Java: selection between overloaded constructors

It’s not based on the number of types that are convertible to the parameter type – it’s whether any value that’s valid for one overload is valid for another, due to implicit conversions.

For example, there’s an implicit conversion from String to Object, but the reverse isn’t true, so String is more specific than Object.

Likewise there’s an implicit conversion from B to A, but the reverse isn’t true, so B is more specific than A.

With A and E however, neither is more specific than the other – there no conversion from A to E, and no conversion from E to A. That’s why overload resolution fails.

The relevant bit of the JLS is actually, which includes this that might make it easier for you to understand:

The informal intuition is that one method is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time error.

So if you have:

void foo(String x)
void foo(Object x)

every invocation handled by foo(String) could be handled by foo(Object), but the reverse is not the case. (For example, you could call foo(new Object()) and that couldn’t be handled by foo(String).)

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