2回答

• 

  竹青牙

  2017-05-25 09:56:49

  已采纳

  Course cr1 = new Course("1","数据结构");

  Course temp = (Course) coursesToSelect.get(0);

  coursesToSelect.add(cr1);

  顺序错了，代码执行是按照写的顺序来的，你还没加进去就要取出来，肯定不对，把 coursesToSelect.add(cr1); 跟

  Course temp = (Course) coursesToSelect.get(0);调换一下顺序就可以了。

  1 0

  清澄白河

  十分感谢！

  2017-05-26 06:29:15

  共 1 条回复 >

• 

  清澄白河

  2017-05-19 15:12:03

  写的代码如下：

  package com.imooc.collection;

  
  

  /**

   * 课程类

   * @author Administrator

   *

   */

  public class Course {

  
  

  public String id;

  public String name;

  public Course(String id,String name){

  this.id = id;

  this.name = name;

  }

  }

  =================================================

  package com.imooc.collection;

  
  

  import java.util.HashSet;

  import java.util.Set;

  /**

   * 学生类

   * @author Administrator

   *

   */

  public class Student {

  public String id;

  public String name;

  public Set courses;

  
  

  public Student(String id,String name){

  this.id = id;

  this.name = name;

  this.courses = new HashSet();

  }

  }

  ======================================

  package com.imooc.collection;

  
  

  import java.util.List;

  import java.util.ArrayList;

  
  

  /**

   * 备选课程类

   * @author Administrator

   *

   */

  public class ListTest {

  
  

  /**

  * 用于存放备选课程的list

  */

  public List coursesToSelect;

  public ListTest(){

  this.coursesToSelect = new ArrayList();

  }

  /**

  * 用于往coursesToSelect中添加备选课程

  */

  public void testAdd(){

  //创建一个课程对象，并且通过调用add方法，添加到备选课程list中

  Course cr1 = new Course("1","数据结构");

  Course temp = (Course) coursesToSelect.get(0);

  coursesToSelect.add(cr1);

  System.out.println("添加了课程:"+temp.id+":"+temp.name);

  Course cr2 = new Course("2","C语言");

  coursesToSelect.add(0,cr2);

  Course temp2 = (Course)coursesToSelect.get(0);

  System.out.println("添加了课程:"+temp2.id+":"+temp2.name);

  }

  public static void main(String[] args){

  ListTest lt = new ListTest();

  lt.testAdd();

  }

  }

  
  

  0 1