原题附上:(为了规范略做改动)
class Hobbits{ String name; } public class HobbitsTest{ public static void main(String[] args){ Hobbits [] h=new Hobbits[3]; int z=0; while (z<4) { z=z+1; h[z]=new Hobbits(); h[z].name="bilbo"; if (z==1) { h[z].name="frodo"; } if (z==2) { h[z].name="sam"; } System.out.println(h[z].name+" is a good Hobbit name"); } } }
问题:
1、代码中h[z]其实也是个引用,否则不能调用name,因此缺少h[z]对象引用的声明 h[z]=new Hobbits();
2、数组是按照h[0]、h[1]、h[2]的顺序,而while循环中却是按h[1]、h[2]、h[3]的顺序,这样导致h[3]为空,会报错:
Exception in thread "main" java.lang.NullPointerException at HobbitsTest.main(HobbitsTest.java:12)
修改:
有两种改法:
1)z从0开始,循环条件改成z<3,z=z+1放在循环的最后一步
2)z从-1开始,循环条件改成z<2,z=z+1位置不变
修改后的代码:
1)
class Hobbits{ String name; } public class HobbitsTest{ public static void main(String[] args){ Hobbits [] h=new Hobbits[3]; int z=0; while (z<3) { h[z]=new Hobbits(); h[z].name="bilbo"; if (z==1) { h[z].name="frodo"; } if (z==2) { h[z].name="sam"; } System.out.println(h[z].name+" is a good Hobbit name"); z=z+1; } } }
2)
class Hobbits{ String name; } public class HobbitsTest{ public static void main(String[] args){ Hobbits [] h=new Hobbits[3]; int z=-1; while (z<2) { z=z+1; h[z]=new Hobbits(); h[z].name="bilbo"; if (z==1) { h[z].name="frodo"; } if (z==2) { h[z].name="sam"; } System.out.println(h[z].name+" is a good Hobbit name"); } } }
运行结果是一样的:
bilbo is a good Hobbit name
frodo is a good Hobbit name
sam is a good Hobbit name