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':
// 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，。
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);
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。
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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