GsonBuilder.setDateFormat(int) is ignored

[Marcono1234]

As described in https://stackoverflow.com/questions/6873020/gson-date-format, calling Gson.setDateFormat(int) has no effect, the created Gson object will not use the date format.

It appears the reason for this is that the method only sets dateStyle, but timeStyle remains unchanged, i.e. if it has not been changed before, then it is still DateFormat.DEFAULT.

gson/gson/src/main/java/com/google/gson/GsonBuilder.java

Lines 451 to 454 in 63ee47c

	public GsonBuilder setDateFormat(int style) {
	this.dateStyle = style;
	this.datePattern = null;
	return this;

However addTypeAdaptersForDate only uses the styles if timeStyle != DateFormat.DEFAULT.

gson/gson/src/main/java/com/google/gson/GsonBuilder.java

Line 615 in 63ee47c

} else if (dateStyle != DateFormat.DEFAULT && timeStyle != DateFormat.DEFAULT) {

[Marcono1234]

Actually maybe it would make sense to do the following:

• Change the dateStyle != DateFormat.DEFAULT && timeStyle != DateFormat.DEFAULT expression in GsonBuilder (see description above) to use || instead of &&, because also for setDateFormat(int, int) this makes a difference:

  // Equivalent to `setDateFormat(DEFAULT, DEFAULT)`
  // > Jan 1, 1970, 1:00:00AM
  DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT, Locale.US).format(new Date(0))
  
  // Equivalent to `setDateFormat(DEFAULT, LONG)` (this currently does not work for Gson)
  // > Jan 1, 1970, 1:00:00AM CET
  DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.LONG, Locale.US).format(new Date(0))

• Deprecate setDateFormat(int) and refer to setDateFormat(int, int) as alternative
  Originally this used DateFormat.getDateInstance and therefore taking only a 'date style' (but no 'time style') made sense; but since commit 2b9fd47 more than 10 years ago it seems this always uses DateFormat.getDateTimeInstance so taking no 'time style' is a bit confusing, and as pointed out in this issue here does not work correctly anyway (also since that same commit probably).
• Maybe remove the following because they are only called from test code and not from the main code (if necessary adjust the test code to test the other remaining methods instead):
  • the methods createAdapterFactory(int) and createDefaultsAdapterFactory() of DefaultDateTypeAdapter.DateType
  • the constructor DefaultDateTypeAdapter(DateType, int)
  • the methods getDateFormatPattern(int) and getUSDateFormat(int) of PreJava9DateFormatProvider