Problem writing "empty" Map key (String with length 0)

[cowtowncoder]

(problem reported by user via email)

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Smile reader doesn’t do well with a map with an empty String key.

Map map = new HashMap();
map.put("foo", Collections.singletonMap("", "bar"));
ObjectMapper smileMapper = new ObjectMapper(new SmileFactory());
byte[] bytes = smileMapper.writer().writeValueAsBytes(map);
smileMapper.reader().readTree(new ByteArrayInputStream(bytes));

I get this exception:
Exception in thread "main" com.fasterxml.jackson.core.JsonParseException: Invalid shared name reference 2; only got 1 names in buffer (invalid content)
at [Source: java.io.ByteArrayInputStream@30c7da1e; line: -1, column: 12]

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[cowtowncoder]

Interesting... looks like a regression; test passes with 2.5, but fails with 2.6(.4).

[cowtowncoder]

So this is a regression from 2.5, due to rewrite of nextFieldName() method; it does not update state properly for the case of empty String (which has its dedicated token byte, separate from others).

The reason it had gone undetected is probably because:

1. Use of empty String as JSON Object key is not very common AND
2. This only caused problems with nextFieldName() but NOT with nextToken().