# TreeSet, TreeMap vs HashSet, HashMap

> Which data structure should be used in which situation!

<br>

### TL;DR

* Unless there is a special reason, use `HashMap` which has good `search performance`
* If you want to `guarantee order`, use LinkedHashMap
* If you want to `iterate` over key values in a consistent manner, use `TreeMap`

<br>

### Differences Between TreeMap and HashMap

* TreeMap is a data structure similar to HashMap, but the difference is that TreeMap's data is sorted
* Since HashMap's data is not sorted, it may be slower than TreeMap for data searching
* However, HashMap uses less memory than TreeMap

<br>

### Order Guarantee Aspect

* `HashMap` does not guarantee order
  * `LinkedHashMap` guarantees the order of insertion!
* `TreeMap` guarantees order using the comparison operation of the class used as the Key value
  * It automatically `sorts` based on the key value

<br>

### Speed Aspect

* The time complexity of `HashMap` is `O(1)`
  * Because it uses `hash values`
* The time complexity of `TreeMap` is `O(log n)`
  * In return, you can get a `sorted order`

<br>

### Whether `null` is Allowed as a Key

* `HashMap` `allows` null as a key
* `TreeMap` does `not allow` null as a key

<br>

### When to Use TreeMap?

* When you need to store and search ordered data
  * It has synchronization handling, making it `Thread-safe`
* When you need to search data in sorted order
* When insertion, deletion, and search operations on data are frequently performed

<br>

### Cases Where TreeMap is Not Efficient

* When the data size is very large
* When insertion, deletion, and search operations on data are not frequently performed


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