假设我在Java 8中有以下功能接口:

interface Action<T, U> {
   U execute(T t);
}

在某些情况下,我需要一个没有参数或返回类型的操作。所以我写 就像这样:

Action<Void, Void> a = () -> { System.out.println("Do nothing!"); };

但是,它给了我编译错误,我需要把它写成

Action<Void, Void> a = (Void v) -> { System.out.println("Do nothing!"); return null;};

这很难看。是否有办法摆脱Void类型参数?


当前回答

我认为这张表简短而有用:

Supplier       ()    -> x
Consumer       x     -> ()
BiConsumer     x, y  -> ()
Callable       ()    -> x throws ex
Runnable       ()    -> ()
Function       x     -> y
BiFunction     x,y   -> z
Predicate      x     -> boolean
UnaryOperator  x1    -> x2
BinaryOperator x1,x2 -> x3

正如在其他回答中所说,这个问题的适当选项是可运行的

其他回答

与@rado回答参数和描述相同的方法:

/*----------------------
Represents an operation
that accepts two input
arguments and returns no
result.
*/
BiConsumer<T,U>         (T x, U y)  -> ()


/*----------------------
Represents a function
that accepts two arguments
and produces a result.
*/
BiFunction<T,U,R>       (T x, U y)   -> R z


/*----------------------
Represents an operation
upon two operands of the
same type, producing a
result of the same type
as the operands.
*/
BinaryOperator<T>       (T x1, T x2) -> T x3


/*----------------------
A task that returns a
result and may throw an
exception.
*/
Callable<V>             ()    -> V x   throws ex


/*----------------------
Represents an operation
that accepts a single
input argument and returns
no result.
*/
Consumer<T>             (T x)   -> ()


/*----------------------
Represents a function that
accepts one argument and
produces a result.
*/
Function<T,R>           (T x)   -> R y


/*----------------------
Represents a predicate
(boolean-valued function)
of one argument.
*/
Predicate<T>            (T x)   -> boolean


/*----------------------
Represents a portion of
executable code that
don't recieve parameters
and returns no result.
*/ 
Runnable                ()    -> ()


/*----------------------
Represents a supplier of
results.
*/
Supplier<T>             ()      -> T x

/*----------------------
Represents an operation 
on a single operand that
produces a result of the
same type as its operand.
*/
UnaryOperator<T>        (T x1)  -> T x2

字体:

[1] https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html

[2] https://docs.oracle.com/javase/8/docs/api/java/lang/Runnable.html

[3] https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Callable.html

你所追求的语法是可以用一个小的帮助函数将一个Runnable转换为Action<Void, Void>(你可以把它放在Action中):

public static Action<Void, Void> action(Runnable runnable) {
    return (v) -> {
        runnable.run();
        return null;
    };
}

// Somewhere else in your code
 Action<Void, Void> action = action(() -> System.out.println("foo"));

我认为这张表简短而有用:

Supplier       ()    -> x
Consumer       x     -> ()
BiConsumer     x, y  -> ()
Callable       ()    -> x throws ex
Runnable       ()    -> ()
Function       x     -> y
BiFunction     x,y   -> z
Predicate      x     -> boolean
UnaryOperator  x1    -> x2
BinaryOperator x1,x2 -> x3

正如在其他回答中所说,这个问题的适当选项是可运行的

这是不可能的。具有非Void返回类型的函数(即使它是Void)必须返回一个值。然而,你可以添加静态方法的动作,让你“创建”一个动作:

interface Action<T, U> {
   U execute(T t);

   public static Action<Void, Void> create(Runnable r) {
       return (t) -> {r.run(); return null;};
   }

   public static <T, U> Action<T, U> create(Action<T, U> action) {
       return action;
   } 
}

这将允许您编写以下内容:

// create action from Runnable
Action.create(()-> System.out.println("Hello World")).execute(null);
// create normal action
System.out.println(Action.create((Integer i) -> "number: " + i).execute(100));

仅供参考,在方法抛出和/或返回值的情况下,哪个函数接口可以用于方法引用。

void notReturnsNotThrows() {};
void notReturnsThrows() throws Exception {}
String returnsNotThrows() { return ""; }
String returnsThrows() throws Exception { return ""; }

{
    Runnable r1 = this::notReturnsNotThrows; //ok
    Runnable r2 = this::notReturnsThrows; //error
    Runnable r3 = this::returnsNotThrows; //ok
    Runnable r4 = this::returnsThrows; //error

    Callable c1 = this::notReturnsNotThrows; //error
    Callable c2 = this::notReturnsThrows; //error
    Callable c3 = this::returnsNotThrows; //ok
    Callable c4 = this::returnsThrows; //ok

}


interface VoidCallableExtendsCallable extends Callable<Void> {
    @Override
    Void call() throws Exception;
}

interface VoidCallable {
    void call() throws Exception;
}

{
    VoidCallableExtendsCallable vcec1 = this::notReturnsNotThrows; //error
    VoidCallableExtendsCallable vcec2 = this::notReturnsThrows; //error
    VoidCallableExtendsCallable vcec3 = this::returnsNotThrows; //error
    VoidCallableExtendsCallable vcec4 = this::returnsThrows; //error

    VoidCallable vc1 = this::notReturnsNotThrows; //ok
    VoidCallable vc2 = this::notReturnsThrows; //ok
    VoidCallable vc3 = this::returnsNotThrows; //ok
    VoidCallable vc4 = this::returnsThrows; //ok
}