I was wondering if there is a straightforward way (one-line, without creating a function) to convert String to Boolean in Java, but in a way that Boolean is null if String is null.
If I see it correctly, all of the methods from the Boolean class are returning false if the input string is null. Why is that?
Why is it better that Boolean.valueOf(String s) returns false in the case of s being null, rather than returning null?
Why is it better that
Boolean.valueOf(String s)returnsfalsein the case ofsbeingnull, rather than returningnull?
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);
Integer.parseInt(null) throw NumberFormatException(null) - doesn't autoboxing also apply there?
boolean from a String, then you'll use Boolean.parseBoolean, not valueOf. How could a misguided idiom, which goes through the redundant process of boxing followed by immediate unboxing, justify any API design decision?NullPointerException and force the client to implement an explicit null-sentinel policy.Why is it better that
Boolean.valueOf(String s)returnsfalsein the case ofsbeingnull, rather than returningnull?
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).
Boolean.getBoolean, that pinnacle of API perversion clearly dating from the earliest of early Java, actually has good reason to return false for a missing system property. Naturally, it relies internally on the API of Boolean.parseBoolean..You can use the ternary operator :
String s = ...
Boolean b = s != null ? Boolean.valueOf(s) : null;
Boolean.valueOf() better than if it returned null?
Optional.ofNullable(s).map(Boolean::valueOf).orElse(null)is an alternative. Mainly useful if your value is already wrapped in an Optional.