6 abstract class PhoneCard double balance; abstract boolean performDialO; double getBalance() return balance;} abstract class None Number PhoneCard extends PhoneCard String phoneSetType; String getSetType() return phoneSetType; 返回6-5 Programming in Java abstract class PhoneCard { double balance; abstract boolean performDial(); double getBalance() { return balance; } } Example(1) abstract class None_Number_PhoneCard extends PhoneCard { String phoneSetType; String getSetType() { return phoneSetType; } } 返回