Java：允许带有自定义注释的对象的集合

发布于 2024-09-27 19:27:49 字数 421 浏览 2 评论 0原文

我想用 @MyEntity 注释来注释一些类

public @interface MyEntity {}

@MyEntity
public class MyClass { ... }

，并定义一个集合，其中只允许具有该注释的类（无需将它们定义为 public class MyClass Implements XXX）：

List<MyEntity> list = new ArrayList<MyEntity>();
list.add(new MyClass())

上面的代码导致编译错误“ArrayList 类型中的方法 add(MyEntity) 不适用于参数 (MyClass)”。有没有办法定义一个集合，只允许具有给定注释的对象？

原文

I'd like to annotate some classes with a @MyEntity annotation

public @interface MyEntity {}

@MyEntity
public class MyClass { ... }

And define a collection where only classes with that annotation are allowed (with no need to define them as public class MyClass implements XXX):

List<MyEntity> list = new ArrayList<MyEntity>();
list.add(new MyClass())

The above code results in a complation error "The method add(MyEntity) in the type ArrayList is not applicable for the arguments (MyClass)". Is there a way to define a collection that only allows objects with a given annotation?

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山人契 2024-10-04 19:27:49

简短的回答是否定的。

您的问题是 List 定义了 MyEntity 或其子类的列表（即，如果我们有 @interface AnotherEntity extends MyEntity 那么我们可以放置 AnotherEntity 到此列表）。

类 MyClass 不扩展/实现 MyEntity，它是用它注释的。

即使有可能，效率也不会很高。您不知道哪些方法或字段可用，MyEntity 不描述对象的接口。因此，它唯一的用途就是过滤错误的插入。您可以轻松地实现它，提供您的 List 实现：

List<?> myEntities = new LinkedList<?> {
   @Override 
   public boolean add(Object obj) {
       if (obj != null && obj.getClass().isAnnotationPresent(MyEntity.class)) {
           return super.add(obj);
       }
       throw new IllegalArgumentException("Only objects annotated with MyEntity are allowed");
   }

   @Override
   public boolean addAll(Collection<?> objs) {
      //something similar, maybe with Collections2.filter from Google Collections
   }
};

The short answer is no.

Your problem is that List<MyEntity> defines a list of MyEntity's or its subclasses (i.e. if we have @interface AnotherEntity extends MyEntity then we could put AnotherEntity to this list).

Class MyClass doesn't extend/implement MyEntity, it's annotated with it.

Even if it was possible, it wouldn't be efficient. You wouldn't know which methods or fields are available, MyEntity doesn't describe your object's interface. So, the only thing it could be used for is filtering wrong insertions. You can implement it easily providing your List implementation:

List<?> myEntities = new LinkedList<?> {
   @Override 
   public boolean add(Object obj) {
       if (obj != null && obj.getClass().isAnnotationPresent(MyEntity.class)) {
           return super.add(obj);
       }
       throw new IllegalArgumentException("Only objects annotated with MyEntity are allowed");
   }

   @Override
   public boolean addAll(Collection<?> objs) {
      //something similar, maybe with Collections2.filter from Google Collections
   }
};

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