「Java」「abstract」 Chapter4 Question1

Given:

abstract class Account{
 abstract void deposit(double atm);
 public abstract boolean withdraw(double atm);
}

class SubAccount extends Account{
// @Override
// void deposit(double atm) {    
// }
//
// @Override
// public boolean withdraw(double atm) {
//  return false;
// }
}

What two changes, made independently,will enable the code to compile?

1. Change the signature of Account to: public class Account.
2. Change the signature of SubAccount to:public abstract class SubAccount.
3. Implement private methods for deposit and withdraw in SubAccount.
4. Implement public methods for deposit and withdraw in SubAccount.
5. Change Signature of SubAccount to: SubAccount implements Account.
6. Make Account an interface.

中文翻譯:

以下哪兩個選項能獨自變更並完成編譯?

1. 更改Account簽章成: public class Account (錯誤，因Account裡有abstract方法)
2. 更改SubAccount簽章成: public abstract class SubAccount(正確，因子類別SubAccount改成abstract，也就不需要覆寫方法)
3. 實作 修飾子為private 的 deposit 與 withdraw 方法(錯誤，因Account最高為default，而private權限更高於default。子類別的覆寫方法權限不可高於父類別 )
4. 實作 修飾子為public 的 deposit 與 withdraw 方法(正確，public權限高於覆類別 deposit 與 withdraw方法。)
5. 更改SubAccount簽章成: SubAccount implements Account(錯誤，Account不是interface)
6. 更改 Account 為 interface 介面。 (錯誤，且毫無異議)