Apache Log4j Ambiguous Overloaded Method

Question

The interface org.apache.logging.log4j.Logger in Apache Log4j 2.12.1 has the following 2 methods:

void info(String message, Object... params);

void info(String message, Supplier<?>... paramSuppliers);

In my code, my intention was to call the second method and the second arg was a lambda paramSupplier. Surprisingly, at runtime, the first method was called, which printed the lambda's obj reference instead of the actual param value.

I am puzzled why these methods are not flagged as ambiguous by the compiler. This is on Java 11. I also see many other methods, in the same Logger interface, where the difference in method signature is just that one method takes an Object and the other method takes a Supplier<?> in the same position in the args list, with the types and order of all other method args matching.

I found the following two questions re: overloaded methods disambiguation but those answers did not seem to explain this.

1. Ambiguous method call when overloading method with generics and lambdas
2. (Java 8) java.util.function.Supplier

Answer 1

By your description, I can suppose that issue is in incorrect package name for Supplier interface, as rule we are using java.util.function.Supplier , but org.apache.logging.log4j.Logger interface expects Supplier interface from org.apache.logging.log4j.util.Supplier package, the second method will be called after changing the package name.

If everything is as I said above, the question of ambiguity is simple, signatures do not match for the second method and void info(String message, Object... params); method will be called as default for unmatched types of parameters, because any class in Java has been extended by Object class and any lambda expression is an object.