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。