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