge two sorted arrays ?

Ans. We can merge two sorted array by using quick sort algorithm

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Q51. Difference between Array and ArrayList ?

Ans. <a href="http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html" rel="nofollow">http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html</a>

Sample Code for ArrayList

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Q52. Implement the following method ?

int findMax(int[] items)

Ans. int findMax(int[] items){
   int maxNumber = 0;
   for(int x:items){
      if(x > maxNumber){
         maxNumber = x;
      }
   }

   return maxNumber;
}

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Q53. 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");

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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Q54. Difference between Vector and ArrayList ?

Ans. Vectors are synchronized whereas Array lists are not.

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Q55. 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");

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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Q56. What is the advantage of using arrays over variables ?

Ans. Arrays provide a structure wherein multiple values can be accessed using single reference and index. This helps in iterating over the values using loops.

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Q57. What are the disadvantages of using arrays ?

Ans. Arrays are of fixed size and have to reserve memory prior to use. Hence if we don't know size in advance arrays are not recommended to use.

Arrays can store only homogeneous elements.

Arrays store its values in contentious memory location. Not suitable if the content is too large and needs to be distributed in memory.

There is no underlying data structure for arrays and no ready made method support for arrays, for every requriment we need to code explicitly

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Q58. Advantage of Collection classes over Arrays ?

Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.

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Q59. What are the Disadvantages of using Collection Classes over Arrays ?

Ans. Collections can only hold objects, It can't hold primitive data types.

Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors.

Collections doesn't allow modification while traversal as it may lead to concurrentModificationException.

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Q60. Difference between ArrayList and LinkedList ?

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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