package?com.hu;?

public?class?Vehicle?{
???public?int?num;???
???public?String?mode;???
???public?String?type;?
?????????
???public?void?ability(){?
??????????System.out.println("交通工具具有載人出行的能力");????????????????
??????????System.out.println("num:"+num+"\nmode:"+mode+"\ntype:"+type);????????????
??????????}????????
??}???
????????????????
public?class?Car?extends?Vehicle?{?
???public?int?num=6;?????
???public?String?mode="陸行";??????
???public?String?type="汽車";??
???????????
???public?void?ability(){
???????????????System.out.println("汽車具有載人陸行的能力");??????????????????
???????????????System.out.println("num:"+num+"\nmode"+mode+"\ntype"+type);????????????????
???????????????}????????????????????
???????????????@Override????????????????????????
???????????????public?String?toString()?{????????????????????????
???????????????return?"Car?[num="?+?num?+?",?mode="?+?mode?+?",?type="?+?type?+?"]";????????????????????????
???????????????}?
???????????????
???????????????public?class?Initail?{???public?static?void?main(String[]?args)?{?
??????????????????????????????Vehicle?vehicle?=?new?Vehicle();????????????????
??????????????????????????????Vehicle?car?=?new?Car();??
????????????????????????????????????????????????????????????????????????????
???????????????????????????????vehicle.ability();??????????????????
???????????????????????????????System.out.println();???
????????????????????????????????????????????????????????????????????
???????????????????????????????car.ability();??????????????????
???????????????????????????????System.out.println();????????????????????????????????????
???????????????????????????????System.out.println(car.num+"\n"+car.mode+"\n"+car.type);//就是這行
???????????????????????????????????????????????
???????????????????????????????System.out.println();????????????????????????????????????????????????????
???????????????????????????????System.out.println(car);????????????????????????
???????????????????????????????}????????????????
???????????????????????????????}

為什么

System.out.println(car.num+"\n"+car.mode+"\n"+car.type);

這句輸出的是

0

null

null

而不是

6

陸行

汽車