Java中的抽象类——创建方法

Question

我有一个关于 Java 中抽象类的问题。 我创建了这样的抽象类：

public class userClass {
private String email;
String accountType;
account account;

public userClass(String email, String accountType){
    this.setEmail(email);
    this.setAccountType(accountType);

     if(accountType.equals("basic")){
          this.account = new basicAccount();
     } 
     else if(accountType.equals("premium")){
          this.account = new premiumAccount();
     }
}

如您所见，我有两种类型的帐户：基本帐户和高级帐户，它们都是帐户抽象类的扩展。现在我只想将方法添加到高级课程，但是当我尝试这样做时，我收到一个错误，说我必须在帐户类中实现它，这是我不想要的。是否有可能以其他方式做到这一点？避免在 account 和 basicAccount 类中放置空方法？

Answer 1

如果你有一个Account 类，比如：

public abstract class Account{
    public abstract void doSomethingAccountLike();
    //more stuff
}

你可以有子类：

public class BasicAccount extends Account{
    public void doSomethingAccountLike(){
        //implementation specific to basic accounts
    }
}

public class PremiumAccount extends Account{
    public void doSomethingAccountLike(){
        //implementation specific to premium accounts
    }

    public void doSomethingPremiumLike(){
        //something that only makes sense
        // in the context of a premium account
    }
}

doSomethingPremiumLike() 方法仅在您有 PremiumAccount 的实例时可用，即：

public class AccountDemo{
    public static void main(String[] args){
        PremiumAccount premium = new PremiumAccount();
        BasicAccount basic = new BasicAccount();
        Account generalAccount = premium;

        //valid- the compiler knows that the
        //premium object is an instance of the
        //premium class
        premium.doSomethingPremiumLike();

        //Would cause a compile error if uncommented.
        //The compiler knows that basic is an instance
        //of a BasicAccount, for which the method
        //doSomethingPremiumLike() is undefined.
        //basic.doSomethingPremiumLike();

        //Would generate a compiler error if uncommented.
        //Even though generalAccount actually refers to
        //an object which is specifically a PremiumAccount,
        //the compiler only knows that it has a reference to
        //an Account object, it doesn't know that it's actually
        //specifically a PremiumAccount. Since the method
        // doSomethingPremiumLike() is not defined for a general
        //Account, this won't compile
        //generalAccount.doSomethingPremiumLike();

        //All compile- all are instances of Account
        //objects, and the method doSomethingAccountLike()
        //is defined for all accounts.
        basic.doSomethingAccountLike();
        premium.doSomethingAccountLike();
        generalAccount.doSomethingAccountLike();
    }
}

您的问题

在我看来，您的问题很可能是在 UserClass 类中，您有一个字段 Account account。在您的构造函数中，您将new BasicAccount() 或new PremiumAccount() 分配给字段account。这很好，但是一旦你这样做了，编译器在任何给定情况下都不再知道account 字段是指PremiumAccount 实例还是BasicInstance。这意味着如果你尝试调用account.doSomethingPremiumLike()，你会得到一个编译器错误。你可以在运行时绕过这个限制：

在UserClass某处：

if(account instanceof PremiumAccount){
    //if we're sure that account is actually a PremiumAccount,
    //cast it to a PremiumAccount here to let the compiler know
    //that doSomethingPremiumLike() can be called 
   ((PremiumAccount)account).doSomethingPremiumLike();
}

---

注意：我已将您的类名称更改为以大写字母开头，这是 Java 和大多数其他 OOP 语言中的通用约定。养成遵循此约定的习惯以帮助他人阅读您的代码是件好事。