[問題] 有關object casting的問題

看板java作者paulcaptain (我不會寫程式)時間15年前 (2010/03/22 00:07)推噓2(2推 0噓 8→)

留言10則, 4人參與討論串1/2 (看更多)

參考了http://programming.im.ncnu.edu.tw/J_Chapter6.htm 我知道upcasting和downcasting的差異 Animal a1, a2, a3, a4; Bird b; Dog d; Fish f; a2 = a1 = new Animal(); b = new Bird(); d = new Dog(); f = new Fish(); System.out.println(a1.moveMethod()); System.out.println(b.moveMethod()); System.out.println(d.moveMethod()); System.out.println(f.moveMethod()); a1 = b; // Correct, we call this upcasting b = a1; // Compile Error, type not compatible b = (Bird)a1; // downcasting, Compile Correct a2 = b; // Correct,we call this upcasting d = a2; // Compile Error, type not compatible d = (Dog)a2; // Compile Correct, but runtime error 但是我有一個問題, 當我於主程式之內new一個B類別名為b, 且有一個副程式名為show(Object obj)比如 public static void show(Object obj) { if( obj instanceof A )//假設有繼承關係為真 { B newobj= (B)obj; newobj.show();//假設B類別有show()此方法 } } 然後在主程式呼叫此副程式show(b), 卻不會有runtime error?? object不是根類別嗎? 為何根類別強制轉型為子類別時(B newobj= (B)obj;) 不會出現runtime error? -- ※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 114.37.76.163

→

rmbrick

03/22 00:10, , 1^F

03/22 00:10, 1^F

→

paulcaptain

03/22 00:17, , 2^F

03/22 00:17, 2^F

→

TonyQ

03/22 00:21, , 3^F

03/22 00:21, 3^F