List,Set,Map三者的区别

List(对付顺序的好帮手)： List接口存储一组不唯一（可以有多个元素引用相同的对象），有序的对象

List in Java provides ordered and indexed collection which may contain duplicates.
Set(注重独一无二的性质): 不允许重复的集合。不会有多个元素引用相同的对象。
Map(用Key来搜索的专家): 使用键值对存储。Map会维护与Key有关联的值。两个Key可以引用相同的对象，但Key不能重复，典型的Key是String类型，但也可以是任何对象。

The Set interface provides an unordered collection of unique objects, i.e. Set doesn’t allow duplicates, while Map provides a data structure based on key-value pair and hashing.

All three List, Set, and Map are interfaces in Java and there are many concrete implementations of them are available in Collection API.
ArrayList and LinkedList are two most popular used List implementation while LinkedHashSet, TreeSet, and HashSet are frequently used Set implementation.

Set vs List vs Map in Java
As I said Set, List and Map are interfaces, which defines core contract e.g. a Set contract says that it can not contain duplicates. Based on our knowledge of List, Set and Map let’s compare them on different metrics.

Duplicate Objects
The main difference between List and Set interface in Java is that List allows duplicates while Set doesn’t allow duplicates. All implementation of Set honor this contract.

While a Map holds two objects per Entry e.g. a key and a value and It may contain duplicate values but keys are always unique. See here for more difference between List and Set data structure in Java.

Order
Another key difference between List and Set is that List is an ordered collection, List’s contract maintains insertion order or element. Set is an unordered collection, you get no guarantee on which order element will be stored.

Though some of the Set implementation e.g. LinkedHashSet maintains order. Also SortedSet and SortedMap e.g. TreeSet and TreeMap maintain a sorting order, imposed by using Comparator or Comparable.