代码是按照老师的写的，运行到student st=students.get(ID);抛出异常

请输入学生ID
1
Exception in thread "main" java.lang.NullPointerException
 at com.zhou.jihe.testMap.testput(testMap.java:24)
 at com.zhou.jihe.testMap.main(testMap.java:43)

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("请输入学生ID");
  String ID=console.next();
  student st=students.get(ID);
   if(st==null)
  {
   System.out.println("请输入学生的名字");
 String name=console.next();
 student newStudent=new student(ID,name);
 students.put(ID, newStudent);
 System.out.println("成功添加学生"+students.get(ID).name);
 i++;    
  }
  else
  {System.out.println("改ID已经被占用");
  continue;
  }
 }
 
 }
public static void main(String[] args) {
 testMap tm=new testMap();
 tm.testput();
 tm.testKeySet();
// student s=new student("1","小明");
// System.out.println(s.id);
}
public void testKeySet()
{
 Set<String> keyset=students.keySet();
 System.out.println("总共有"+students.size()+"个学生");
 for(String stuID:keyset)
 {
  student st=students.get(stuID);
  if(st!=null)
  {
   System.out.println("学生："+st.name);
  }
 }
 }
}

求大神帮忙解答