在java中使用没有“循环”的标签

发布于 2024-10-18 09:56:07 字数 356 浏览 8 评论 0原文

我一直认为标签必须仅与循环一起使用，但似乎并非如此。给出这样的代码：

public class LabelTest {
    public static void main(String[] args) {
        label1: System.out.println("");
        label2: LabelTest t = new LabelTest();  
    }                                               
}

当编译标记为“label1”的行时，可以编译，但“label2”处的代码会出现错误。这是为什么呢？为什么我要标记不是“循环”的语句？

原文

I always thought that the labels must be used only with loops but it seems not. Giving such code:

public class LabelTest {
    public static void main(String[] args) {
        label1: System.out.println("");
        label2: LabelTest t = new LabelTest();  
    }                                               
}

When compiled line labeled "label1" compiles but the code at "label2" gives errors. Why's that? And why would I want to label statements which are not "loops"?

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老子叫无熙 2024-10-25 09:56:07

您会收到错误，因为标签无法应用于变量声明，这就是语言语法已定义（标签只能在Statement之前，并且LocalVariableDeclarationStatement不是Statement）。原因可能是它可能会导致变量范围的混乱。这有效：

    label1: System.out.println("");
    label2: { LabelTest t = new LabelTest(); }

You get an error because a label cannot be applied to variable declarations, that's just how the language grammar is defined (a label can only precede a Statement, and a LocalVariableDeclarationStatement is not a Statement). The reason is probably that it could cause confusion concerning variable scope. This works:

    label1: System.out.println("");
    label2: { LabelTest t = new LabelTest(); }

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陈甜 2024-10-25 09:56:07

为了补充 Michael Borgwardt 的答案，为了方便起见，您可以这样做（我前几天在阅读 Java rt.jar 源代码时刚刚发现了这一点）：

BlockSegment:
if (conditionIsTrue) {
    doSomeProcessing ();
    if (resultOfProcessingIsFalse()) break BlockSegment;
    otherwiseDoSomeMoreProcessing();
    // These lines get skipped if the break statement
    // above gets executed
}
// This is where you resume execution after the break
anotherStatement();

现在，这在逻辑上相当于：

if (conditionIsTrue) {
    doSomeProcessing ();
    if (!resultOfProcessingIsFalse()) {
        otherwiseDoSomeMoreProcessing();
        // More code here that gets executed
    }
}
anotherStatement();

但是，您可以跳过一些额外的大括号（以及大括号附带的缩进）。也许它看起来更干净（在我看来确实如此），并且在某些地方这种编码风格可能是合适的并且不那么令人困惑。

因此，您可以使用除循环之外的标签，甚至可以超出 if 语句。例如，这是有效的 Java 语法（也许您可以想出一个这样做的理由）：

statementOne();
statementTwo();
BlockLabel: {
    statementThree();
    boolean result = statementFour();
    if (!result) break BlockLabel;
    statementFive();
    statementSix();
}
statementSeven();

如果 break 在此处执行，则执行跳到由label，并且 statementFive() 和 statementSix() 被跳过。

当您必须跳过块中的块时，这种样式（没有 if 语句）的有用性变得更加明显。 一般来说，您可以通过足够聪明地使用循环来完成所有事情。但是，在某些情况下，不带循环的标签可以更轻松地阅读代码。例如，如果您需要顺序检查参数，您可以这样做或抛出异常。这最终是代码整洁度和个人风格的问题。

To add to Michael Borgwardt's answer, you can do things like this for convenience (I just discovered this the other day while reading through the Java rt.jar source code):

BlockSegment:
if (conditionIsTrue) {
    doSomeProcessing ();
    if (resultOfProcessingIsFalse()) break BlockSegment;
    otherwiseDoSomeMoreProcessing();
    // These lines get skipped if the break statement
    // above gets executed
}
// This is where you resume execution after the break
anotherStatement();

Now, this is logically equivalent to:

if (conditionIsTrue) {
    doSomeProcessing ();
    if (!resultOfProcessingIsFalse()) {
        otherwiseDoSomeMoreProcessing();
        // More code here that gets executed
    }
}
anotherStatement();

But, you get to skip some of the extra braces (and indentations that come with braces). Perhaps it looks cleaner (it does in my opinion), and there are some places where this style of coding may be appropriate and less confusing.

So, you may use labels beyond just loops, and even beyond if statements. For example, this is valid Java syntax (and maybe you could conjure up a reason to do something like this):

statementOne();
statementTwo();
BlockLabel: {
    statementThree();
    boolean result = statementFour();
    if (!result) break BlockLabel;
    statementFive();
    statementSix();
}
statementSeven();

If the break gets executed here, then execution skips to the end of the block denoted by the label, and statementFive() and statementSix() get skipped.

The usefulness of this style (without an if statement) becomes more evident when you have blocks within blocks where you must skip around. In general, you can accomplish everything with smart enough use of loops. However, there are a few cases where labels without loops make for easier reading of code. For example, if you need to sequentially check parameters, you may either do this or throw an exception. It ends up being a matter of cleanliness of code and personal style.

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