?student st=students.get(ID);package com.zhou.jihe;import java.util.HashMap;import java.util.Map;import java.util.Scanner;import java.util.Set;public class testMap {public Map<String,student> students;public void testMap(){??? this.students=new HashMap<String,student>();?}public void testput()?{ ??Scanner console=new Scanner(System.in);?int i=0;?while(i<3)?{??System.out.println("請輸入學生ID");??String ID=console.next();??student st=students.get(ID);??//System.out.println(st);??if(st==null)??{???System.out.println("請輸入學生的名字");?String name=console.next();?student newStudent=new student(ID,name);?students.put(ID, newStudent);?System.out.println("成功添加學生");?i++;? ?????}??else??{System.out.println("改ID已經被占用");??continue;??}?}??}public static void main(String[] args) {?testMap tm=new testMap();?tm.testput();}public void testKeySet(){?Set<String> keyset=students.keySet();?for(String stuID:keyset)?{??student st=students.get(stuID);?} ?}}請輸入學生ID1Exception in thread "main" java.lang.NullPointerException?at com.zhou.jihe.testMap.testput(testMap.java:24)?at com.zhou.jihe.testMap.main(testMap.java:46)