【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;
    }    
}

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());
        }
    }
}

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 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);