Collection summary

1.Collection Is a general interface of the collection class .Collection There are two sub interfaces ：List and Set.Map It's an interface , But don't inherit Collection.

2.List Subclass of interface ：ArrayList、LinkedList、Vector

3.Set Subclass of interface ：TreeSet、HashSet、LinkedHashSet.

4.Map Subclass of interface ：TreeMap、HashMap、LinkedHashMap.

5. Underlying implementation

• ArrayList： Array
• LinkedList： Linked list
• Vector： Array
• TreeSet：TreeMap                       TreeMap: Red and black trees
• HashSet：HashMap                     HashMap： Array + Linked list （jdk1.7） Array + Linked list + Red and black trees （jdk：1.8 in the future ）
• LinkedHashSet： Red and black trees + Linked list

6. If you go TreeSet Add elements to the tree storage structure （ Reference object ）, The comparator needs to be defined . Comparators are divided into external comparators （Comparable） And internal comparator （Comparator）

7. Add elements to the collection , You can use generics to contract the type of element you want to add . Generic class , Generic interfaces and generic methods . Yes 3 Kinds of wildcards ：<?>、<? extends class>（ The upper limit of generics ）、<? super class>（ The lower bound of generics ）

8.HashMap Find elements in faster than TreeMap fast .

9.TreeMap What's stored is Key-value value ,TreeSet Yes, actually TreeMap structure , Only store key value .