package gg;

public class Course {

public String id;

public String name;

public Course(String id,String name)

{

this.id=id;

this.name=name;

}

}

package gg;

import java.util.HashSet;

import java.util.Set;

public class Student {

public String id;

public String name;

public Set courses;

public void Student(String id,String name)

{

this.id=id;

this.name=name;

this.courses=new HashSet();

}

}

package gg;

import java.util.ArrayList;

import java.util.List;

public class ListTest {

public List coursestoselect;

public void Listest()

{

this.coursestoselect=new ArrayList();

}

public void testAdd()

{

Course c1=new Course("1", "数学");

coursestoselect.add(c1);

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

System.out.println("添加课程："+temp.id+","+temp.name);

}

public static void main(String[] args) {

ListTest lt=new ListTest();

lt.testAdd();

}

}

为什么编译出错啊？

Exception in thread "main" java.lang.NullPointerException

at gg.ListTest.testAdd(ListTest.java:16)

at gg.ListTest.main(ListTest.java:24)

这是什么错误啊？