Java collections ----- Map Detailed

      Map Collection and exist side by side. For storing data having a mapping relationship: Key-Value

•      Map of the key and value can be any type of reference data
•      Map of the key with Set to store, not allowed to repeat, that is the same, common String class as the Map "key"
•      There is a one-way one to one relationship between the key and value, that is, always find unique value determined by the specified key

   1.HashMap

      HashMap is not thread safe

package com.gather;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

public class HashMapPractise {
    public static void main(String[] args) {
        Map<String, Person> map = new HashMap<>();
        Person p1 = new Person("张三", 22);
        Person p2 = new Person("李四", 23);
        Person p3 = new Person("王五", 22);
        map.put ( "AA" , P1); 
        map.put ( "BB" , P2); 
        map.put ( "the CC" , P3); 
        
        // first traversal: can be taken out simultaneously on the key 
        for (String STR: map.keySet ()) { 
           System.out.println ( "bond is:" + str + ", the value:" + as map.get (STR)); 
        } 
        System.out.println ( "----- ------------------------- " ); 
        
        // The second way to navigate: just take the keys or only the values 
        for (String str: the Map. keySet ()) { 
            System.out.println ( "bond is:" + STR); 
        } 
        
        for (the Person Person: map.values ()) { 
            the System.out.the println ( "value:" +the Person); 
        } 
        System.out.println ( "------------------------------" ); 
        
        // Recommended: Third species traversal: can be taken out simultaneously on the key 
        for (the entry <String, the Person> entry: EnumMap.entrySet ()) { 
            System.out.println ( "bond is:" + entry.getKey () + " , the value of: "+ entry.getValue ()); 
        } 
        System.out.println ( " ------------------------------ " ); 
        
        // fourth traversal: key-value pair can be taken out simultaneously 
        the Iterator <the Entry <String, the Person entries It >> = . EnumMap.entrySet () Iterator (); 
         the while (entries.hasNext ()) { 
            the Entry <String, the Person> = entry entries.next ();
            System.out.println ( "bond is:" + entry.getKey () + " , the value:" + entry.getValue ()); 
        } 
    } 
}

   2.Hashtable

     Hashtable is thread safe

package com.gather;

import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Map;
import java.util.Map.Entry;

public class HashtablePractise {
    public static void main(String[] args) {
        Map<String, Person> map = new Hashtable<>();
        Person p1 = new Person("张三", 22);
        Person p2 = new Person("李四", 23);
        map.put("AA", p1);
        map.put("BB", p2);

        for (Entry<String, Person> entry : map.entrySet()) {
            System.out.println("键为:" + entry.getKey() + ",值为:" + entry.getValue());
        }
        System.out.println("------------------------------");

        Hashtable<String, Person> hashtable = new Hashtable<>();
        hashtable.put("AA", p1);
        hashtable.put("BB", p2);

        // Hashtable的另外一种遍历方式：Enumeration
        Enumeration<String> enuKey = hashtable.keys();
        while (enuKey.hasMoreElements()) {
            System.out.println(enuKey.nextElement());
        }

        Enumeration<Person> enuValue = hashtable.elements();
        while (enuValue.hasMoreElements()) {
            System.out.println(enuValue.nextElement());
        }
    }
}