Java 8 lambda 语法中的大括号何时是可选的? [英] When are braces optional in Java 8 lambda syntax?

问题描述

我意识到 Java 8 lambda 实现可能会发生变化，但在 lambda build b39 中，我发现只有当 lambda 表达式返回非 void 类型时才能省略大括号.例如，这会编译:

I realise that the Java 8 lambda implementation is subject to change, but in lambda build b39, I've found that braces can only be omitted when the lambda expression returns a non-void type. For example, this compiles:

public class Collections8 {
        public static void main(String[] args) {
                Iterable<String> names = Arrays.asList("Alice", "Bob", "Charlie");
                names.filter(e -> e.length() > 4).forEach(e -> { System.out.println(e); });
        }
}

但是像这样去掉大括号:

But removing the braces like this:

names.filter(e -> e.length() > 4).forEach(e -> System.out.println(e));

给出错误

Collections8.java:6: error: method forEach in interface Iterable<T> cannot be applied to given types;
        names.filter(e -> e.length() > 4).forEach(e -> System.out.println(e));
                                         ^
  required: Block<? super String>
  found: lambda
  reason: incompatible return type void in lambda expression
  where T is a type-variable:
    T extends Object declared in interface Iterable

谁能解释一下这里发生了什么?

Can anyone explain what's going on here?

推荐答案

当 lambda 主体是单个表达式或 void 方法调用时，您可以省略大括号.每个表达式的计算结果都是一个值，因此不能为空.

You may omit the braces when the lambda body is a single expression or a void method invocation. Every expression evaluates to a value, and thus cannot be void.

如果 lambda 的主体是一个语句块(例如，一系列计算后跟一个 return 语句)，或者 lambda 没有值(即有一个 void返回类型)并且不是单一的void方法调用，必须使用块形式，需要括号.

If the body of the lambda is a block of statements (e.g. a series of calculations followed by a return statement), or the lambda has no value (i.e. has a void return type) and is not a single void method invocation, you must use the block form, which requires brackets.

在块式 lambda 中，如果一个值是 returned，那么所有可能的代码路径必须return 一个值或 throw一个 Throwable.

In a block-style lambda, if a value is returned, then all possible code paths must either return a value or throw a Throwable.

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