getMaximum wird immer 31 liefern, wie es auch in der API dazu steht..
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public int getActualMaximum(int field)
Return the maximum value that this field could have, given the current date. For example, with the date "Feb 3, 1997" and the DAY_OF_MONTH field, the actual maximum would be 28; for "Feb 3, 1996" it s 29. Similarly for a Hebrew calendar, for some years the actual maximum for MONTH is 12, and for others 13.
Overrides:
getActualMaximum in class Calendar
Parameters:
field - the field to determine the maximum of
Returns:
the maximum of the given field for the current date of this Calendar