12_Collection.md

Collection

What is a Collection?

• Various implementations exist based on the List, Map, and Set interfaces
• Reasons for use
  • Because standardized classes are provided for handling large amounts of data, you can use them conveniently without directly implementing DataStructures
  • Unlike arrays, you don't need to pre-define the space for storing objects, so the number of objects can be determined dynamically based on the situation
    • This also improves the spatial efficiency of the program

Types of Collections

List

• The List interface can be directly defined and used by the user through @Override, and the representative implementation is ArrayList.
  • This is an improvement of the existing Vector
• LinkedList
  • A data structure that stores data in a manner where each node has data and a pointer, connected in a single line
  • Nodes containing data are connected, and the node's pointer is responsible for the connection to the next or previous node
  • Advantages
    • Adding/removing data at the middle of the lined-up nodes is possible in O(1) time
  • Disadvantages
    • Unlike arrays or tree structures, searching for data at a specific position takes O(n) time

Map

• The representative implementation is HashMap
• It has a key-value structure, and the specific details about Map are consistent with the hashtable in the DataStructure section
• Duplicate values are not stored based on the key, and order is not guaranteed
• To guarantee order for keys, use LinkedHashMap

Set

• The representative implementation is HashSet.
• Duplicate values for value are not stored
• In fact, the Set data structure is simply a data structure where the value is used as the key, replacing the key in the Map's key-value structure
• Likewise, order is not guaranteed, and LinkedHashSet is used to guarantee order