This effectively forces instances of Either to either hold an A or a B (unless they consciously break the contract associated with the accept() method), and client code gets pattern matching “for free” by implementing suitable Visitors. Of course it’s much more verbose than in Haskell and ML, but nevertheless quite useful. No need to trouble oneself with instanceof, casts and non-exhaustive matches.

As for Maybe, I have this “library solution” in Java:
interface Optional<T> {
T orNull();
T orElse(T defaultValue);
T orThrow() throws RuntimeException;
boolean isSome();
boolean isNone();
}
Usage:
class Test {
static Optional<Number> getNumber() { … }

public static void main(String[] args) {
Number a = getNumber().orNull();
Number b = getNumber().orElse(Double.NaN);
Number c = getNumber().orThrow();
Optional<Number> d = getNumber();
if (d.isSome()) System.out.println(d.orThrow());
}
}