See also: Set vs bag

In programming, a collection is a class used to represent a set of similar data type items as a single unit. These unit classes are used for grouping and managing related objects. A collection has an underlying data structure that is used for efficient data manipulation and storage. Read more here: What Does Collection Mean?

A set and a bag are both collections of items in programming, but they differ in their characteristics and behaviors.

A set is a collection of distinct and unordered elements - each item can only appear once in the collection. Sets are commonly used when the order of elements is unimportant, but uniqueness is. In sets, duplicates are not allowed, and any attempts to add an already existing item are simply ignored. Sets are usually implemented using hash tables, which provide efficient membership testing and set operations such as union, intersection, and difference.

A bag, also known as a multiset, is a collection of elements that allows duplicates and does not impose any order on the items. In a bag, elements can appear multiple times, and the count of each item represents the number of times it occurs in the collection. Bags are useful when we need to keep track of the frequency of occurrence of each element. For example, a bag can be used to store a histogram of a set of values. Bags are typically implemented using arrays or hash tables, where each element is associated with a count.