编译器如何知道在 ExecutorService 中提交（Callable<T> 任务）或提交（Runnable 任务）时使用哪个 lambda？
How compiler knows which lambda to use in submit(Callable<T> task) or submit(Runnable task) in ExecutorService?
submit() method accepts both Runnable and callable as parameters. but
it returns Future when we send callable interface as a parameter.
所以当 lambda 发布时
() -> {}
编译器如何知道要使用哪个功能接口？可调用还是可运行？ AFAIK 编译器不够聪明，无法分析表达式 return 是否存在，或者我错了吗？
对于任何 T，lambda 表达式 () -> {} 与 Callable<T> 不兼容，而它 与 Runnable 兼容.因此选择 Runnable 重载。
JLS 的 15.27.3 and 15.27.2 部分解释了为什么会这样。
A lambda expression is compatible in an [...] invocation context [...] with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.
这里Callable<T>的地面目标类型还是Callable<T>。让我们看看“一致”是什么意思：
A lambda expression is congruent with a function type if all of the following are true:
[...]
- If the function type's result is a (non-void) type
R, then either i) the lambda body is an expression that is compatible with R in an assignment context, or ii) the lambda body is a value-compatible block, and each result expression (§15.27.2) is compatible with R in an assignment context.
“值兼容块”定义为：
A block lambda body is value-compatible if it cannot complete normally (§14.21) and every return statement in the block has the form return Expression;.
还有：
In a value-compatible block lambda body, the result expressions are any expressions that may produce an invocation's value. Specifically, for each statement of the form return Expression ; contained by the body, the Expression is a result expression.
因此，“编译器不够智能，无法分析表达式 return 是否存在”的说法是错误的。任何 Java 编译器都需要遵循规范所说的内容，并分析块是否正常完成（大致意味着完成 运行 而没有到达 return 或 throw 语句）。
在() -> {}的情况下。没有 return 语句，因此块总是正常完成。
submit() method accepts both Runnable and callable as parameters. but it returns Future when we send callable interface as a parameter.
所以当 lambda 发布时
() -> {}
编译器如何知道要使用哪个功能接口？可调用还是可运行？ AFAIK 编译器不够聪明，无法分析表达式 return 是否存在，或者我错了吗？
对于任何 T，lambda 表达式 () -> {} 与 Callable<T> 不兼容，而它 与 Runnable 兼容.因此选择 Runnable 重载。
JLS 的 15.27.3 and 15.27.2 部分解释了为什么会这样。
A lambda expression is compatible in an [...] invocation context [...] with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.
这里Callable<T>的地面目标类型还是Callable<T>。让我们看看“一致”是什么意思：
A lambda expression is congruent with a function type if all of the following are true:
[...]
- If the function type's result is a (non-void) type
R, then either i) the lambda body is an expression that is compatible withRin an assignment context, or ii) the lambda body is a value-compatible block, and each result expression (§15.27.2) is compatible withRin an assignment context.
“值兼容块”定义为：
A block lambda body is value-compatible if it cannot complete normally (§14.21) and every return statement in the block has the form
return Expression;.
还有：
In a value-compatible block lambda body, the result expressions are any expressions that may produce an invocation's value. Specifically, for each statement of the form
return Expression ;contained by the body, theExpressionis a result expression.
因此，“编译器不够智能，无法分析表达式 return 是否存在”的说法是错误的。任何 Java 编译器都需要遵循规范所说的内容，并分析块是否正常完成（大致意味着完成 运行 而没有到达 return 或 throw 语句）。
在() -> {}的情况下。没有 return 语句，因此块总是正常完成。