练习哈希集 ，不能循环输出，求指点

Question

public class Course { int id; String name; public Course(int id,String name){ this.id=id; this.name=name; } }import java.util.HashSet;import java.util.Set;public class SetTest { public Set settest; public void SetTest(){ settest=new HashSet(); } /** * 添加课程信息 */ public void testAdd(){ Course cu1=new Course(0,"离散数学"); settest.add(cu1); Course cu2=new Course(1,"数据结构"); settest.add(cu2); Course cu3=new Course(2,"近代史"); settest.add(cu3); Course cu4=new Course(3,"数据库"); settest.add(cu4); Course cu5=new Course(5,"大学英语"); settest.add(cu5); } /** * 用for each方法循环遍历输出课程信息 */ public void testForEach(){ System.out.println("有如下待选课程："); //判断是否为空 while(settest.isEmpty()){ for(Object ob:settest){ Course cu=(Course) ob; System.out.println(cu.id+":"+cu.name); } } } public static void main(String[] args) { SetTest set=new SetTest(); set.testAdd(); set.testForEach(); }}运行结果如下：Exception in thread "main" java.lang.NullPointerException at collection.SetTest.testAdd(SetTest.java:16) at collection.SetTest.main(SetTest.java:45)

Answer 1

public void SetTest(){
    settest=new HashSet();
}  
改成
public SetTest() {
    settest = new HashSet();
}
不然类加载的时候找不到无参构造，就不能new HashSet();就会报空指针。