方法參照一元靜態方法是功能及雙功能參數類型

Question

考慮以下降低測試用例之間曖昧：

import java.util.AbstractList; 
import java.util.Collection; 
import java.util.Iterator; 
import java.util.List; 
import java.util.function.BiFunction; 
import java.util.function.Function; 
public final class Example { 
    static class PairList<A, B> { 
     public void replaceAllSecond(Function<? super B, ? extends B> secondFunction) {} 
     public void replaceAllSecond(BiFunction<? super A, ? super B, ? extends B> secondFunction) {} 
    } 

    static class ImmutableList<E> extends AbstractList<E> { 
     public static <E> ImmutableList<E> copyOf(Iterable<? extends E> elements) {return null;} 
     public static <E> ImmutableList<E> copyOf(Collection<? extends E> elements) {return null;} 
     public static <E> ImmutableList<E> copyOf(Iterator<? extends E> elements) {return null;} 
     public static <E> ImmutableList<E> copyOf(E[] elements) {return null;} 

     @Override public E get(int index) {return null;} 
     @Override public int size() {return 0;} 
    } 

    public static void foo() { 
     PairList<Integer, List<Integer>> list = new PairList<>(); 
     list.replaceAllSecond(x -> ImmutableList.copyOf(x)); //accepted 
     list.replaceAllSecond(ImmutableList::copyOf); //error 
    } 
} 

與從Oracle JDK 8u40，調用​​用的λ的javac編譯被接受，但調用傳遞方法引用被拒絕，並出現以下錯誤：

Example.java:26: error: reference to replaceAllSecond is ambiguous 
       list.replaceAllSecond(ImmutableList::copyOf); //error 
        ^
    both method replaceAllSecond(Function<? super B,? extends B>) in PairList and method replaceAllSecond(BiFunction<? super A,? super B,? extends B>) in PairList match 
    where B,A are type-variables: 
    B extends Object declared in class PairList 
    A extends Object declared in class PairList 
1 error 

我不明白爲什麼超載服用BiFunction是可能適用在這裏。從JLS 15.12.2.1（省略一些子彈）：

A member method is potentially applicable to a method invocation if and only if all of the following are true:

• If the member is a fixed arity method with arity n, the arity of the method invocation is equal to n, and for all i (1 ≤ i ≤ n), the i'th argument of the method invocation is potentially compatible, as defined below, with the type of the i'th parameter of the method.

An expression is potentially compatible with a target type according to the following rules:

• A method reference expression (§15.13) is potentially compatible with a functional interface type if, where the type's function type arity is n, there exists at least one potentially applicable method for the method reference expression with arity n (§15.13.1), and one of the following is true:

  • The method reference expression has the form ReferenceType :: [TypeArguments] Identifier and at least one potentially applicable method is i) static and supports arity n, or ii) not static and supports arity n-1.

正如我解釋它，BiFunction的功能類型元數是2，但copyOf所有重載是靜態的，具有元數1，因此該方法引用不是潛在的兼容與BiFunction參數，因此replaceAllSecond(BiFunction)不適用。

我誤解了JLS，還是這是一個javac錯誤？ JDK-8026231描述了更新javac以實現該規範，但該錯誤在2013年第一版Java 8之前（2014年3月）得到了解決。

Answer 1

你的例子可以進一步減少到以下幾點：

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

public final class Example { 
    static class PairList<A, B> { 
     public void replaceAllSecond(Function<? super B, ? extends B> secondFunction) {} 
     public void replaceAllSecond(BiFunction<? super A, ? super B, ? extends B> secondFunction) {} 
    } 

    public static <E> List<E> copyOf(Iterable<? extends E> elements) {return null;} 
    public static <E> List<E> copyOf(Collection<? extends E> elements) {return null;} 

    public static void foo() { 
     PairList<Integer, List<Integer>> list = new PairList<>(); 
     list.replaceAllSecond(x -> Example.copyOf(x)); //accepted 
     list.replaceAllSecond(Example::copyOf); //error 
    } 
} 

我想這是一個javac的問題，因爲這段代碼編譯罰款與Java-9月初訪問構建（甚至是很老的像9ea57）而Java-8失敗（即使使用最新的更新）。