FORMATTING DECIMAL NUMBERS
These tips were developed using Java(tm) 2 SDK, Standard Edition,
v 1.2.2.
Suppose that you're developing a Java(tm) application that uses
decimal numbers, and you'd like to control the formatting of
these numbers for output purposes. How do you do this using the
Java library?
Or perhaps you don't care about formatting, but you do care about
making your application work in an international context. For
example, a simple statement like:
System.out.println(1234.56);
is locale-dependent; "." is used as a decimal point in the United
States, but not necessarily everywhere else. How do you deal with
this concern?
A couple of classes in the java.text package deal with these kinds
of issues. Here's a simple example that uses these classes to
tackle the problem mentioned in the previous paragraph:
import java.text.NumberFormat;
import java.util.Locale;
public class DecimalFormat1 {
public static void main(String args[]) {
// get format for default locale
NumberFormat nf1 = NumberFormat.getInstance();
System.out.println(nf1.format(1234.56));
// get format for German locale
NumberFormat nf2 =
NumberFormat.getInstance(Locale.GERMAN);
System.out.println(nf2.format(1234.56));
}
}
If you live in the United States and run this program, the output
is:
1,234.56
1.234,56
In other words, different locales, in this case locales for the
United States and for Germany, use different conventions for
representing numbers.
NumberFormat.getInstance returns an instance of NumberFormat
(actually a concrete subclass of NumberFormat such as
DecimalFormat), that is suited for formatting numbers according
to the default locale. You can also specify a non-default locale,
such as "Locale.GERMAN". Then the format method is called to
format a number according to the rules of a specific locale.
Note that the program could have done the formatting using
a single expression:
NumberFormat.getInstance().format(1234.56)
but it's more efficient to save a format and then reuse it.
Internationalization is a big issue when formatting numbers.
Another is the ability to exercise fine control over formatting,
for example, by specifying the number of decimal places. Here's
another example that illustrates this idea:
import java.text.DecimalFormat;
import java.util.Locale;
public class DecimalFormat2 {
public static void main(String args[]) {
// get format for default locale
DecimalFormat df1 = new DecimalFormat("####.000");
System.out.println(df1.format(1234.56));
// get format for German locale
Locale.setDefault(Locale.GERMAN);
DecimalFormat df2 = new DecimalFormat("####.000");
System.out.println(df2.format(1234.56));
}
}
In this example, a specific number format is set, using a notation
like "####.000". This pattern means "four places before the decimal
point, which are empty if not filled, and three places after the
decimal point, which are 0 if not filled". The output of this
program is:
1234.560
1234,560
In a similar way, it's possible to control exponent formatting, for
example:
import java.text.DecimalFormat;
public class DecimalFormat3 {
public static void main(String args[]) {
DecimalFormat df = new DecimalFormat("0.000E0000");
System.out.println(df.format(1234.56));
}
}
The output here is:
1.235E0003
You can also work with percentages:
import java.text.NumberFormat;
public class DecimalFormat4 {
public static void main(String args[]) {
NumberFormat nf = NumberFormat.getPercentInstance();
System.out.println(nf.format(0.47));
}
}
The output from this program is:
47%
So far, you've seen various techniques for formatting numbers.
What about going the other direction, that is, reading and parsing
strings that contain formatted numbers? Parsing support is included
in NumberFormat. For example, you can say:
import java.util.Locale;
import java.text.NumberFormat;
import java.text.ParseException;
public class DecimalFormat5 {
public static void main(String args[]) {
// get format for default locale
NumberFormat nf1 = NumberFormat.getInstance();
Object obj1 = null;
// parse number based on format
try {
obj1 = nf1.parse("1234,56");
}
catch (ParseException e1) {
System.err.println(e1);
}
System.out.println(obj1);
// get format for German locale
NumberFormat nf2 =
NumberFormat.getInstance(Locale.GERMAN);
Object obj2 = null;
// parse number based on format
try {
obj2 = nf2.parse("1234,56");
}
catch (ParseException e2) {
System.err.println(e2);
}
System.out.println(obj2);
}
}
This example has two parts, both of them concerned with parsing
an identical string: "1234,56". The first part uses the default
locale, the second the German locale. When this program is run
in the United States, the result is:
123456
1234.56
In other words, the string "1234,56" is interpreted as a large
integer "123456" in the United States, but as a decimal number
"1234.56" in the German locale.
There's one final point to be covered in this discussion of
formatting. In the examples above, DecimalFormat and NumberFormat
are both used. DecimalFormat is used to gain fine control over
formatting, while NumberFormat is used to specify a locale other
than the default. How do you combine these two classes?
The answer centers around the fact that DecimalFormat is a
subclass of NumberFormat, a subclass whose instances are specific
to a particular locale. So you can use NumberFormat.getInstance to
specify a locale, and then cast the resulting instance to a
DecimalFormat object. The documentation says that this technique
will work in the vast majority of cases, but that you need to
surround the cast with a try/catch block just in case it does not
(presumably in a very obscure case with an exotic locale). Such an
approach looks like this:
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.Locale;
public class DecimalFormat6 {
public static void main(String args[]) {
DecimalFormat df = null;
// get a NumberFormat object and cast it to
// a DecimalFormat object
try {
df = (DecimalFormat)
NumberFormat.getInstance(Locale.GERMAN);
}
catch (ClassCastException e) {
System.err.println(e);
}
// set a format pattern
df.applyPattern("####.00000");
// format a number
System.out.println(df.format(1234.56));
}
}
The getInstance method obtains the format, then applyPattern is
called to set a particular formatting pattern. The output of this
program is:
1234,56000
If you don't care about internationalization, it makes sense to use
DecimalFormat directly.
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