Decoding floating point number in simple.json Java

Question

I am trying to read and parse a json file using simple.json in Java. However, on floating point numbers I get error. How should I parse floating point numbers?

The JSON File is like:

[
  {
    "region":"NF",
    "destination":"d1",
    "source":"s1",
    "time":2003,
    "value":0.1
  },
  {
    "region":"NF",
    "destination":"d2",
    "source":"s2",
    "time":2004,
    "value":0.002
  },
]

My code to parse it is:

JSONArray jsonArray = (JSONArray)obj;
Iterator<JSONObject> iterator = jsonArray.iterator();

while(iterator.hasNext()){
    JSONObject jsonObject = iterator.next();
    String region = (String) jsonObject.get("region");
    String src = (String) jsonObject.get("source");
    String dst = (String) jsonObject.get("destination");
    long time = (long) jsonObject.get("time");
    long val = (long) jsonObject.get("value");
}

what is the error you are getting? A sample JSON input, and code reading this input would help understand the problem better! — Vikdor
– Vikdor, Commented Feb 10, 2014 at 23:57
Can you show us something that would enable someone to answer this? Like the JSON file that you're trying to parse? Or the code that you're using? Maybe even the error message? Anything, so we can see what the problem is. Thank you. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Feb 10, 2014 at 23:57
Don't know anything about "simple.json", but a floating-point number should generally come through from JSON as a subclass of Number. You should be able to apply doubleValue() to it without having to determine the specific class returned. — Hot Licks
– Hot Licks, Commented Feb 11, 2014 at 0:03
You're receiving a Double, not a double. You must use one of the methods of Number to retrieve the value. — Hot Licks
– Hot Licks, Commented Feb 11, 2014 at 0:27

Joel · Accepted Answer · 2014-02-11 01:50:20Z

11

If you want to store a floating point number, then you need a variable of that type, i.e., a double.

double val = ((Number)jsonObject.get("value")).doubleValue();

In this case, the get() method should return an instance of java.lang.Number. Then you can call the doubleValue() method to store the floating point value.

edited Feb 11, 2014 at 1:50

answered Feb 11, 2014 at 0:12

Joel

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6 Comments

maerics Over a year ago

Note that OP is (presumably) asking about the json-simple library, not the JSON.org library.

Joel Over a year ago

@maerics Edited answer to fit the library requirement, thanks!

Hot Licks Over a year ago

Does get return a Number? If it returns Object then doubleValue will not apply.

Dawood ibn Kareem Over a year ago

In which case, this won't actually compile.

hAlE Over a year ago

Thanks! Issue solved. In simple.json values are returned as Object. So a casting to Number is required. Number val = (Number)jsonObject.get("value"); double value = val.doubleValue();

|

CatSleeping · Accepted Answer · 2019-01-14 02:03:14Z

1

In Java EE 7, use jsonObject.getJsonNumber("key").doubleValue() to get the double value.

See: https://docs.oracle.com/javaee/7/api/javax/json/JsonNumber.html

answered Jan 14, 2019 at 2:03

CatSleeping

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Comments

maerics · Accepted Answer · 2014-02-11 00:13:12Z

0

I would guess that this library (I'm guessing it's json-simple from your tag) returns numeric types as type double.

Double value = (Double) jsonObject.get("value");

For example (tested and working with json-simple-1.1.1):

String jsonString = "{\"foo\":1.23}";
JSONObject obj = (JSONObject) JSONValue.parse(jsonString);
Double d = (Double) obj.get("foo"); // => 1.23

edited Feb 11, 2014 at 0:13

answered Feb 11, 2014 at 0:07

maerics

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3 Comments

hAlE Over a year ago

I get this error when I do this: Exception in thread "main" java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Double

maerics Over a year ago

@user1720860: note that both the type and the cast must be "Double".

Hot Licks Over a year ago

The class used should be Number, which is "agnostic" and will cover either Double or Long. Generally the parser will return integer values as Long and fractional values as Double.

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