運行報錯，空指針異常，怎么解決

package com.XuanKe;

import java.util.List;
import java.util.ArrayList;

/**
?* 備選課
?* @author Administrator
?*
?*/
public class BeiXUanKeTest {
?private List CourseToSelect;

?public List getCourseToSelect() {
??return CourseToSelect;
?}
?public void setCourseToSelect(List courseToSelect) {
??this.CourseToSelect = new ArrayList();
?}
?public void CourseAdd(){//添加備選課程
??Course cr1=new Course();
??cr1.setId("1");
??cr1.setName("數據結構");
??CourseToSelect.add(cr1);
??Course temp =(Course) CourseToSelect.get(0);
??System.out.println("添加課程為："+temp.getId()+" :"+temp.getName());
?}
?public static void main(String[] args) {
??BeiXUanKeTest bx=new BeiXUanKeTest();
??bx.CourseAdd();
?}
?
}

package com.XuanKe;

import java.util.HashSet;
import java.util.Set;
/**
?* 學生類
?* @author Administrator
?*
?*/
public class Student {
?private String id;
?private String name;
?private Set course;
?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 Set getCourse() {
??return course;
?}
?public void setCourse(Set course) {
??this.course = new HashSet();
?}
?
}
package com.XuanKe;
/**
?* 課程類
?* @author Administrator
?*
?*/
public class Course {
?private String id;
?private String name;
?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;
?}
}

慕粉LH1324198599

2016-10-07

源自：Java入門第三季 4-4

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3 回答

大世界小人生回答被采納 +3 積分
2016-10-07

package?com.XuanKe;
import?java.util.List;
import?java.util.ArrayList;
/**
?*?備選課
?*?@author?Administrator
?*
?*/
public?class?BeiXUanKeTest?{
?private?List?CourseToSelect=new?ArrayList();//直接初始化，簡單明了，不會報異常
?//public?List?getCourseToSelect()?{
??//return?CourseToSelect;
?//}
//?public?void?setCourseToSelect(List?courseToSelect)?{
?//?this.CourseToSelect?=?new?ArrayList();
?//}
?public?void?CourseAdd(){//添加備選課程
??Course?cr1=new?Course();
??cr1.setId("1");
??cr1.setName("數據結構");
??CourseToSelect.add(cr1);
??Course?temp?=(Course)?CourseToSelect.get(0);
??System.out.println("添加課程為："+temp.getId()+"?:"+temp.getName());
?}
?public?static?void?main(String[]?args)?{
??BeiXUanKeTest?bx=new?BeiXUanKeTest();
??bx.CourseAdd();
?}
?
}
package?com.XuanKe;
import?java.util.HashSet;
import?java.util.Set;
/**
?*?學生類
?*?@author?Administrator
?*
?*/
public?class?Student?{
?private?String?id;
?private?String?name;
?private?Set?course;
?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?Set?getCourse()?{
??return?course;
?}
?public?void?setCourse(Set?course)?{
??this.course?=?new?HashSet();
?}
?
}
package?com.XuanKe;
/**
?*?課程類
?*?@author?Administrator
?*
?*/
public?class?Course?{
?private?String?id;
?private?String?name;
?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;
?}
}