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Q31. 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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Q32. 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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Q33. 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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Q34. 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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Q35. 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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Q36. 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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Q37. 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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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 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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Q40. 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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Q41. 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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Q42. Which of the two - Arrays or LinkedList - is a better data structure for implementing Queue ? and Why ?

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Q43. Write an efficient program for printing k largest elements in an array. Elements in array can be in any order.

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Q44. 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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Q45. Find Intersection/union between two arrays.

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Q46. How arrays work internally ?

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Q47. Can we change the size of array once created ?

Ans. No. Arrays cannot resize dynamically.

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Q48. What is ArrayStoreException ?

Ans. It's an exception that is thrown when we attempt to add value of an incompatible type to an array.

For example -

Object[] strArray = new String[2];
strArray[0] = 5;

In this code, strArray reference of type Object has currently been assigned the String array but at line 2 we are trying to add an integer value.

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Q49. What is an anonymous array in Java ?

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Q50. What are jagged arrays ?

Ans. an array of different sizes

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Q51. Are Arrays treated like primitives or like objects in Java ?

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Q52. How can we check equality for Arrays in Java ?

Ans. You call the method java.util.Arrays.equals(Object[] a, Object[] a2)?

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Q53. Are arrays Thread Safe ?

Ans. Yes

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Q54. What is the time and space complexity for different operations for arrays ?

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Q55. Difference between Arrays and LinkedList ?

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Q56. Can we have 3 dimensional arrays in Java ?

Ans. Yes

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Q57. Can we make array volatile in Java ?

Ans. Yes, you can make an array (both primitive and reference type array e.g. an int array and String array) volatile in Java but only changes to reference pointing to an array will be visible to all threads, not the whole array

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Q58. Is this array declaration correct ? If not , Why ?

String[] strArray = new String[];

Ans. No, We haven't specified the size of array to be initialized.

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Q59. Is this array initialization correct ? If Yes, What will be the size of array ?

String[] strArray = new String[]{"Buggy","Bread"};

Ans. Yes, size of the array will be 2.

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Q60. Is this array initialization correct ? If Yes, What will be the size of array ?

String[] strArray = new String[3]{"Buggy","Bread"};

Ans. No. It will result in error saying "Cannot define dimension expressions when an array initializer is provided"

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