按照老师教的，为什么会报错？
张帆9
2016-11-09 09:28
错误如图所示：
14回答
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- 逐梦流星
- 2017-03-16 18:09:17
把coursesToSelect全部改成couresToSelect试试
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- 慕粉4353249
- 2016-12-12 19:33:34
你的包到错了，把import java.util.ArrayDeque; 改为：import java.util.Arrays;
将代码中ArrayDeque.aslist改为Arrays.asList
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- 晴颜
- 2016-12-06 22:27:05
你看你的报错信息指向第38行，你的38addAll()这个括号里面为什么有数字2？
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- 张帆9
- 2016-11-10 16:03:46
最终将系统重装，就好了！奇怪！
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- TommySEA
- 2016-11-10 03:41:02
空指针异常了，你试一试把导入的：import java.util.ArrayDeque; 改为：import java.util.Arrays; 再运行看看
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- 张帆9
- 2016-11-09 14:16:50
把你的全盘复制到我的eclipse运行，还是报错！
Exception in thread "main" java.lang.NullPointerException
at java.util.ArrayList.addAll(ArrayList.java:577)
at com.imooc.collection.ListTest.testAdd(ListTest.java:31)
at com.imooc.collection.ListTest.main(ListTest.java:38)
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- 半度微凉非比晴空
- 2016-11-09 14:13:01
package com.imooc.collection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;/**
* 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");
coursesToSelect.add(cr1);
Course temp = (Course) coursesToSelect.get(0);
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);
coursesToSelect.add(cr1);
Course temp0 = (Course) coursesToSelect.get(2);
System.out.println("添加了课程：" + temp.id + ":" + temp.name);
//以下方法会抛出数组下标越界异常
// Course cr3 = new Course("3", "test");
// coursesToSelect.add(4, cr3);
Course[] course = {new Course("3", "离散数学"), new Course("4", "汇编语言")};
coursesToSelect.addAll(Arrays.asList(course));
Course temp3 = (Course) coursesToSelect.get(2);
Course temp4 = (Course) coursesToSelect.get(3);
System.out.println("添加了两门课程：" + temp3.id + ":" +
temp3.name + ";" + temp4.id + ":" + temp4.name);
Course[] course2 = {new Course("5", "高等数学"), new Course("6", "大学英语")};
coursesToSelect.addAll(2, Arrays.asList(course2));
Course temp5 = (Course) coursesToSelect.get(2);
Course temp6 = (Course) coursesToSelect.get(3);
System.out.println("添加了两门课程：" + temp5.id + ":" +
temp5.name + ";" + temp6.id + ":" + temp6.name);
}
/**
* 取得List中的元素的方法
* @param args
*/
public void testGet() {
int size = coursesToSelect.size();
System.out.println("有如下课程待选：");
for(int i= 0 ; i < size; i++) {
Course cr = (Course) coursesToSelect.get(i);
System.out.println("课程：" + cr.id + ":" + cr.name);
}
}
/**
* 通过迭代器来遍历List
* @param args
*/
public void testIterator() {
//通过集合的iterator方法，取得迭代器的实例
Iterator<Course> it = coursesToSelect.iterator();
System.out.println("有如下课程待选(通过迭代器访问)：");
while(it.hasNext()) {
Course cr = it.next();
System.out.println("课程：" + cr.id + ":" + cr.name);
}
}
/**
* 通过for each方法访问集合元素
* @param args
*/
public void testForEach() {
System.out.println("有如下课程待选(通过for each访问)：");
for (Object obj : coursesToSelect) {
Course cr = (Course) obj;
System.out.println("课程：" + cr.id + ":" + cr.name);
}
}
/**
* 修改List中的元素
* @param args
*/
public void testModify() {
coursesToSelect.set(4, new Course("7", "毛概"));
}
/**
* 删除List中的元素
* @param args
*/
public void testRemove() {
// Course cr = (Course) coursesToSelect.get(4);
System.out.println("即将删除4位置和5位置上的课程！");
Course[] courses = {(Course) coursesToSelect.get(4), (Course) coursesToSelect.get(5)};
coursesToSelect.removeAll(Arrays.asList(courses));
// coursesToSelect.remove(4);
System.out.println("成功删除课程！");
testForEach();
}
/**
* 往List中添加一些奇怪的东西
* @param args
*/
public void testType() {
System.out.println("能否往List中添加一些奇怪的东西呢！？");
coursesToSelect.add("我不是课程，我只是一个无辜的字符串！！");
}
public static void main( String[] args) {
ListTest lt = new ListTest();
lt.testAdd();
lt.testType();
lt.testForEach();
// lt.testGet();
// lt.testIterator();
// lt.testForEach();
// lt.testModify();
// lt.testForEach();
// lt.testRemove();
}
} -
- 半度微凉非比晴空
- 2016-11-09 14:09:01
这是字符串问题
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- 半度微凉非比晴空
- 2016-11-09 14:05:59
package com.imooc.collection; import java.util.Arrays; import java.util.List; import java.util.ArrayList; import java.util.Scanner; public class SetTest { public List<Course> coursesToSelect; public SetTest(){ coursesToSelect=new ArrayList<Course>(); } public void testAdd() { //创建一个课程对象，并通过调用add方法，添加到备选课程List中 Course cr1 = new Course("1" , "数据结构"); coursesToSelect.add(cr1); Course temp = (Course) coursesToSelect.get(0); //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); //coursesToSelect.add(cr1); //Course temp0 = (Course) coursesToSelect.get(2); //System.out.println("添加了课程：" + temp.id + ":" + temp.name); //以下方法会抛出数组下标越界异常 // Course cr3 = new Course("3", "test"); // coursesToSelect.add(4, cr3); Course[] course = {new Course("3", "离散数学"), new Course("4", "汇编语言")}; coursesToSelect.addAll(Arrays.asList(course)); Course temp3 = (Course) coursesToSelect.get(2); Course temp4 = (Course) coursesToSelect.get(3); //System.out.println("添加了两门课程：" + temp3.id + ":" + // temp3.name + ";" + temp4.id + ":" + temp4.name); Course[] course2 = {new Course("5", "高等数学"), new Course("6", "大学英语")}; coursesToSelect.addAll(2, Arrays.asList(course2)); Course temp5 = (Course) coursesToSelect.get(2); Course temp6 = (Course) coursesToSelect.get(3); // System.out.println("添加了两门课程：" + temp5.id + ":" + //temp5.name + ";" + temp6.id + ":" + temp6.name); } /** * 通过for each方法访问集合元素 * @param args */ public void testForEach() { System.out.println("有如下课程待选(通过for each访问)："); for (Object obj : coursesToSelect) { Course cr = (Course) obj; System.out.println("课程：" + cr.id + ":" + cr.name); } } /** * @param args */ public static void main(String[] args) { SetTest st=new SetTest(); st.testAdd(); st.testForEach(); //创建一个新的学生对象 Student student=new Student("1","小明"); System.out.println("欢迎学生:"+student.name+"选课！"); //创建一个Scanner对象，用来接收从键盘输入的课程ID Scanner console=new Scanner(System.in); for(int i=0;i<3;i++){ System.out.println("请输入课程ID"); String courseId=console.next(); for(Course cr:st.coursesToSelect){ if(cr.id.equals(courseId)){ student.courses.add(cr); /** * Set中，添加某一个对象 * 无论添加多少次，最终只会保留一个该对象的引用 * 并且保留的的是第一次添加的那个 */ //student.courses.add(cr); } } } st.testForEachSet(student); } public void testForEachSet(Student student){ //打印输出，学生所选的课程! System.out.println("共选择了:"+student.courses.size()+"门课程!"); for(Course cr:student.courses){ System.out.println("选择了课程:"+cr.id+":"+cr.name); } // TODO Auto-generated method stub } } -
- 张帆9
- 2016-11-09 13:43:02
The import java.util.Arrays cannot be resolved
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- 张帆9
- 2016-11-09 13:33:47
package com.imooc.collection;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;
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");
coursesToSelect.add(cr1);
Course temp=(Course) coursesToSelect.get(0);
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);
Course[] course={new Course("3","离散数学"),new Course("4","汇编语言")};
coursesToSelect.addAll(ArrayDeque.aslist(course));
Course temp3=(Course) coursesToSelect.get(2);
Course temp4=(Course) coursesToSelect.get(3);
System.out.println("添加了两门课程："+temp3.id+":"+temp3.name+";"+temp4.id+":"+temp4.name);
Course[] course2={new Course("5","高等数学"),new Course("6","大学英语")};
coursesToSelect.addAll(2,ArrayDeque.aslist(course2));
Course temp5=(Course) coursesToSelect.get(2);
Course temp6=(Course) coursesToSelect.get(3);
System.out.println("添加了两门课程："+temp5.id+":"+temp5.name+";"+temp6.id+":"+temp6.name);
}
public static void main(String[] args){
ListTest lt=new ListTest();
lt.testAdd();
}
}
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- 半度微凉非比晴空
- 2016-11-09 12:16:31
导入import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.Scanner; -
- 半度微凉非比晴空
- 2016-11-09 12:15:32
导入包
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- HHHHHHyyyy
- 2016-11-09 11:24:21
能不能发一下ListTest的源码。。互相学习
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