What is the structure of the Java Collections Framework? Why is Map not a Collection?

A Collection is a storage for individual values, whereas a Map is a key-value storage. This fundamental difference leads to different methods in these interfaces. To put it simply, they have different method signatures such as put for Map and add for Collection.

The Collection itself is divided into three main groups, each corresponding to a specific interface:

• List – Ordered tuples that can contain duplicates and allow index-based (random) access;
• Queue – Typically FIFO collections, which implies adding/removing elements from the end. There is a sub-interface called Deque – double-ended queue adapted for manipulation on both ends;
• Set – Collections that are not necessarily ordered and contain unique (from equals perspective) elements .

A HashMap can be converted into a form of Collection by invoking methods like keySet(), entrySet(), or values().