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(); } /** * 添加課程信息 */ public void testAdd(){ Course cu1=new Course(0,"離散數(shù)學(xué)"); settest.add(cu1); Course cu2=new Course(1,"數(shù)據(jù)結(jié)構(gòu)"); settest.add(cu2); Course cu3=new Course(2,"近代史"); settest.add(cu3); Course cu4=new Course(3,"數(shù)據(jù)庫(kù)"); settest.add(cu4); Course cu5=new Course(5,"大學(xué)英語(yǔ)"); settest.add(cu5); } /** * 用for each方法循環(huán)遍歷輸出課程信息 */ public void testForEach(){ System.out.println("有如下待選課程："); //判斷是否為空 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(); }}運(yùn)行結(jié)果如下：Exception in thread "main" java.lang.NullPointerException at collection.SetTest.testAdd(SetTest.java:16) at collection.SetTest.main(SetTest.java:45)