Java注释：在变量初始化时，但不在赋值时？

发布于 2024-12-20 09:09:36 字数 1341 浏览 0 评论 0原文

我一直无法理解注释必须或可以放置的确切位置。

使用此方法的类可以编译，但会给出警告“未选中”：

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

此编译时没有警告：

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

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

此不会编译，但会用错误标记cloneBoard：

<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Sun关于注释的教程中，我找不到关于为什么会这样的答案是： http://docs.oracle.com/javase/tutorial/java/javaOO/annotations.html。

语法定义也没有帮助我，因为我不太确定我是否正确理解它： http://java.sun.com/docs/books/jls/third_edition/html/syntax.html#18.1

在我看来，这里有什么问题注释可以专门用于变量，但只能在声明变量时使用；任何后续作业都不会被注释覆盖。这是正确的吗？有没有比抑制整个方法的未经检查的警告更优雅的解决方案？

原文

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

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
}

This compiles without warning:

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

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

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
}

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.

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

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?

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丢了幸福的猪 2024-12-27 09:09:36

来自 Java 语言规范 - 接口 >注释

注释可以在任何声明中用作修饰符，无论是包、类、接口、字段、方法、参数、构造函数还是局部变量。

它只能在声明中使用。

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甜尕妞 2024-12-27 09:09:36

注释是声明的一部分；就像你不能写 Object obj 除非在声明 obj 的地方，也不能写 final obj 除了作为 final Object obj，@Deprecated obj 也是被禁止的。

至于优雅 - 无论如何，理想情况下你的方法不应该很长和复杂，但是如果你确实发现你想用这个注释来标记特定的赋值，你总是可以使用一个简单的包装方法：（

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

尽管在在这种特定情况下，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）。

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.

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(); }

(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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