Java Callable<Void> 作为带有 lambda 的函数式接口
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Callable<Void> as Functional Interface with lambdas
提问by ferrerverck
I am just learning about new java8 features. And here is my question:
我只是在学习新的 java8 特性。这是我的问题：
Why is it not allowed to use Callable<Void>as a functional interface for lambda expressions? (Compiler complains about returning value)
And it is still perfectly legal to use Callable<Integer>there. Here is the sample code:
为什么不允许Callable<Void>用作 lambda 表达式的功能接口？（编译器抱怨返回值）而且在Callable<Integer>那里使用它仍然是完全合法的。这是示例代码：
public class Test {
public static void main(String[] args) throws Exception {
// works fine
testInt(() -> {
System.out.println("From testInt method");
return 1;
});
testVoid(() -> {
System.out.println("From testVoid method");
// error! can't return void?
});
}
public static void testInt(Callable<Integer> callable) throws Exception {
callable.call();
}
public static void testVoid(Callable<Void> callable) throws Exception {
callable.call();
}
}
How to explain this behavior?
如何解释这种行为？
采纳答案by Thilo
For a Voidmethod (different from a voidmethod), you have to return null.
对于Void方法（与方法不同void），您必须返回null.
Voidis just a placeholder stating that you don't actually have a return value (even though the construct -- like Callable here -- needs one). The compiler does not treat it in any special way, so you still have to put in a "normal" return statement yourself.
Void只是一个占位符，表明您实际上没有返回值（即使构造 - 就像这里的 Callable - 需要一个）。编译器不会以任何特殊方式处理它，因此您仍然必须自己放入“正常”的 return 语句。
