import java.util.HashMap;

import java.util.Map;

import java.util.Map.Entry;

import java.util.Scanner;

import java.util.Set;

public class MapTest {

public Map<String,Student> students;

public MapTest(){

this.students=new HashMap<String,Student>();

}

public void put(){

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("请输入学生姓名:");

String name = console.next();

Student newStudent = new Student(ID,name);

students.put(ID, newStudent);

System.out.println("成功添加学生:"+newStudent.id+":"+newStudent.name);

i++;

}else{

System.out.println("该学生ID已被占用!");

continue;

}

}

}

// 测试Map keySet方法

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);

}

}

}

public void testRemove(){

System.out.println("请输入要删除的学生ID:");

Scanner sc = new Scanner(System.in);

while(true){

String ID = sc.next();

if(students.get(ID) != null){

students.remove(ID);

System.out.println("成功删除学生:"+students.get(ID).name);

break;

}else{

System.out.println("输入的ID不存在!");

continue;

}

}

}

public void testEntrySet(){

Set<Entry<String,Student>> entrySet = students.entrySet();

for (Entry<String, Student> entry : entrySet) {

System.out.println("取得key:"+entry.getKey());

System.out.println("取得value"+entry.getValue().name);

}

}

public static void main(String[] args){

MapTest mt=new MapTest();

mt.put();

mt.testKeySet();

mt.testRemove();

mt.testEntrySet();

}

}

请输入学生id:

1

请输入学生姓名:

aaa

成功添加学生:1:aaa

请输入学生id:

2

请输入学生姓名:

bbb

成功添加学生:2:bbb

请输入学生id:

3

请输入学生姓名:

ccc

成功添加学生:3:ccc

总共有:3个学生

学生:aaa

学生:bbb

学生:ccc

请输入要删除的学生ID:

7

输入的ID不存在!

1

Exception in thread "main" java.lang.NullPointerException

at MapTest.testRemove(MapTest.java:54)

at MapTest.main(MapTest.java:79)