Json helper for nullable properties

SafeJson.java

import javax.json.*;

/**
 * Usage:
 * class SomeClass {
 *     //...
 *     public JsonValue toJson() { 
 *         return Json.createObjectBuilder().add("id", this.someId)
 *                    .add("zipCode", SafeJson.nvl(this.someNullableInt))
 *                    .add("phone", SafeJson.nvl(this.someNullableString))
 *                    .add("date", SafeJson.nvl(this.someNullableDate))
 *                    .build(); 
 *     }
 *     //...
 * }
 * Note: This strange construction `Json.createObjectBuilder().add(KEY, ref).build().get(KEY);` 
 *       is needed to get value wrapped in `JsonValue` (the only portable way I've found so far).
 *       In JavaEE 8 however it is possible using `Json.createValue()` or underlying 
 *       `JsonProvider.provider().createValue()` methods.
 */
public final class SafeJson {
    private static final String KEY = "1";
    
    public static JsonValue nvl(final String ref) {
        if (ref == null) {
            return JsonValue.NULL;
        }
        return Json.createObjectBuilder().add(KEY, ref).build().get(KEY);
    }
    
    public static JsonValue nvl(final Integer ref) {
        if (ref == null) {
            return JsonValue.NULL;
        }
        return Json.createObjectBuilder().add(KEY, ref).build().get(KEY);
    }

    public static JsonValue nvl(final java.sql.Date ref) {
        if (ref == null) {
            return JsonValue.NULL;
        }
        return Json.createObjectBuilder().add(KEY, ref.getTime()).build().get(KEY);
    }
}