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

Question

I have this class called Container:

public class Container {

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

    publ         


        

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) e -> String.valueOf(e)));

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

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^{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.}