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# Collection

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## 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

<br>

## 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

<br>

### 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`

<br>

### 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