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Java float Precision Changes When Unexpectedly Up-casting to double

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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();


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

Now when I cast the values to String’s such as


The precision of the values are not cast to double’s:

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();




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

Another solution is to ensure the valueObject uses double’s then no casting is performed when putting the values into the JSONObject.





Published at DZone with permission of Mick Knutson, author and DZone MVB. (source)

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)


Jonathan Fisher replied on Fri, 2012/05/18 - 9:45am

The real mistake here is using floating point arithmitic to represent currency! Use a long that represents cents.

Balázs Bessenyei replied on Sat, 2012/05/19 - 6:31am in response to: Jonathan Fisher

A very good advice since Java FP types are unable to represent 1/10ths for example cents. Beside using long BigDecimal can be used as well.

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