Short Answer
The return cancel() statement must return a valid value, but the method declaration void run() declares that run() does not return a value; hence, return cancel() in run() is an error. The return statement (without an expression) attempts to transfer control to the caller and is used when the method return type is void; hence, not an error.
Long Answer
rel=“nofollow”>JLS ∗return * 声明 国家:
A return statement with no Expression attempts to transfer control to the invoker of the method or constructor that contains it. [...] A return statement with an Expression must be contained in a method declaration that is declared to return a value (§8.4) or a compile-time error occurs. The Expression must denote a variable or value of some type T, or a compile-time error occurs. The type T must be assignable (§5.2) to the declared result type of the method, or a compile-time error occurs.
The JLS Method型回归/code> section 国家:
一种方法的回报类型宣布,如果方法回报价值,其价值类型即为无效。 如果并且只有在以下条件允许的情况下,R1号回归类型R1即为另一种回归类型R2的方法的回归类型替代物:[......]R1无效,R2无效。
The JLS Types, Values, and Variables chapter, first paragraph states:
Java节目使用的语言是很强的类型,这意味着每个变数和每个表达方式都有汇编时已知的类型。 类型限制变数(第412段)可以持有的或表达能够产生的价值,限制这些价值支持的行动,并确定行动的含义。
The JLS The Kinds of Natures and Values:
Java方案拟订语言有两种类型:原始类型(第4.2节)和参考类型(第4.3节)。 相应地,可以储存在变数中的两种数据值,作为论据,通过方法回馈,并按原始价值(第4.2段)和参考价值(第4.3段)操作。
现在仅举几个例子。 缩略语: 国家:
与C和C++不同的是, Java方案语言只允许某些形式的表述作为表述。 注: Java方案措辞不允许“更改为无效”即不属于类型
The JLS Method Body section 国家:
如果一种方法被宣布无效,则其身体不得包含任何带有表述的返回声明(第14.17条)。
最后,JLS Method Declarations 国家:
一种方法声明要么具体规定了方法回报的价值类型,要么使用关键词表示该方法不退还价值。
现在,当我们齐心协力,我们就能够提出以下几点:
- If a
return statement contains an expression, the expression must evaluate to a valid value.
- A valid
return expression value must be a primitive type or a reference type.
void is not a valid value type.- A method declared with a
void return type, returns no value.
- Method
void run() does not return a value.
- In
run(), return, without an expression, will happily transfer control to the caller.
- In
run(), return some expression is an error because some expression must be a valid value and run() does not return a value.
