NumberFormat
is the abstract base class for all number
formats. This class provides the interface for formatting and parsing
numbers.
NumberFormat
also provides methods for determining
which locales have number formats, and what their names are.
NumberFormat
helps you to format and parse numbers for any locale.
Your code can be completely independent of the locale conventions for
decimal points, thousands-separators, or even the particular decimal
digits used, or whether the number format is even decimal.
To format a number for the current Locale, use one of the factory
class methods:
myString = NumberFormat.getInstance().format(myNumber);
If you are formatting multiple numbers, it is
more efficient to get the format and use it multiple times so that
the system doesn't have to fetch the information about the local
language and country conventions multiple times.
NumberFormat nf = NumberFormat.getInstance();
for (int i = 0; i < myNumber.length; ++i) {
output.println(nf.format(myNumber[i]) + "; ");
}
To format a number for a different Locale, specify it in the
call to
getInstance
.
NumberFormat nf = NumberFormat.getInstance(Locale.FRENCH);
You can also use a
NumberFormat
to parse numbers:
myNumber = nf.parse(myString);
Use
getInstance
or
getNumberInstance
to get the
normal number format. Use
getIntegerInstance
to get an
integer number format. Use
getCurrencyInstance
to get the
currency number format. And use
getPercentInstance
to get a
format for displaying percentages. With this format, a fraction like
0.53 is displayed as 53%.
You can also control the display of numbers with such methods as
setMinimumFractionDigits
.
If you want even more control over the format or parsing,
or want to give your users more control,
you can try casting the NumberFormat
you get from the factory methods
to a DecimalFormat
. This will work for the vast majority
of locales; just remember to put it in a try
block in case you
encounter an unusual one.
NumberFormat and DecimalFormat are designed such that some controls
work for formatting and others work for parsing. The following is
the detailed description for each these control methods,
setParseIntegerOnly : only affects parsing, e.g.
if true, "3456.78" -> 3456 (and leaves the parse position just after index 6)
if false, "3456.78" -> 3456.78 (and leaves the parse position just after index 8)
This is independent of formatting. If you want to not show a decimal point
where there might be no digits after the decimal point, use
setDecimalSeparatorAlwaysShown.
setDecimalSeparatorAlwaysShown : only affects formatting, and only where
there might be no digits after the decimal point, such as with a pattern
like "#,##0.##", e.g.,
if true, 3456.00 -> "3,456."
if false, 3456.00 -> "3456"
This is independent of parsing. If you want parsing to stop at the decimal
point, use setParseIntegerOnly.
You can also use forms of the parse
and format
methods with ParsePosition
and FieldPosition
to
allow you to:
- progressively parse through pieces of a string
- align the decimal point and other areas
For example, you can align numbers in two ways:
- If you are using a monospaced font with spacing for alignment,
you can pass the
FieldPosition
in your format call, with
field
= INTEGER_FIELD
. On output,
getEndIndex
will be set to the offset between the
last character of the integer and the decimal. Add
(desiredSpaceCount - getEndIndex) spaces at the front of the string.
- If you are using proportional fonts,
instead of padding with spaces, measure the width
of the string in pixels from the start to
getEndIndex
.
Then move the pen by
(desiredPixelWidth - widthToAlignmentPoint) before drawing the text.
It also works where there is no decimal, but possibly additional
characters at the end, e.g., with parentheses in negative
numbers: "(12)" for -12.
Number formats are generally not synchronized.
It is recommended to create separate format instances for each thread.
If multiple threads access a format concurrently, it must be synchronized
externally.