java.text.Format and its subclasses in java.text have format methods that accept FieldPosition arguments. The specifications and implementations disagree on the behavior resulting when a NullPointerException is passed in:

Format.format(Object, StringBuffer, FieldPosition):
specification: NPE; no implementation

DateFormat.format(Object, StringBuffer, FieldPosition):
specification: unspecified; implementation: pass-through

DateFormat.format(Date, StringBuffer, FieldPosition):
specification: unspecified; no implementation

SimpleDateFormat.format(Date, StringBuffer, FieldPosition):
specification: unspecified; implementation: NPE

MessageFormat.format(Object, StringBuffer, FieldPosition):
specification: unspecified; implementation: no exception

MessageFormat.format(Object[], StringBuffer, FieldPosition):
specification: unspecified; implementation: no exception

NumberFormat.format(Object, StringBuffer, FieldPosition):
specification: NPE; implementation: pass-through

NumberFormat.format(double, StringBuffer, FieldPosition):
specification: unspecified; no implementation

NumberFormat.format(long, StringBuffer, FieldPosition):
specification: unspecified; no implementation

DecimalFormat.format(Object, StringBuffer, FieldPosition):
specification: NPE; implementation: pass-through

DecimalFormat.format(double, StringBuffer, FieldPosition):
specification: unspecified; implementation: NPE

DecimalFormat.format(long, StringBuffer, FieldPosition):
specification: unspecified; implementation: NPE

ChoiceFormat.format(double, StringBuffer, FieldPosition):
specification: unspecified; implementation: no exception

ChoiceFormat.format(long, StringBuffer, FieldPosition):
specification: unspecified; implementation: pass-through

The specifications and implementations need to be made consistent. Allowing null FieldPositions in general seems the best approach.
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