What types of collections are used in Java for data storage, and which ones have you worked with in your projects?

In Java, various types of collections are used to store data, which can be divided into three main groups:

• List — an ordered list of elements allowing duplicates. Examples: ArrayList, LinkedList.
• Set — a collection of unique elements, without duplicates. Examples: HashSet, TreeSet.
• Map — a collection of key-value pairs. Examples: HashMap, TreeMap.

In projects, the following are often used:

• ArrayList for dynamic lists with quick access by index.
• HashSet for storing unique elements with fast search.
• HashMap for associative arrays when quick retrieval of values by key is needed.

Also, LinkedList is used for frequent insertions/deletions in the middle of the list, and TreeSet and TreeMap for sorted collections.

The choice of collection depends on performance requirements and functionality.