How to convert String to Boolean in Java, but to treat null differently than false?

Why is it better that Boolean.valueOf(String s) returns false in the case of s being null, rather than returning null?

Because out the build-in feature in Java, called autoboxing.

Let's suppose that Boolean.valueOf(String s) was actually returning null, when s is null. Then, the following statement would be throwing NullPointerException, because null cannot be assigned to a primitive:

boolean b = Boolean.valueOf(null);

Why is it better that Boolean.valueOf(String s) returns false in the case of s being null, rather than returning null?

It is a matter of opinion, and (to be frank) pointless to ask ... unless you happen to be designing your own language and runtime libraries. The design decision was made more than 20 years ago, and there is zero chance that it will be changed while the language is called "Java".

The javadocs for Boolean.valueOf(String) don't offer any clues as to why the designers designed it this way.

However, it is clear that the behavior of Boolean.valueOf(String) is inconsistent with the behavior of valueOf(String) for the other primitive wrapper classes. For example Integer.valueOf(String) throws NumberFormatException if the string argument is null. So it is possible that the real explanation is that the Boolean.valueOf(String) semantics are just an unfortunate anomaly that is the result of an oversight that occurred in the rush to release Java 1.0.

However, that is speculation. For a real answer you would need to consult relevant internal Sun / Oracle documentary sources (if they still exist) and / or talk to someone on the original Java team (if they can still remember).

You can use the ternary operator :

String s = ...
Boolean b = s != null ? Boolean.valueOf(s) : null;