Java compiler: How can two methods with the same name and different signatures match a method call?

I have this class called 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));
    }
}

and the class called 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;
    }
}

and finally a test class for the Token class

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());
    }
}

The test compiles and runs just fine in eclipse.

When building on the commad line

mvn clean test

a compile error is raised:

[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

The compilation also fails when I change line 21 to one of

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

When I change the line to one of

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

compilation is successful.

I cannot undestand why this compiliation error is raised.

I came across the follwong links boxing and unboxing, multiple generic types and intersection types and this eclipse compiler bug but I still cannot relate to the mentioned causes.

My question is, what makes the compiler think that both signatures of the verify method are matching when the mapper String::valueOf is passed to the get method?

For compilation the following jdk is used (with maven and gradle):

$ 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

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.}

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Answer