英文:
Is it possible to change an object which was originally created as a subclass object to another subclass object?
问题
例如，我有一个类（Account）和两个子类（BasicAccount 和 PremiumAccount）。
如果我像这样创建一个对象：
Account account1 = new BasicAccount();
是否可能将 account1 的子类更改为 PremiumAccount？
英文:
For example, I have a Class (Account) and 2 subclasses (BasicAccount and PremiumAccount).
If I create an object like this
Account account1 = new BasicAccount();
Is it possible to change account1 subclass to PremiumAccount?
答案1
得分: 1
正如评论中所指出的，这从根本上是不可能的。
解决底层问题的实际方法是通过包信封-信件模式（也称为句柄-主体习语）。
也就是说，您创建一个包装器类，该类实现了公共接口，并将所有方法分派给可以重新赋值的实例变量。
至少，这看起来如下：
class AccountWrapper implements Account {
private Account instance;
private AccountWrapper(Account instance) {
this.instance = instance;
}
public static AccountWrapper createBasicAccount() {
return new AccountWrapper(new BasicAccount());
}
public static AccountWrapper createPremiumAccount() {
return new AccountWrapper(new PremiumAccount());
}
public void upgrade() {
if (instance instanceof PremiumAccount) throw new InvalidStateException();
this.instance = new PremiumAccount(instance); // 复制状态
}
// … 实现 Account 方法并转发到 `instance`。
}
然后您可以像这样使用它：
final AccountWrapper account = AccountWrapper.createBasicAccount();
// …
account.upgrade();
英文:
As noted in comments, that’s fundamentally not possible.
A practical solution to the underlying problem is via the envelope–letter pattern (also known as handle–body idiom).
That is, you create a wrapper class that implements the common interface and dispatches all methods to an instance variable which can be reassigned.
At minimum, this would look as follows:
class AccountWrapper implements Account {
private Account instance;
private AccountWrapper(Account instance) {
this.instance = instance;
}
public static AccountWrapper createBasicAccount() {
return new AccountWrapper(new BasicAccount());
}
public static AccountWrapper createPremiumAccount() {
return new AccountWrapper(new PremiumAccount());
}
public void upgrade() {
if (instance instanceof PremiumAccount) throw new InvalidStateException();
this.instance = new PremiumAccount(instance); // copy state
}
// … implement Account methods and forward to `instance`.
}
Then you can use it like this:
final AccountWrapper account = AccountWrapper.createBasicAccount();
// …
account.upgrade();
</details>
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