【java】HashMap、Map、Set、HashMap.put()、HashMap.keySet()、HashMap.entrySet()、Map.Entry内部类

 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;
     }
 }

Student

 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());
         }
     }
 }

HashMapDemo

通过HashMap计算字符串中各字母出现次数：

 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);

snippet