用Java中的通用参数重写一个方法？

问题描述

我有一个抽象类 Monitor.java ，它被Class EmailMonitor.java 分类。

方法：

  public abstract List   

EmailMonitor.java 。

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）{
 //...无关逻辑
返回emailAccounts; 
} 
  

然而，这会产生编译时错误： $ b $名称冲突：类型为EmailMonitor的方法performMonitor（List< EmailAccount>）与Monitor类型的performMonitor（Lis<？extends MonitorAccount> emailAccounts）具有相同的擦除，但不包含覆盖它

EmailAccount 是 MonitorAccount ，所以（至少在我看来）以这种方式覆盖它是非常有意义的。虽然看到编译器对我的逻辑不满意，但我应该如何正确地进行此操作，同时仍然保持编译时间检查以确保对 EmailMonitor.performMonitor（）接收 EmailAccount 列表，而不是某些其他类型的 MonitorAccount ？

解决方案

不，它不会正确覆盖它。覆盖意味着你应该能够处理任何有效的基类输入。考虑一下如果客户这样做会发生什么：

列表< NonEmailAccount> nonEmailAccounts = ...;
x.performMonitor（nonEmailAccounts）;

在你的描述中，没有任何东西会给编译时错误带来麻烦 - 但这显然是错误的。

听起来像 Monitor 应该是它可以监控的账户类型的通用名称，所以您的 EmailMonitor 应该扩展 Monitor< EmailAccount> 。所以：

  public abtract class Monitor< T extends MonitorAccount> 
 {
 ... 
 public abstract List  List< ;? extends T>帐户）; 
} 
 
公共类EmailMonitor扩展了Monitor< EmailAccount> 
 {
 @Override 
 public abstract List  List< ;? extends EmailAccount>账户）
 {
 // Code goes here 
} 
} 
  

您可能想仔细考虑 performMonitor 调用中的泛型 - 返回值是什么意思来表示？

I have an abstract Class Monitor.java which is subclassed by a Class EmailMonitor.java.

The method:

public abstract List<? extends MonitorAccount> performMonitor(List<? extends MonitorAccount> accounts)

is defined in Monitor.java and must be overridden in EmailMonitor.java.

I currently have the method overridden in EmailMonitor.java as follows:

@Override
public List<EmailAccount> performMonitor(List<EmailAccount> emailAccounts) {
    //...unrelated logic
    return emailAccounts;
}

However, this produces the compile time error:

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

EmailAccount is 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 EmailAccount rather than some other type of MonitorAccount?

解决方案

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:

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.

It sounds to me like Monitor should be generic in the type of account it can monitor, so your EmailMonitor should extend Monitor<EmailAccount>. So:

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 performMonitor call though - what's the return value meant to signify?

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