Collection in Salesforce?

How many Type Collection in Salesforce?

Ans:-  3 types 

a)      Lists

b)      Set

c)      Map

What is list ?

Ans:- A list is an ordered collection of typed primitives, sObjects, user-defined objects, Apex objects or collections that are distinguishedby their indices.The index position of the first element in a list is always 0.

Example:- List<String>mylist = new List<String>();

What is Set ?

Ans:- A set is an unordered collection of primitives or sObjects that do not contain any duplicate elements.

To access elements in a set, use the system methods provided by Apex.

Example:-

Set<Integer> st = new Set<Integer> (); // Define a new set

st.add (1); // Add an element to the set

st.remove (1); // Remove the element from the set

What is Map?

Ans:- A map is a collection of key-value pairs where each unique key maps to a single value. Keys can be any primitive data type, while values can be a primitive, sObject, collection type or an Apex object.

To declare a map, use the Map keyword followed by the data types of the key and the value within <> characters.

Example :-

Map<String, String>country_currencies = new Map<String, String>();

Map<ID, Set<String>> m = new Map<ID, Set<String>>();

Map<ID, Map<ID, Account[]>> m2 = new Map<ID, Map<ID, Account[]>>();