@Override
    public B apply(A x) {
        if (pf.isDefinedAt(x)) {
            return pf.apply(x);
        } else {
            return null;
        }
    }
Work exactly same as scala's PartialFunction.

@param <A> argument type
@param <B> result type
indexr-common/src/main/java/io/indexr/util/function/PartialFunction.java