// 創(chuàng)建新的學(xué)生對(duì)象

Student newStudent = new Student(ID, name);

// 通過(guò)調(diào)用students的put方法，添加ID-學(xué)生映射

students.put(ID, newStudent);

為什么新建的學(xué)生對(duì)象里面已經(jīng)有了ID和name 兩個(gè)參數(shù)了。在put方法里面還要添加ID這個(gè)屬性？

個(gè)人感覺(jué)得你的問(wèn)不是很大，你仔細(xì)看看 其實(shí)你在沒(méi)有添加對(duì)象的時(shí)候就開(kāi)始從student的對(duì)象中取元素了！你再看看TestPut方法中這句STUDENT ST=Students.get(ID)的位置！

package student;

import java.util.HashMap;

import java.util.Map;

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 Testput(){

Scanner console=new Scanner(System.in);

int i=0;

while(i<3){

System.out.println("請(qǐng)輸入學(xué)生ID:");

String ID=console.next();

STUDENT ST=Students.get(ID);

if(ST==null){

System.out.println("請(qǐng)輸入學(xué)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(xué)生："+Students.get(ID).getName());

i++;

}

else{

System.out.println("該ID已被占用！");

continue;

}

}

}

public void Testkeyset(){

Set<String> Keyset=Students.keySet();

System.out.println("總共有："+Keyset.size());

for(String stuID:Keyset){

STUDENT st=Students.get(stuID);

if(st!=null){

System.out.println("學(xué)生："+st.getName());

}

}

}

public static void main(String[] args) {

Maptest mp=new Maptest();

mp.Testput();

mp.Testkeyset();

}

}

package student;

import java.util.HashSet;

import java.util.Set;

public class STUDENT {

private String id;

private String name;

public static Set<CLASS> Class;

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;

}

public STUDENT(String id,String name){

this.id=id;

this.name=name;

this.Class=new HashSet<CLASS>();

}

}

STUDENT類(lèi)

解決了，，在STUDENT類(lèi)里的Class加了一個(gè)static就解決了，，雖然不知為什么得加static

System.out.println("成功添加學(xué)生："+Students.get(ID).getName());

請(qǐng)輸入學(xué)生ID:

1

請(qǐng)輸入學(xué)生姓名：

小明

Exception in thread "main" java.lang.NullPointerException

at student.Maptest.Testput(Maptest.java:25)

at student.Maptest.main(Maptest.java:49)