What is the difference between set() and add() method in collection of Salesforce?

Hi Aditya,

add() method adds your specified element to the list at the specified index, the element which previously existed at that particular index and all the following elements will shift 1 index , and the previous element still exists.

Whereas in set() method the element is replaced by the element we want to insert at that particular index.

ex:-

a list - [1,2,3,4,5,6]

on add(0, 9)

the list will be [9,1,2,3,4,5,6]

on set(0,9)

the list will be

[9,2,3,4,5,6]

hope it helps.