default ToStringNullable<ENTITY> asString() {
        return mapToStringIfPresent(val -> {
            if (val instanceof String) {
                return (String) val;
            } else throw new SpeedmentFieldException(format(
                "Expected field %s to be of type String, but it was not.",
                identifier()
            ));
        });
    }
Returns an {@link ToBooleanNullable} expression that has the value of
this field, casted to a {@code boolean} by first casting it to
{@link Boolean}.

@return  expression for this value after mapper has been applied

@since 3.1.0
runtime-parent/runtime-field/src/main/java/com/speedment/runtime/field/ReferenceField.java