package com.tn.hashMap;

 public class Student {

     private String id;

     private String name;

     public Student(String id, String name) {

         super();

         this.id = id;

         this.name = name;

     }

     public String getId() {

         return id;

     }

     public void setId(String id) {

         this.id = id;

     }

     public String getName() {

         return name;

     }

     public void setName(String name) {

         this.name = name;

     }

     @Override

     public String toString() {

         return  name;

     }

 }

 package com.tn.hashMap;

 import java.util.HashMap;

 import java.util.Map;

 import java.util.Set;

 public class HashMapDemo {

     public static void main(String[] args){

         Student student1=new Student("0001","哪吒");

         Student student2=new Student("0002","杨戬");

         Student student3=new Student("0003","雷震子");

         HashMap<String,Student> hm=new HashMap<String,Student>();

         //HashMap的键值对都是Object类型，如果需要用到基本类型可用Integer之类的包装类

         hm.put(student1.getId(), student1);

         hm.put(student2.getId(), student2);

         hm.put(student3.getId(), student3);

         //put方法如果key相同，则后面的value会覆盖前面的

         System.out.println(hm.size());

         System.out.println(hm);

         //通过keySet方法用Set遍历

         Set<String> set=hm.keySet();

         for(String str:set){

             System.out.println(hm.get(str));//打印出来的顺序是不可预知的

         }

         //通过entrySet方法用set遍历

         /*放入HashMap集合中的key、Value会被包装成Map.Entry内部类的属性

          * 一个键值对成为一个Map.Entry实例对象

          *

          */

         Set<Map.Entry<String, Student>> set1=hm.entrySet();

         //Set<Map<K,V>.Entry<K, V>> 注：Map后的泛型可不指明，不用写。

         for(Map.Entry<String, Student> mEntry:set1){

             System.out.println(mEntry.getKey()+":"+mEntry.getValue());

         }

     }

 }

 String str="abbcccddddeeeeeffffff";

         HashMap<String,Integer> hashMap=new HashMap<String,Integer>();

         for(int i=0;i<str.length();i++){

             if(!hashMap.containsKey(str.substring(i, i+1))){

                 hashMap.put(str.substring(i, i+1), 1);

             }else{

                 hashMap.put(str.substring(i,i+1), hashMap.get(str.substring(i, i+1))+1);

             }

         }

         System.out.println(hashMap);