Over a million developers have joined DZone.
{{announcement.body}}
{{announcement.title}}

Java Float Precision Changes When Unexpectedly Up-Casting to Double

DZone's Guide to

Java Float Precision Changes When Unexpectedly Up-Casting to Double

A quick reminder: Remember to match your variable types when working with object containers. Here's an example of values gone wild when upcasting.

· Java Zone
Free Resource

What every Java engineer should know about microservices: Reactive Microservices Architecture.  Brought to you in partnership with Lightbend.

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.

Microservices for Java, explained. Revitalize your legacy systems (and your career) with Reactive Microservices Architecture, a free O'Reilly book. Brought to you in partnership with Lightbend.

Topics:
java ,float ,double ,precision ,tutorial

Published at DZone with permission of Mick Knutson, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

THE DZONE NEWSLETTER

Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

X

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}