Java Float Precision Changes When Unexpectedly Up-Casting to Double

I ran across a little gotcha today where a float value being inserted into another object container (JSONObject) was not holding the precision of the original value. The JSONObject actually takes a double, not a float, and I overlooked the up-casting initially until I started unit testing. (Have to love unit tests!)

Here is a print of the values I started with:

07:46:15.108 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - ===== abv:: calculateAbv =====
07:46:15.117 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - volume: 16
07:46:15.118 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - abv: 3.2
07:46:15.118 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - price: 5.99
07:46:15.118 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - value_r: 0.117
07:46:15.118 [http-apr-8080-exec-49] INFO c.b.javaee6.service.AbvResource - score_r: 12.0

Here is how I was creating my JSONObject:

JSONObject json = new JSONObject();
json.put("abv_r", valueObject.getAbv());
json.put("volume_r", valueObject.getVolume());
json.put("price_r", valueObject.getPrice());
json.put("value_r", valueObject.getValue());
json.put("score", valueObject.getScore());
result = json.toString();

And the result:

 {
   "abv_r": 3.200000047683716,
   "volume_r": 16,
   "price_r": 5.989999771118164,
   "value_r": 0.11699999868869781,
   "score": 12
 }

The JSONObject.put method does not take a float, but takes a double:

/**
* Put a key/double pair in the JSONObject.
*
* @param key A key string.
* @param value A double which is the value.
* @return this.
* @throws JSONException If the key is null or if the number is invalid.
*/
public JSONObject put(String key, double value)
throws JSONException {
    put(key, new Double(value));
    return this;
}

See org.codehaus.jettison.json.JSONObject for more details.

Now when I cast the values to Strings such as...

“”+valueObject.getAbv()

... the precision of the values are not cast to doubles:

JSONObject json = new JSONObject();
json.put("abv_r", ""+valueObject.getAbv());
json.put("volume_r", ""+valueObject.getVolume());
json.put("price_r", ""+valueObject.getPrice());
json.put("value_r", ""+valueObject.getValue());
json.put("score", ""+valueObject.getScore());
result = json.toString();

And the result:

{
    "abv_r": "3.2",
    "volume_r": "16"",
    "price_r": "5.99",
    "value_r": "0.117"",
    "score": "12.0""
}

So care needs to be taken to remember that other Objects can cast your objects to have undesirable effects.

Another solution is to ensure the valueObject uses doubles. That way, no casting is performed when putting the values into the JSONObject.