Fix interpretation of trailing comma when setLenient() is true.

The following code exhibits the issue:

    StringReader reader = new StringReader("{foo: ['bar', 'baz',]}");
    JsonReader jsonReader = new JsonReader(reader);
    jsonReader.setLenient(true);
    JsonParser parser = new JsonParser();
    JsonElement root = parser.parse(jsonReader);
    assertEquals(
        "Trailing comma should be ignored like in ECMAScript 5.",
        2,
        root.getAsJsonObject().get("foo").getAsJsonArray().size());

Currently, GSON returns an array that corresponds to:

    ["bar", "baz", null]

Instead of:

    ["bar", "baz"]

This is atrocious. The current behavior is consistent with bad browser behavior 
from the last decade. The expected behavior is consistent with ECMAScript 5, as 
well as any programmer who has ever wanted trailing commas to be ignored so 
that it is easier to write lists of things. Here is more information on the 
badness of the current JSON spec:

    http://bolinfest.com/essays/json.html

Please, please fix this. Anyone who uses JSON for a config language wants to be 
able to write data like this:

   [
     "foo",
     "bar",
   ]

such that it is possible to add something to the list without having to clean 
up the comma from the previous line. The current behavior is incredibly 
counterintuitive.

Also, while we're on the subject, the trailing comma should also be supported 
for map entries:

  {
    "foo": 42,
    "bar": 24,
  }

Again, this makes it considerably easier to maintain JSON data files.

Original issue reported on code.google.com by [e-mail] on 6 Jan 2013 at 11:58

[inder123]

We will not support this behavior change by default. Write better JSON. We could consider a GsonBuilder property ignoreTrailingCommas() but that looks hidious.

[BoD]

You mean more hideous than the current “happily parse invalid JSON without a warning and result in a very surprising results that nobody should reasonably expect” behavior?