当实例作为其超接口传递时,Java 8默认实现不可用

Question

我AbstractMethodError在调用一个方法时会收到一个,我认为该方法应该在目标实例中有一个默认实现.

我在三个参数中创建了一个功能接口,但也派生自java.util.function.Function并提供了一个默认实现Function#apply(..).然后我使用3参数lambda表达式创建我的接口实例.3参数方法都Function#apply(..)可以在创建的实例中正常工作.

当我将创建的实例传递给期望我的接口的方法时,我可以Function#apply(..)从方法中调用它并且它工作正常.

但是当我将实例传递给期望a的方法时Function,我会在AbstractMethodError尝试调用时收到Function#apply(..).

我似乎错过了对默认方法如何以及何时绑定到实例的理解.我做错了什么？

展示:

package spike;

import java.util.function.BiFunction;
import java.util.function.Function;

public class ReductionProblem {

    interface F3<T, U, V, R> extends Function<T, BiFunction<U, V, R>> {

        default BiFunction<U, V, R> apply(final T t) {
            return (U u, V v) -> apply(t, u, v);
        }

        R apply(T t, U u, V v);

    }

    private static <T, U, V, R> BiFunction<U, V, R> workingReduce(
            F3<T, U, V, R> f, T t) {
        return f.apply(t);
    }

    private static <T, U, V, R> BiFunction<U, V, R> brokenReduce(
            Function<T, BiFunction<U, V, R>> f, T t) {
        return f.apply(t);
    }

    public static void main(String[] args) {

        /*
         * Object is instantiated here, right? So, Function#apply(Integer)
         * should be defined and ready to go.
         */
        final F3<Integer, Integer, String, Integer> f3 = (a, b, c) -> a * b
                * c.length();

        final Integer a = 3, b = 13;
        final String c = "see";

        final Integer expected = a * b * c.length();

        Integer v1 = f3.apply(a, b, c);
        display("invoke with 3", expected, v1);

        /*
         * It appears that f3 can indeed behave as a Function<Integer,
         * BiFunction<>> ...
         */
        Integer v2 = f3.apply(a).apply(b, c);
        display("invoke with 1 then 2", expected, v2);

        /*
         * From inside a method, no problem...
         */
        Integer v3 = workingReduce(f3, a).apply(b, c);
        display("invoke with 1 inside special reducer", expected, v3);

        /*
         * But when passed explicitly as a Function, AbstractMethodError is
         * thrown from inside the reduction method. So, it seems that the
         * instantiation of the object with the default method implementation
         * does not occur where I am supposing it does.
         */
        Integer v4 = brokenReduce(f3, a).apply(b, c);
        display("invoke with 1 inside general reducer", expected, v4);

    }

    private static void display(String label, Object expected, Object actual) {

        System.out.println(label + ":"
                + (actual.equals(expected) ? "pass" : "fail"));

    }

}

Answer 1

这似乎是您正在使用的任何编译器中的错误.我只能用Eclipse编译器重现它.它工作得很好

[~/workspace/Example/bin]$ javac -version
javac 1.8.0_05
[~/workspace/Example/bin]$ 

这里的错误似乎是编译器没有添加invokeinterface用于调用方法但是引用抽象方法本身.或者它可能在没有default方法实现的情况下构建由lambda生成的实例,但由于其他调用工作,这似乎不太可能.