Collection

Question 1

What is Java collection?

Answer 1

A collection is an object that contains multiple elements of the same type in a single unit.

Question 2

What is Java Collections Framework?

Answer 2

Collections framework is a library that provides common architecture for creating, updating and accessing different types of collections.

Question 3

What are the main benefits of collections framework?

Answer 3

Reusability: common classes and utility methods that can be used with different types of collections.
Quality: improves the program quality, since the code is already tested.
Speed: developers speed increased since they can focus on core logic and use the collections provided.
Maintenance: Since collections are open source and widely documented, it is easy to maintain the code.

Question 4

What is the root of the collections framework?

Answer 4

Collection interface is the root of Collections Framework.

Question 5

What are the thread safe classes in the Collections framework?

Answer 5

Stack, Vector, Hashtable, BlockingQueue, Concurrent Map, ConcurrentNavigableMap

Question 6

How to convert a list to a set?

Answer 6

Use HashSet. We can put a list into a HashSet, internally the hashCode() method is used to identify duplicate elements.

Question 7

How to remove duplicate elements from an ArrayList?

Answer 7

If ordering of elements is not important, then we can put the elements of ArrayList into a HashSet and then add them back to the ArrayList.

Question 8

How can you make the elements of a Collection be in sorted order?

Answer 8

You can use the utility method Collections.sort to sort a list. This is nLog(n) order of performance.

Question 9

In which scenario is LinkedList better than ArrayList?

Answer 9

LinkedList is better in the scenario when we do not need random access to elements, or if there are a lot of insertion or deletion actions on elements.

Question 10

What are the differences between list & set?

Answer 10

Order: list is an ordered sequence of elements. Set is just a distinct collection that is unordered.
Positional Access: When using a list we can specify where we want to insert an element. In a set there is no order.
Duplicate: in a list we can store duplicates. In a set we cannot store duplicates.

Question 11

How to decide when to use List, Set or Map?

Answer 11

If we do not want duplicates, use a set. If we want to frequently access elements operations based on an index value or we want to maintain insertion order then we used a list. For fast key-based search we use a HashMap.

Question 12

Difference between HashMap & HashTable?

Answer 12

Synchronisation: HashMap is not synchronised. HashTable is synchronised.
Null values: HashMap only allows one null key and n null values. HashTable does not allow null keys or values.
Ordering: HashMap maintains the order of elements. A HashTable does not guarantee any kind of order.

Question 13

What is the reason for overriding the equals method?

Answer 13

The equals method in Object class is used to check wether two objects are the same or not. If we want a custom implementation we override it. In HashMap implementation, if we want to use an object as a key, we override it.

Question 14

How does hashCode() method work in java?

Answer 14

Object class in Java has hashCode() method which returns a hash code value, which is an integer. If two objects are the same, their hashcode is also the same. Object generates a hashcode based on the memory address of the instance of the object.

Question 15

What is the difference between Iterator and Enumeration?

Answer 15

Both are interfaces in java to access Data Structures. Enumeration is an older interface. Enumeration can only traverse legacy collections. Iterator can traverse both legacy and newer collections.Iterator is fail-fast, enumeration is not fail-fast.

Question 16

What is the difference between an ArrayList and LinkedList

Answer 16

ArrayList is an index based dynamic array. LinkedList is a Doubly Linked List data structure. It is easier to insert into LinkedList, a since there is no need to resize an Array. LinkedList has better performance for removing elements. LinkedList is slower in accessing an element, since we have to traverse the list one by one to access the right location.

Question 17

What is the default size of the load factor in a HashMap?

Answer 17

0.75

Question 18

What is a Hash collision?

Answer 18

A hash collision occurs when two objects may have the same HashCode but they are different. Java will have an issue storing the two different objects in a HashMap in this case.

Question 19

Difference between queue and stack?

Answer 19

Queue is FIFO. Stack is LIFO.

Question 20

What method do we need to override to use an object as a key in a hashmap?

Answer 20

equals() and hashCode()

Question 21

What is the difference between peek(), poll() and remove() methods of Queue?

Answer 21

poll() and remove() can be used for removing the head object of a queue. If the list is empty the poll() method returns null, but the remove() throws a NoSuchElementException. peek() is used to retrieve the head of the queue, but not remove it.

Question 22

How to insert, delete & retrieve elements from a HashMap?

Answer 22

Retrieve: get() method, passing object key.
Insert: put() method, passing key & value
Delete: remove() method, passing object key.

Question 23

What is the difference between fail-fast and fail-safe Iterator in java?

Answer 23

Fail-fast iterator throws ConcurrentModificationException, fail safe does not throw this exception. Fail-fast does not clone the original collection, fail safe creates a copy of the original collection of objects.

Question 24

How to convert a map to a list in java?

Answer 24

A map has three collection sets, key set, value set, key-value set. We can use List keyList = new ArrayList(map.keySet());