怎么回事，一樣的啊

package?collection_map_List;

import?java.util.ArrayList;
import?java.util.Arrays;
import?java.util.List;
import?java.util.Scanner;

public?class?SetTest?{
public?static?List<Course>?courses;
public?SetTest()?{
this.courses=new?ArrayList<Course>();
}
public?void?addcourse()?{
Course?course1=new?Course("0102",?"數(shù)學(xué)");
Course?course2=new?Course("0103",?"數(shù)據(jù)結(jié)構(gòu)");
courses.add(course1);
courses.add(course2);

Course?c1=new?Course("2",?"英語");	??
courses.add(2,c1);
}
public?void?add2(){
Course[]?course={?new?Course("001",?"english"),new?Course("002",?"math")};
???//2.1、addAll()添加方式
courses.addAll(Arrays.asList(course));
Course[]?c1={?new?Course("05",?"心理學(xué)"),new??Course("06",?"歷史")};
courses.addAll(Arrays.asList(c1));
}
public?void?allCourse(){	???//遍歷集合
for(Course?c:?courses){
System.out.println(c.id+"\t"+c.name);
}
}
public?static?void?stuCourse(Student?student1){
for(Course?course?:?student1.course){
System.out.println(course.id+","+course.name);
}
}
public?static?void?main(String[]?args){
Scanner?scanner=new?Scanner(System.in);
SetTest?lt=new?SetTest();
lt.addcourse();
lt.add2();
System.out.println("....所有課程清單.....................");
lt.allCourse();
System.out.println(".................................");

Student?student1=new?Student("311309030","噠噠");
System.out.println("···········輸入要添加課程的編號(hào)············");
for(int?i=0;i<3;i++){
String?sc=scanner.next();
for(Course?cou:courses){
if(cou.id.equals(sc)){
//System.out.println(cou.name);
student1.course.add(cou);
if(i<2){
System.out.println("成功添加一門課程，請(qǐng)繼續(xù)添加課程：");
}
else{
System.out.println("添加課程完畢！");
}
}
}
}
System.out.println("學(xué)生:"+student1.id+"\t"+student1.name+"?的課程有：");
stuCourse(student1);
}

}


001	english
002	math
05	心理學(xué)
06	歷史
.................................
···········輸入要添加課程的編號(hào)············
001
Exception?in?thread?"main"?java.lang.NullPointerException
	at?collection_map_List.SetTest.main(SetTest.java:55)
	
為什么?一添加課程?報(bào)空指針錯(cuò)誤，55行

慕仰3473226

2016-08-09

源自：Java入門第三季 4-11

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2 回答

慕粉3236370
2016-08-26

/**
?*?課程類
?*?@author?Administrator
?*
?*/
public?class?Course?{

	public?String?id;
	
	public?String?name;
	
	public?Course(String?id,?String?name)?{
		this.id?=?id?;
		
		this.name?=?name;
	}
	
	public?Course()?{
		
	}
}


import?java.util.HashSet;
import?java.util.Set;

/**
?*?學(xué)生類
?*?@author?Administrator
?*
?*/
public?class?Student?{

	public?String?id;
	
	public?String?name;
	
	public?Set<Course>?courses;

	public?Student(String?id,?String?name)?{
		this.id?=?id;
		this.name?=?name;
		this.courses?=?new?HashSet<Course>();
	}
}

35.for(Course course : student1.courses){

55.student1.courses.add(cou);

....所有課程清單.....................

0102 數(shù)學(xué)

0103 數(shù)據(jù)結(jié)構(gòu)

2 英語

001 english

002 math

05 心理學(xué)

06 歷史

.................................

···········輸入要添加課程的編號(hào)············