Summary of collection: (to be updated)

The class set has the following interfaces ：Collection,List,Set,Map,Iterator,Enumeration,Queue,ListIterator
ps: It is very necessary to clarify these interfaces ！！！

collection（interface）： Interface of single value data
|collection Methods of interface definition | Method type |
| public boolean add(E e)|–|

public boolean add(E e)
public boolean addAll(Collection<? extends E> c)
public void clear()
public boolean contains(Object o)
public boolean remove(Object o)
public int size()
public Object[] toArray()
public Iterator iterator()

The two most commonly used interfaces are add and iterator

List（ Interface ）
public interface List extends Collection

public E get(int index)
public public set (int index,E element)
public ListIterator listIterator()

The most common is get(index) Method ,
List： Elements have subscripts , Orderly , Element repeatable
List There are three subclasses ：ArrayList,LinkedList,Vector
ArrayList
public class ArrayList extends AbstractList implements List,RandomAccess,Cloneable,Serializable
ArrayList Implementation relationship of interface ：

ArratList and LinkedList The difference between （ Interview questions ）
ArrayList：
The underlying data structure is an array , Memory space is continuous
Elements have subscripts , Orderly , Allow repetition
Usually subscript
Adding and deleting operations are slow , Query operation is relatively fast （ Large amount of data ）
LinkedList：
The underlying data structure is linked list , Memory space is not continuous
Elements have subscripts , Orderly , Allow repetition
Usually carry out the first and last operations
Adding and deleting operations are faster , Query operation is slow （ When there's a lot of data ）
Be careful ：Linklist Not all queries are slow , The head and tail operation is still relatively fast

 Insert a code chip here 

set： Element has no subscript , disorder , Element is not repeatable （ Reference types are rewritten equals() and hashCode() Methodical , So if we write generic types by ourselves , in order to set The element of is not repeatable , You need to rewrite it yourself equals() and hashCode()）
Map: Yes key value , disorder ,key Do not repeat , however value Can be repeated （ Reference types are rewritten equals() and hashCode() Methodical , So if we are generic key It's my own type , in order to keyt Non repeatability of , You need to rewrite it yourself equals() and hashCode()）

HashMap：

HashSet： Packaged HashMap

List Iteration ： Among them, according to list With subscript , Orderly features we can use for To iterate , in addition List Iterators that inherit the parent interface ：iterator, In addition, it also has its own iterator listIterator（listIterator(int Index) It realizes the iteration from the beginning of the list ）listIterator You can also iterate from back to front .

Iterations of sets ：

List<String>list=new ArrayList<>();
        list.add(" A dream of red mansions ");
        list.add(" Water margin ");
        list.add(" Journey to the west ");
        list.add(" The romance of The Three Kingdoms ");
        System.out.println("--------------------------- Traverse set one -------------------------------");
        for (int i = 0; i <list.size(); i++) {
    
                System.out.println(list.get(i));
            }
        System.out.println("--------------------------- Traversal Set II -------------------------------");
        for (String i:
                 list) {
    
                System.out.println(i);
            }
        System.out.println("--------------------------- Traverse set three -------------------------------");
        Iterator it=list.iterator();
        while(it.hasNext()){
    
            System.out.println(it.next());
        }
        System.out.println("--------------------------- Traversal set four traversal in reverse order -------------------------------");
        ListIterator<String> it2 = list.listIterator();
        while(it2.hasNext())
            it2.next();
            if(!it.hasNext()){
    
                while(it2.hasPrevious())
                System.out.println(it2.previous());
            }

Note that the last case is traversal in reverse order , In reverse traversal , You must traverse to the last element in positive order before traversing in reverse order , Otherwise, the pointer of the iterator is in the first , The pointer cannot find the previous point , You can't iterate .

ListIterator<String> it2 = list.listIterator();
                while(it2.hasPrevious())
                System.out.println(it2.previous());
 The result is ：
--------------------------- Traversal set four traversal in reverse order -------------------------------

Map( The parent interface )HashMap（ Subclass implementation ）
Map Iteration ：Map You cannot iterate directly , because Map There is no iterator , and hashMap Yes keySet and entrySet The two methods , So use the above two methods to iterate .

public static void main(String[] args) {
    
        Map<Integer,String>map=new HashMap<>();
        map.put(1," Bones jing ");
        map.put(2," Pipa essence ");
        map.put(3," Bubble essence ");
        map.put(4," Carp essence ");
        System.out.println("================= Iteration method 1 =======================");
        Set<Integer> set1 = map.keySet();
        Iterator<Integer> it1 = set1.iterator();
        while(it1.hasNext()){
    
            Integer key=it1.next();
            System.out.println(key+"="+map.get(key));
        }
        System.out.println("================= Iteration mode 2 =======================");
        Set<Map.Entry<Integer, String>> set2 = map.entrySet();
        Iterator<Map.Entry<Integer, String>> it2 = set2.iterator();
        while(it2.hasNext()){
    
            Map.Entry<Integer, String> entry = it2.next();
            //System.out.println(entry);
            System.out.println(entry.getKey()+"="+entry.getValue());
        }
    }