Core Java - Interview Questions and Answers for 'Array' | Search Interview Question

Interview Questions and Answers - Order By Newest

Q11. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve vectors arraylist collections list synchronization synchronized}

Q12. Difference between Arrays and ArrayList ?

Core Java

Ans. Both Arrays and ArrayLists are used to store elements. Elements can be either primitives or objects in case of Arrays, but only objects can be stored in Arraylist. Array is a fixed length data structure while arraylist is variable length collection class. Once created, you cannot change the size of the arrays, but arraylists can dynamically resize itself when needed.Another notable difference between Arrays and Arrayslist is that arary is part of core java programming and array list is part of collection classes

Sample Code for arrays arraylist

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve array arraylist array vs arraylist Asked in 7 Companies basic frequent}

Q13. Can we declare an array without assigning the size of an array?

Core Java

Ans. No, It will throw compile time error saying "must provide either dimension expressions or an array initializer"

Alternatively we can provide array initializer like

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

which will initialize it to size 2 with values as "Buggy" and "Bread"

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays array initialization}

Q14. Why doesn't this code give compile time error when we are clearly trying to add an integer element to an array of Strings

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

Core Java

Ans. Because the reference is of Object Array and the resolution of what it holds is identified at the runtime.

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

would have given the compile time array as the reference is of String array.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays}

Q15. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java arrays loops variables basic interview question}

Q16. What is the difference between int[] x; and int x[]; ?

Core Java

Ans. No Difference. Both are the acceptable ways to declare an array.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java array arrays array declaration difference between Asked in 3 Companies basic}

Q17. What is CopyOnWriteArrayList ?

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list arraylist copyonwritearraylist ConcurrentModificationException Asked in 1 Companies}

Q18. What are the advantages and disadvantages of CopyOnWriteArrayList ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list arraylist copyonwritearraylist advantages-disadvantages ConcurrentModificationException Asked in 4 Companies Expert}

Q19. What is ArrayIndexOutOfBoundException ?

Core Java

Ans. Exception thrown by the application is we try to access an element using an index which is not within the range of array i.e lower than 0 or greater than the size of the array.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java exceptions arrayindexoutofboundexception Asked in 1 Companies}

Q20. 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));

Ans. Compile time error at last line as the get method expect argument of type native int.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arraylist list coding collections}

Q21. 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));

Ans. Yes but the last line will throw NullPointerException upon execution.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collections arraylist list coding}

Q22. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve ebay collections arraylist final keyword}

Q23. 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 Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arraylist list collections}

Q24. Explain what happens when insertion is performed in case of ArrayList and LinkedList.

Data Structure

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arraylist vs linkedlist collections list Asked in 2 Companies}

Q25. Difference between Array and ArrayList ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve array arraylist Asked in 11 Companies basic frequent}

Q26. Write code to find second largest number in an array of integers.

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve code coding find in array Asked in 1 Companies}

Q27. Write a Program to merge two sorted arrays ?

Data Structure

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays merge sorted arrays Asked in 2 Companies}

Q28. Is array an object in Java ? How can you prove that ?

Core Java

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)

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays}

Q29. How can we get an array out of ArrayList ?

Core Java

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

String a[] = arrayList.toArray();

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve array arraylist}

Q30. What is the trade off between using Collections.synchronizedList() on an ArrayList and a CopyOnWriteArrayList?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve Collections.synchronizedList() CopyOnWriteArrayList}

Q31. why aren't arrays better than array list ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays arraylist arrays vs arraylist Asked in 3 Companies}

Q32. Why iterators of an array list are fail fast ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve fail fast arraylist collections}

Q33. What problem we could have with ArrayList which aren't possible with Vectors ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve vector arraylist list collections Basic}

Q34. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collections arraylist set}

Q35. Why do we pass an array of strings to main method ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve main method main method string array argument}

Q36. What does the following initialization mean ?

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

What could be the use of such a collection.

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collections linkedlist arraylist}

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]);
}

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays}

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]);
}

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arrays}

Q39. How can we convert String into Char array and vice versa ?

Core Java

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);

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve string char array}

Q40. How can we convert a character or a character array into a String ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve character char char array String char to String char array to String}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.