二Xap23 6 class D200 Card extends Number PhoneCard { double balance;//Number PhoneCard double balance; double additoryFee; boolean performDialO if (balance >(0.5+additoryFee)) { balance -=(0.5 additoryFee); return true; else reurn false; } 返回6-10 Programming in Java class D200_Card extends Number_PhoneCard { double balance; double additoryFee; boolean performDial() { if (balance >(0.5+additoryFee)) { balance -= (0.5 + additoryFee); return true; } else reurn false; } } Example(3) 返回 double balance;//Number_PhoneCard