Collections

Question 1

Collection

USED FOR:

Answer 1

it is used to store multiples of the same item or type in the same place (collection)

Question 2

Array

DEFINE & RULES

Answer 2

• It is used to store both primitives and reference types as collections.
• fixed sized. Once the size is declared, you cannot add or remove elements (size cannot be changed).
• Array does not provide methods to manipulate the data.
• Array allows duplicates
• Array allows null objects

NOTE: Except arrays, all other collections are used to store objects only (not primitives).

Question 3

Duplication - Nulls - Order

LIST INTERFACE

Answer 3

• Allows duplication
• Allows null objects (data)

-Keeps insertion order.
NOTE: Provides all the methods with indexes.

-List implementation classes are Vector, ArrayList and LinkedList

(Iterable)

Question 4

Duplication - Nulls - Order

MAP INTERFACE

Answer 4

• Map always stores data with key-value pairs
• Each key-value pair is also known as entry
• Keys -> NO duplicates
• Values -> duplicate
• put() method is used to add key-value pairs to the collection

-get(key) method is used to get value from the collection
NOTE: Maps do not work with methods that requires index.

Map implementation classes are HashMap, LinkedHashMap, HashTable, and TreeMap

Question 4

Duplication - Nulls - Order

SET INTERFACE

Answer 4

• No duplicates
• One null object (data). BUT, TreeSet does not allow any null elements because it sorts.

-Set does not preserve insertion order.
NOTE: Set does not provide any methods that use index as arguments.

Set implementation classes are HashSet, LinkedHashSet, and TreeSet

(Iterable)

Question 5

ArrayList vs Vector

Answer 5

Vector is the thread-safe version of ArrayList. Thread-safe means synchronized.

Question 6

ArrayList vs LinkedList

Answer 6

LinkedList stores data with nodes (previous - itself - next) and this makes it faster when you remove or add elements. On the other hand, ArrayList stores data with indexes which takes less memory compared to LinkedList but slower when adding or removing elements.

LinkedList implements Queue and Dequeue which allows LinkedList to have more methods

Question 7

HashSet vs LinkedHastSet

Answer 7

The only difference between them is LinkedHashSet keeps insertion order.

Question 8

HashSet vs TreeSet

Answer 8

TreeSet -> NO nulls

HashSet -> a single null element.

TreeSet -> sorts elements implicitly.
Numbers: ascending
Strings: lexicographically

Question 8

HashSet vs TreeSet

Answer 8

TreeSet -> NO nulls

HashSet -> a single null element.

TreeSet -> sorts elements implicitly.
Numbers: ascending
Strings: lexicographically

Question 9

HashMap vs LinkedHashMap

Answer 9

The only difference is LinkedHashMap keeps the insertion order

Question 10

HashMap vs TreeMap

Answer 10

HashMap -> one null key & many null values

TreeMap -> NO null keys & many null values. TreeMap -> sorts keys.

Question 11

HashMap vs HashTable

Answer 11

HashMap -> 1 null key & many null values.

HashTable -> NO null keys or values.
HashTable -> thread-safe.

Question 12

Hash vs Linked vs Tree

Answer 12

Hash -> allows nulls

Linked -> keeps insertion order

Tree -> No nulls & sorts

Question 13

Iterator

EXAMPLE

Answer 13

Collections a = new ArrayList<>();

a. add(5);
a. add(7);
a. add(9);

Iterator b = a.iterator();

while(b.hasNext()){
System.out.println(b.next());
}