在 Java 中使用通用参数覆盖方法？
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Overriding a method with Generic Parameters in Java?
提问by Robert Ngetich
I have an abstract Class Monitor.javawhich is subclassed by a Class EmailMonitor.java.
我有一个抽象类Monitor.java，它是由类EmailMonitor.java子类化的。
The method:
方法：
public abstract List<? extends MonitorAccount> performMonitor(List<? extends MonitorAccount> accounts)
is defined in Monitor.javaand must be overridden in EmailMonitor.java.
在Monitor.java 中定义并且必须在EmailMonitor.java 中覆盖。
I currently have the method overridden in EmailMonitor.javaas follows:
我目前在EmailMonitor.java 中重写了该方法，如下所示：
@Override
public List<EmailAccount> performMonitor(List<EmailAccount> emailAccounts) {
//...unrelated logic
return emailAccounts;
}
However, this produces the compile time error:
但是，这会产生编译时错误：
Name clash: The method performMonitor(List<EmailAccount>) of type EmailMonitor has the same erasure as performMonitor(Lis<? extends MonitorAccount> emailAccounts) of type Monitor but does not override it
Name clash: The method performMonitor(List<EmailAccount>) of type EmailMonitor has the same erasure as performMonitor(Lis<? extends MonitorAccount> emailAccounts) of type Monitor but does not override it
EmailAccountis a subclass of MonitorAccount, so (in my mind at least) overriding it in this way makes perfect sense. Seeing as the compiler is not happy with my logic though, How should I go about this correctly while still keeping my compile time checks to make sure that all calls to EmailMonitor.performMonitor()receive Lists of EmailAccountrather than some other type of MonitorAccount?
EmailAccount是 的子类MonitorAccount，所以（至少在我看来）以这种方式覆盖它是完全合理的。看到编译器对我的逻辑不满意，我应该如何正确地解决这个问题，同时仍然保持我的编译时检查以确保所有调用都是EmailMonitor.performMonitor()接收列表的EmailAccount而不是其他类型的MonitorAccount？
采纳答案by Jon Skeet
No, it's not overriding it properly. Overriding means you should be able to cope with any valid input to the base class. Consider what would happen if a client did this:
不，它没有正确覆盖它。覆盖意味着您应该能够处理对基类的任何有效输入。考虑如果客户这样做会发生什么：
Monitor x = new EmailMonitor();
List<NonEmailAccount> nonEmailAccounts = ...;
x.performMonitor(nonEmailAccounts);
There's nothing in there which should give a compile-time error given your description - but it's clearly wrong.
根据您的描述，那里没有任何东西应该给出编译时错误 - 但这显然是错误的。
It sounds to me like Monitorshould be generic in the type of account it can monitor, so your EmailMonitorshould extend Monitor<EmailAccount>. So:
在我看来Monitor，它可以监控的帐户类型应该是通用的，因此您EmailMonitor应该扩展Monitor<EmailAccount>. 所以：
public abtract class Monitor<T extends MonitorAccount>
{
...
public abstract List<? extends T> performMonitor(
List<? extends T> accounts);
}
public class EmailMonitor extends Monitor<EmailAccount>
{
@Override
public abstract List<? extends EmailAccount> performMonitor(
List<? extends EmailAccount> accounts)
{
// Code goes here
}
}
You might want to think carefully about the generics in the performMonitorcall though - what's the return value meant to signify?
不过，您可能需要仔细考虑performMonitor调用中的泛型——返回值意味着什么？
回答by Robert Ngetich
Here is my own solution. I suspect this is the same thing Jon Skeet was trying to get at... without the typo (see my comment in reply to his answer).
这是我自己的解决方案。我怀疑这与 Jon Skeet 试图解决的问题相同......没有打字错误（请参阅我对他的回答的评论）。
the Monitor.javaclass:
在Monitor.java类：
public abstract class Monitor <T extends MonitorAccount> {
...
public abstract List<T> performMonitor(List<T> accounts);
..
}
EmailMonitor.java
电子邮件监视器.java
public class EmailMonitor extends Monitor<EmailAccount> {
...
public List<EmailAccount> performMonitor(List<EmailAccount> emailAccounts) {
..//logic...logic...logic
return emailAccounts;
}
...
}
In this configuration, EmailMonitor.performMonitor()will always check at compile time that it receives a list of EmailAccountrather than any of my other types FTPAccount, DBAccount,etc... It's much cleaner than the alternative, which would have been receiving/sending a raw list and then having to coerce it the required type resulting in potential runtime type casting exceptions.
在此配置中，EmailMonitor.performMonitor()将始终在编译时检查它是否接收到一个EmailAccount列表，而不是我的任何其他类型的FTPAccount、DBAccount等......它比替代方案更清晰，后者本来会接收/发送原始列表，然后必须将其强制为所需类型，从而导致潜在的运行时类型转换异常。
