Java编译器：具有相同名称和不同签名的两个方法如何与一个方法调用匹配？

Question

我有一个叫做Container：

public class Container {

    private final Map<String, Object> map = new HashMap<>();

    public void put(String name, Object value) {
        map.put(name, value);
    }

    public Container with(String name, Object value) {
        put(name, value);
        return this;
    }

    public Object get(String name) {
        return map.get(name);
    }

    public <R> R get(String name, Function<Object, R> mapper) {

        Object value = get(name);

        if (null == value) {
            return null;
        }

        return mapper
            .apply(value);
    }

    public <R> R get(String name, Class<R> type) {

        Object value = get(name);

        if (null == value) {
            return null;
        }

        if (type.isAssignableFrom(value.getClass())) {
            return type
                .cast(value);
        }

        throw new ClassCastException(String
            .format("%s -> %s", value.getClass(), type));
    }
}

而该类称为Token：

public class Token {

    private String value;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    public Token withValue(String value) {
        setValue(value);
        return this;
    }
}

最后是该Token班的测试班

public class TokenTest {

    @Test
    public void verifyToken() {
        verify("bar", new Token()
            .withValue("bar"));
    }

    @Test
    public void verifyContainer() {
        Container tokens = new Container()
            .with("foo", "bar")
            .with("baz", "bat");

        verify("bar", tokens.get("foo", String.class));
        verify("bat", tokens.get("baz", String::valueOf));  // line 21
    }

    private void verify(String expected, String actual) {
        verify(expected, new Token()
            .withValue(actual));
    }

    private void verify(String expected, Token actual) {
        Assert
            .assertEquals(expected, actual.getValue());
    }
}

该测试仅编译并运行Eclipse中的文件。

在通用线路上构建时

mvn clean test

出现编译错误：

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:testCompile (default-testCompile) on project ambiguous: Compilation failure
[ERROR] /C:/data/projects/java/ambiguous/src/test/java/ambiguous/TokenTest.java:[21,9] reference to verify is ambiguous
[ERROR]   both method verify(java.lang.String,java.lang.String) in ambiguous.TokenTest and method verify(java.lang.String,ambiguous.Token) in ambiguous.TokenTest match

当我将行更改21为以下之一时，编译也将失败

verify("bat", tokens.get("baz", e -> String.valueOf(e)));
verify("bat", tokens.get("baz", e -> e.toString));

当我将行更改为

verify("bat", tokens.get("baz", String.class));
verify("bat", tokens.get("baz", Object::toString));

编译成功。

我无法理解为什么会出现这种编译错误。

我遇到了follwong链接装箱和拆箱，多种泛型类型和交集类型以及此eclipse编译器错误，但我仍然与所提到的原因无关。

我的问题是，verify当将映射器String::valueOf传递给get方法时，是什么使编译器认为该方法的两个签名都匹配？

对于编译，使用以下jdk（与maven和gradle一起使用）：

$ java -version
openjdk version "1.8.0_201-1-ojdkbuild"
OpenJDK Runtime Environment (build 1.8.0_201-1-ojdkbuild-b09)
OpenJDK 64-Bit Server VM (build 25.201-b09, mixed mode)

Answer 1

According to the JLS §15.12.2.2:

An argument expression is considered pertinent to applicability for a potentially applicable method m unless it has one of the following forms:

• An implicitly typed lambda expression¹.
• An inexact method reference expression².
• [...]

Therefore:

verify("bar", tokens.get("foo", e -> String.valueOf(e)));

an implicitly typed lambda expression e -> String.valueOf(e) is skipped from the applicability check during overload resolution - both verify(...) methods become applicable - hence the ambiguity.

In comparison, here are some examples that will work, because the types are specified explicitly:

verify("bar", tokens.get("foo", (Function<Object, String>) e -> String.valueOf(e)));

verify("bar", tokens.get("foo", (Function<Object, String>) String::valueOf));

^{1 - An implicitly typed lambda expression is a lambda expression, where the types of all its formal parameters are inferred.
2 - An inexact method reference - one with multiple overloads.}