JsonReader API call to determine whether Number was specified as whole number (no period) or floating-point (period)

[jernst]

I'd like to be able to tell the difference between
{ "x" : 17 }
and
{ "x" : 17.0 }
You internally distinguish between those two, but there is no API that exposes this. It might be as simple as adding a call:
boolean peekIsWholeNumber()

[Marcono1234]

Possibly a duplicate of #1621

Are you really working directly with JsonReader or are you using one of the Gson methods and actually want to deserialize a Java Number (or Object)? In the latter case you can probably use ToNumberStrategy / ToNumberPolicy introduced by #1290.

If you are working directly with a JsonReader you could use its nextString() method. This will also work for JSON numbers and lets you determine whether the number is an integer or a floating point number and perform the parsing yourself.

It would be good if you could explain your use case a bit more in detail or provide the code you have so far.

[jernst]

I'm attempting to make best possible sense out of a wide variety of JSON files whose provenance is not entirely clear, and there is no documentation. First thing I need is to faithfully "read it in" so then I can throw some smarts at it to figure out what it might possibly mean. If the first step is already losing information, the second steps become harder.

I read through the code and I'm now attempting to read as Long, and if that throws, I retry as Double. That gives me a workaround.

[Marcono1234]

I read through the code and I'm now attempting to read as Long, and if that throws, I retry as Double. That gives me a workaround.

Your workaround might not work as desired when the number has 0 as decimal place, such as the 17.0 in your example. nextLong() internally falls back to reading as Double:

gson/gson/src/main/java/com/google/gson/stream/JsonReader.java

Lines 976 to 980 in e614e71

	double asDouble = Double.parseDouble(peekedString); // don't catch this NumberFormatException.
	long result = (long) asDouble;
	if (result != asDouble) { // Make sure no precision was lost casting to 'long'.
	throw new NumberFormatException("Expected a long but was " + peekedString + locationString());
	}

I think checking first for peek() == JsonToken.NUMBER and then calling nextString() as proposed above would give you better results. You could then also check if the number has an exponent, maybe that provides further insight into the structure of your data.

[jernst]

Gotcha. You are right :-)

[Marcono1234]

Do you think this issue can be closed? I think for inspecting the exact number format the approach of checking nextString() is the most versatile one.

One issue with a peekIsWholeNumber() method (or similar) is that its name might be misleading; for example one might consider 17.0 a whole number, but in your use case that is not desired.

I am not sure what other situations exist where it is really necessary to peek the number type. In all cases the nextString() approach will likely give you more possibilities and allows you to decide how to parse it, e.g. as BigDecimal. peekIsWholeNumber() might also give the wrong impression that calling nextDouble() will succeed while it could in theory be the case that the number is too large for a double, causing precision loss.

[jernst]

Whatever I can suggest now would break the existing API, and so I won't. Ok, closing.