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import java.util.Optional; | |
import java.util.function.Function; | |
class Main { | |
public static void main(String[] args) { | |
Optional<String> o = Optional.of("foo"); | |
Function<String, String> f = s -> null; | |
Function<String, String> g = String::valueOf; | |
//lawless lolz | |
System.out.println( o.map(g.compose(f)) ); //Optional[null] | |
System.out.println( o.map(f).map(g) ); //Optional.empty | |
//This is because | |
System.out.println( o.map(f) ); //Optional.empty | |
//Once again for clarity | |
System.out.println( o.map(s -> null) ); //Optional.empty | |
//And now for more lolz | |
System.out.println( o.flatMap(s -> null) ); //throws NullPointerException | |
} | |
} |
FWIW, we tried this (failed) experiment about 12 years ago. The correct solution is to assume that null
does not exist, at all.
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That's correct.
Optional
has been designed to explicitly never be able to contain anull
value. Arguably more correct behaviour would have been to havemap
to throw anNPE
if the function application returnednull
- but you'd still have the issue thato map f map g
ando map (g compose f)
would be different.Presumably one "advantage" for explicitly disallowing
Optional(null)
might raise its head when the JVM gets value classes.