Java批注:在变量初始化时，而不是赋值时? [英] Java annotations: at variable initialization, but not assignment?

问题描述

我一直在理解必须将注解确切放置在何处或可以放置注解的地方.

I've been having trouble understanding where exactly an annotation has to or can be placed.

具有此方法的类可以编译，但会发出未选中"警告:

The class with this method compiles, but gives a warning "unchecked":

<B extends BitSet> void doStuff(LinkedList<B> list) {
    B board = list.getFirst();
    B cloneBoard;
    cloneBoard = (B) board.clone(); //unchecked
}

此编译没有警告:

<B extends BitSet> void doStuff(LinkedList<B> list) {
    B board = list.getFirst();

    @SuppressWarnings("unchecked")
    B cloneBoard = (B) board.clone();
}

这不会编译，但会将cloneBoard标记为错误:

This doesn't compile but marks cloneBoard with an error:

<B extends BitSet> void doStuff(LinkedList<B> list) {
    B board = list.getFirst();
    B cloneBoard;

    @SuppressWarnings("unchecked")
    cloneBoard = (B) board.clone(); // cloneBoard cannot be resolved to a type;
                                    // VariableDeclaratorID expected
}

在有关注释的Sun教程中，我找不到为什么的答案: http://docs.oracle.com/javase/tutorial/java/javaOO/annotations.html .

In the Sun tutorial on annotations, I couldn't find an answer as to why this is: http://docs.oracle.com/javase/tutorial/java/javaOO/annotations.html.

语法定义也没有帮助我，因为我不确定我是否正确理解它:

The grammar definition didn't help me either, since I'm not quite sure I understand it correctly: http://java.sun.com/docs/books/jls/third_edition/html/syntax.html#18.1

在我看来，这里的问题是注释可以专门用于变量，但只能在声明它们时使用.以后的任何作业都不会包含在注释中.这样对吗?除了抑制整个方法的未经检查的警告之外，还有一种更优雅的解决方案吗?

It seems to me that what's the problem here is that annotations can be specifically used for variables, but only when they are declared; any later assignment will not be covered by the annotation. Is this correct? Is there a more elegant solution than suppressing unchecked warnings for the whole method?

推荐答案

注释是声明的一部分；就像您不能在声明obj的位置写Object obj一样，也不能写final obj以外的final obj一样，也禁止写@Deprecated obj.

An annotation is part of a declaration; just as you can't write Object obj except at the point where obj is declared, nor final obj except as final Object obj, so too is @Deprecated obj forbidden.

至于优雅—理想情况下，无论如何，您的方法都不应太长且复杂，但是，如果您确实希望使用此批注标记特定的分配，则可以始终使用简单的包装器方法:

As for elegance — ideally your methods should not be very long and complicated, anyway, but if you do find that you'd like to mark a specific assignment with this annotation, you can always make use of a simple wrapper method:

@SuppressWarnings("unchecked")
private static <T extends ClassThatDeclaresCloneAsPublic> T cloneObj(T obj)
    { return (T) obj.clone(); }

(尽管在这种特定情况下，我想您可以编写cloneBoard = board.getClass().cast(board.clone());并完全省去注释，如果您愿意的话.)

(Though in this specific case, I suppose you could write cloneBoard = board.getClass().cast(board.clone()); and dispense with the annotation altogether, if you wanted.)

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