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("請輸入學生id:");

String ID=console.next();

Student st=students.get(ID);

if(st==null){

System.out.println("請輸入學生姓名:");

String name = console.next();

Student newStudent = new Student(ID,name);

students.put(ID, newStudent);

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

i++;

}else{

System.out.println("該學生ID已被占用!");

continue;

}

}

}

// 測試Map keySet方法

public void testKeySet(){

Set<String> keySet = students.keySet();

System.out.println("總共有:"+students.size()+"個學生");

for (String stuId : keySet) {

Student st=students.get(stuId);

if(st!=null){

System.out.println("學生:"+st.name);

}

}

}

public void testRemove(){

System.out.println("請輸入要刪除的學生ID:");

Scanner sc = new Scanner(System.in);

while(true){

String ID = sc.next();

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

students.remove(ID);

System.out.println("成功刪除學生:"+students.get(ID).name);

break;

}else{

System.out.println("輸入的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();

}

}

請輸入學生id:

1

請輸入學生姓名:

aaa

成功添加學生:1:aaa

請輸入學生id:

2

請輸入學生姓名:

bbb

成功添加學生:2:bbb

請輸入學生id:

3

請輸入學生姓名:

ccc

成功添加學生:3:ccc

總共有:3個學生

學生:aaa

學生:bbb

學生:ccc

請輸入要刪除的學生ID:

7

輸入的ID不存在!

1

Exception in thread "main" java.lang.NullPointerException

at MapTest.testRemove(MapTest.java:54)

at MapTest.main(MapTest.java:79)