Below are the most frequently asked Java Collections interview questions with answers. This will help you to crack collections interview questions in 2021.
Table of Contents
1. What is a framework?
A framework is a predefined architecture that contains a set of several classes and interfaces which can be used directly without any modification.
2. What is a Collection framework?
In Java, a collection is a single entity that represents a group of objects and a framework is a defined architecture that contains interfaces and classes. Hence, the Java Collection framework is an architecture that defines a set of interfaces, classes, and algorithms. This framework was available from JDK 1.2. It can also perform operations like search, insert, sort, delete, manipulate, etc.
3. What is a Collections class?
Collections is a class in Java that is part of java.util package and contains various static methods that we can use to perform operations on the Collections. It also contains methods for various algorithms like sorting, binary search, etc.
4. What are the differences between an Array and Collections?
| Array | Collection |
|---|---|
| It is fixed size and cannot change the size in runtime | The size can be dynamically changed and is not fixed. |
| It can store only same type of data | It can store combination of data type values |
| It does not have built-in methods like sorting or searching | It has built-in methods for sorting and searching |
| Performance is fast | Performance is slow |
| Memory efficiency is less | Memory efficiency is more |
| It is not implemented based on any data structure | Every class in Collection is implemented based on some data structure |
5. What are the various interfaces present in the Collection framework?
Collection interface: This is the root interface of the Collection framework hierarchy. The other interfaces which are present in the Collection framework are Map and Iterator interfaces. These interfaces further have subinterfaces since there is no direct implementation for the Collection interface.
The Collection interfaces contain List, Set, and Queue interfaces.
List interface: This is an ordered collection of elements that allows storing duplicate values.
Set interface: This is an unordered collection of elements and does not allow duplicate values.
Queue interface: It implements the queue data structure and follows the FIFO(First-In-First-Out) concept.
Deque interface: This is a double-ended queue that implements both stack and queue and hence supports both FIFO and LIFO.
Map interface: It stores data in the form of key-value pairs and we can store only unique keys.
6. What are the differences between ArrayList and Vector?
| ArrayList | Vector |
|---|---|
| ArrayList is not synchronized | Vector is synchronized |
| It is not a legacy class | It is a legacy class |
| It can increase its size by 50% of the array size | It can increase its size by doubling |
| It is not thread-safe | It is thread-safe |
| Uses only Iterator for traversing | Uses both Enumeration and Iterator for traversing |
| ArrayList performance is faster | Vector performance is comparatively slower |
7. What are the differences between ArrayList and LinkedList?
| ArrayList | LinkedList |
|---|---|
| Uses dynamic array to store elements | Uses Double LinkedList to store elements |
| Performance is slower | Performance is faster |
| It can be used to implement only List | It can be used to implement List and Queue |
| Provides random access | Does not provide random access |
| Occupies less memory since it stores only object | Occupies more memory since it stores both object and reference of the object |
| get(int index) gives performance of O(1) | get(int index) gives performance of O(n) |
| Removal operation performance in worst case is O(n) and best case is O(1) | Removal operation gives performance of O(1) |
| Add method gives performance of O(n) in worst case | Add method gives performance of O(1) |
8. What are the differences between Iterator and ListIterator?
| Iterator | ListIterator |
|---|---|
| It is a universal iterator | It is not a universal iterator |
| It can be used for any type of Collection | It can be used only for List implementation classes |
| Supports only read and delete operation | Supports add, update, read and delete operations. |
| Can be used only for forward navigation | Can be used for forward and reverse direction navigation |
| It is not a bidirectional iterator | It is a bidirectional iterator |
| We use the iterator() method | We use the listiterator() method. |
| We cannot specify the index for iteration | We can specify the index for iteration |
9. What are the differences between Iterator and Enumeration?
| Enumeration | Iterator |
|---|---|
| Available from JDK 1.0 | Available from JDK 1.2 |
| It is not a universal iterator | It is a universal iterator |
| It can be used to iterate only legacy collections like Vector, hashtable | It can be used to iterate any Collections |
| Supports only read operation | Supports both read and delete operation |
| It has lengthy method names Eg: hasMoreElements() | It has short method names Eg: hasNext() |
10. What are the differences between List and Set?
| List | Set |
|---|---|
| It can store duplicate elements | It can store only unique elements |
| It is an ordered collection | It is an unordered collection |
| It has a single legacy class which is Vector | It does not have any legacy class |
| It can store any number of null values | It can store only a single null value |
11. What are the differences between HashSet and TreeSet?
| TreeSet | HashSet |
|---|---|
| Maintains ascending order | Does not maintain any order |
| Computation is slower | Computation is faster |
| Time cost is log(n) | Time cost is constant |
12. What are the differences between Set and Map?
| Set | Map |
|---|---|
| Set stores only values | Map stores key and values |
| Set can contain unique values | Map can contain unique keys and duplicate values |
| It can hold single null value | It can hold single null key and multiple null values |
13. What are the differences between HashSet and HashMap?
| HashSet | HashMap |
|---|---|
| It implements the Set interface | It implements the Map interface |
| It stores only values | It stores data in the form of key-value pair |
| It cannot have duplicate values | It can contain duplicate values with unique keys |
| It can have only a single null value | It can hold a single null key with multiple null values |
14. What are the differences between HashMap and TreeMap?
| TreeMap | HashMap |
|---|---|
| Maintains ascending order | Does not maintain any order |
| Cannot contain null key | Can contain one null key |
| It is a tree structure based implementation | It is a hashtable based implementation |
| It does not allow null values | It allows multiple null values |
| Sorting is slower | Sorting is faster |
| Uses red-black tree data structure | It uses a hashtable data structure |
15. What are the differences between HashMap and HashTable?
| HashMap | Hashtable |
|---|---|
| It is not synchronized | It is synchronized |
| It is not thread safe and cannot be shared between threads | It is thread safe and can be shared between threads |
| Allows one null key and multiple null values | Does not allow null key and null value |
| Introduced in JDK 1.2 | It is a legacy class |
| We use Iterator to traverse through Hashmap | We use Iterator or Enumeration to traverse through Hashtable |
| Inherits AbstractMap class | Inherits Dictionary class |
| Computation is fast | Computation is slow |
16. What are the differences between Collection and Collections?
| Collections | Collection |
|---|---|
| This is a class | This is an interface |
| Provides methods for sorting, searching and synchronization | Provides data structure for queue, list, set |
| Provides static methods for various operations on Collection | Provides methods to support data structure |
17. What are the differences between Comparator and Comparable?
| Comparator | Comparable |
|---|---|
| Provides multiple sorts of sequences | Provides single sort of sequences |
| It has an in-built method named compare() | It has an in-built method names compareTo() |
| It is part of the java.util package | It is part of the java.lang package |
| The actual class is not modified | The actual class is modifying on implementation |
18. What is BlockingQueue?
BlockingQueue is an interface that provides concurrency in operations like insert and delete elements. This means it inserts an element by waiting till the queue has available space and deletes an element by waiting till the queue is non-empty.
19. What is the advantage of the Properties file?
A property file is a file that helps in changing or updating the file without the need to recompile the java class. It helps in managing data and is useful when there is frequent update to file data.
20. What is the hashCode() method?
The hashCode() method is used in the hashing technique that returns an integer number that is used to store key-value pairs. It is used in Collections like HashSet, HashMap, etc. It returns the same integer value when both keys are identical. In case both objects are different, then it returns a different integer value for both the objects.
21. Why do we override the equals() method?
The equals method checks if two objects are equal. This is present in the Object class and we need to override it in order to check the equality.
22. How to synchronize List, Set, and Map elements?
We can synchronize the elements using the synchronized method.
23. What are the advantages of generic collections?
- It does not require typecasting
- It checks during compile-time and is type-safe.
- Also checks for bugs during compile time itself
24. What is hash-collision and how is it handled?
Hash collision is the process when two different keys produce the same integer value or hash value. We can avoid hash collision by using the below 2 techniques:
- Separate chaining
- Open addressing
25. What is the default size of the load factor in hashing?
The default size of the load factor is 0.75.
26. What is fail-fast?
The fail-fast is a condition in which the iterator throws the ConcurrentModificationException when there is a structural modification.
27. What is the difference between Array and ArrayList?
| Array | ArrayList |
|---|---|
| It is of fixed-size | It is of dynamic size |
| It is static | It is dynamic |
| It can store primitive values and objects | It can store only objects |
28. What is the difference between the length of an array and the size of ArrayList?
We can retrieve the length of an array using the length property for an array. But the ArrayList does not support this. Hence it uses the size() method to find the size of the ArrayList. Both return the number of elements in an array or ArrayList.
29. How to convert an array to ArrayList and vice versa?
We can use the asList() method to convert an array to ArrayList and we can use the toArray() method to convert the ArrayList to an array.
30. How to make Java ArrayList as read-only?
When an ArrayList is read-only, we can only read the values and cannot modify or update it. We can do this by using the Collections.unmodifiableCollection() method.
31. How can we remove duplicates from ArrayList?
We can remove duplicates from ArrayList by using the HashSet or LinkedHashSet.
32. What are the new features of Collections in Java 8?
- Java Stream API support
- Many new methods for improvements like forEachRemaining() in Iterator, replaceAll(), compute() and merge() methods in Map.
- Extension of Iterable interface with forEach() default method.
33. Why Map interface does not extend the Collection interface?
A Map interface represents data in the form of key-value pairs while the Collections do not support this. Hence a Map interface cannot be considered as a Collection and hence not part of the Collection framework.
34. What is an Iterator?
An Iterator is an interface that has an iterator() method that helps us to iterate or traverse through all the elements in the Collection.
35. What are the different ways to iterate through a list?
We can iterate through a list in two different ways:
- Using the for-each loop
- Using iterator
36. What are the different Collection views provided by the Map interface?
The Map interface provides the below Collection views:
- keySet(): This method returns a Set view of the elements.
- values(): This method returns a Collection of values in the map.
- entrySet(): This method returns a Set view of the key-value pair mapping present in the map.
37. Which Collection class provides random access to elements?
ArrayList, HashMap, TreeMap, HashTable, and Vector classes.
38. What is EnumSet?
EnumSet stores Enumeration data types and implements the Set interface. All the elements should belong to the single Enum type and does not allow to store null values.
39. Which Collection classes are thread-safe?
Vector, Hashtable, Properties, and Stack classes.
40. How can we sort a list of Objects?
We can sort a list of Objects using the Arrays.sort() or Collections.sort()
41. Why Collection does not extend the Cloneable and Serializable interfaces?
The Collection interface contains elements that are nothing but a group of objects. The order of the elements is based on the implementation of the Collection interface. Hence it is not required to extend the Cloneable and Serializable interface.
42. How to reverse a list?
We can use the reverse() method to reverse the elements in the list.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
public class ArrayListReverse {
public static void main(String[] args) {
ArrayList<Integer> al = new ArrayList<Integer>();
al.add(30);
al.add(10);
al.add(50);
al.add(40);
al.add(20);
System.out.println("Elements in the ArrayList");
System.out.println(al);
System.out.println(" in descending order:");
Collections.reverse(al);
System.out.println(al);
}
}
Elements in the ArrayList [30, 10, 50, 40, 20] Sort Elements in descending order: [20, 40, 50, 10, 30]
43. How many types of LinkedList does Java support?
LinkedList in Java supports two types: Single LinkedList, Doubly LinkedList
44. What is a Set and what are the various implementations?
A Set is unordered collection of elements which means it does not maintain any insertion order and cannot store any duplicate values.
The HashSet, LinkedHashSet, and TreeSet are the classes that implement the Set interface.
45. What is an emptySet()?
An emptySet() returns empty immutable set whenever we try to remove null elements. This set is serializable.
46. What is a Vector?
A vector is similar to array but was part of the legacy framework and has separate methods apart from the Collections framework.
47. What is a dictionary class?
A dictionary class that can store key-value pairs.
48. What is an UnsupportedOperationException?
The UnsupportedOperationException is thrown when the method is not supported by the Collection type.
49. What is a peek() method in Queue?
The peek() method returns the head element but does not remove it. If the queue is empty then it returns null.
50. What are the best practices in the Collection framework?
- Select the correct type of collection based on the requirement
- Avoid rehashing or resizing
- Use more interfaces
- Use generics for type-safety
- Enhance code reusability by using the Collection utility class
- Use immutable classes
51. What are the different types of queue?
- PriorityQueue
- Double-ended queue(deque)
- Circular queue
52. What are the differences between stack and queue?
| Stack | Queue |
|---|---|
| Follows LIFO concept | Follows FIFO concept |
| Performs insertion or deletion operation through one end | Performs insertion through one end and deletion through the other end |
| Uses a single pointer | Uses double pointers |
| There is no variant types | There are variants like PriorityQueue, CircularQueue, Double-ended Queue |
53. What are the differences between array and stack?
| Array | Stack |
|---|---|
| Stores elements of same data type | Stores elements of different data types |
| We can use random access operation to remove or add elements | We can use LIFO concept to add or remove elements |
| Identifies the elements based on the index | Used for push and pop operations |
54. What are the advantages of a stack?
- Uses LIFO concept to manage the data
- Automatically cleans up the object
- Memory allocation and deallocation management is easier
- Uses stack when the variable is not used outside the function
- Does not easily get corrupted
55. What is a Big-O notation?
The Big-O notation is to calculate the performance of the algorithm based on the number of elements. We can use this to compare and check which collection type is adaptable to implement.
For example, the getIndex(index i) method of the ArrayList performs a constant time and does not depend on the number of elements. Hence the Big-O notation is O(1).
56. What is a LinkedList?
The LinkedList is a class that implements both deque and List using the double linked list. Each element is a node that has 2 parts, one stores the value, and the other stores the address reference of the previous and next nodes.
57. What is Map.entry in Map?
Map.entry is an interface that supports working with a map entry. It returns a view of the collection. We can use the entrySet() method to retrieve the set view of the map that has the key-value pairs. It also has separate methods like getKey() to retrieve the key and getValue() to retrieve the value.
58. What are the different methods to remove elements from an ArrayList?
- clear(): Removes all the elements and empties the ArrayList
- remove(int index): Removes the element at the specified index
- remove(Object o): Removes the specified element
- removeAll(): Removes all the elements in the list
- removeIf(Predicate filter): Removes the elements that satisfy the predicate
59. How to join multiple ArrayList?
We can join or merge the elements of different ArrayList using the addAll(Collection c) method.
ArrayList<String> al = new ArrayList<String>(); ArrayList<String> arr = new ArrayList<String>(); al.addAll(arr);
60. What is a diamond operator?
The diamond operator supports to retrieve the argument types of the generic class. It is represented as <> and can replace the parameterized constructor with empty parameter sets.