Lambda'特殊的void-compatibility rule' - 语句表达式 [英] Lambda 'special void-compatibility rule' - statement expression

问题描述

我正在阅读Java 8 in Action。在3.5.2节中有一个关于'void-compatibility rule'的段落：

Im reading Java 8 in Action. In section 3.5.2 there is a paragraph about 'void-compatibility rule':

如果一个lambda有一个语句表达式作为它的主体，它与函数描述符兼容
，返回void（假设参数
list也兼容）。例如，以下两行都是
合法，即使List的方法添加返回布尔值而不是消费者上下文中预期的
void（T - > void）：

If a lambda has a statement expression as its body, it’s compatible with a function descriptor that returns void (provided the parameter list is compatible too). For example, both of the following lines are legal even though the method add of a List returns a boolean and not void as expected in the Consumer context (T -> void):

// Predicate has a boolean return 
Predicate<String> p = s -> list.add(s); 
// Consumer has a void return 
Consumer<String> b = s -> list.add(s);

您如何描述'语句表达'？我以为这是陈述或表达。此虚空兼容性规则对我来说也不是100％清楚，你能想到其他任何例子吗？

How would you describe 'statement expression' in general? I thought it was either statement or expression. Also this void-compatibility rule is not 100% clear to me, can you think of any other examples?

推荐答案

这个词语句表达式或表达式语句是指也允许用作语句的表达式。它们在Java语言规范中描述，§14.8。表达式陈述。

The term "statement expression" or "expression statement" refers to expressions that are also allowed to be used as a statement. They are described in the Java Language Specification, §14.8. Expression Statements.

它们包括

• 方法调用


• 作业


• 增量/减量表达式


• 类实例创建表达式

• Method Invocations
• Assignments
• Increment/Decrement expressions
• Class Instance Creation expressions

所以其他例子是

Consumer<String> b = s -> counter++;
Function<String,Integer> f = s -> counter++;

或

Consumer<String> b = s -> new BigDecimal(s);
Function<String,BigDecimal> f = s -> new BigDecimal(s);

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根据经验，lambda表达式为表格 x - >表达式仅对消费者（或一般的 void 函数类型）合法，如果 x - > {表达; } 也是合法的。

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As a rule of thumb, a lambda expression of the form x -> expression is only legal for a Consumer (or void function type in general), if x -> { expression; } would be legal too.

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