public final class

Formatter

extends Object
implements Closeable Flushable
java.lang.Object
   ↳ java.util.Formatter

Class Overview

Formats arguments according to a format string (like printf in C).

It's relatively rare to use a Formatter directly. A variety of classes offer convenience methods for accessing formatter functionality. Of these, format(String, Object...) is generally the most useful. PrintStream and PrintWriter both offer format and printf methods.

Format strings consist of plain text interspersed with format specifiers, such as "name: %s weight: %03dkg\n". Being a Java string, the usual Java string literal backslash escapes are of course available.

Format specifiers (such as "%s" or "%03d" in the example) start with a % and describe how to format their corresponding argument. It includes an optional argument index, optional flags, an optional width, an optional precision, and a mandatory conversion type. In the example, "%s" has no flags, no width, and no precision, while "%03d" has the flag 0, the width 3, and no precision.

Not all combinations of argument index, flags, width, precision, and conversion type are valid.

Argument index. Normally, each format specifier consumes the next argument to format. For convenient localization, it's possible to reorder arguments so that they appear in a different order in the output than the order in which they were supplied. For example, "%4$s" formats the fourth argument (4$) as a string (s). It's also possible to reuse an argument with <. For example, format("%o % results in "100 64 40".

Flags. The available flags are:

Flags
, Use grouping separators for large numbers. (Decimal only.) format("%,d", 1024); 1,234
+ Always show sign. (Decimal only.) format("%+d, %+4d", 5, 5); 
+5,   +5
A space indicates that non-negative numbers should have a leading space. (Decimal only.) format("x% d% 5d", 4, 4); 
x 4    4
( Put parentheses around negative numbers. (Decimal only.) format("%(d, %(d, %(6d", 12, -12, -12); 
12, (12),   (12)
- Left-justify. (Requires width.) format("%-6dx", 5);
format("%-3C, %3C", 'd', 0x65);		 
5      x

D  ,   E
0 Pad the number with leading zeros. (Requires width.) format("%07d, %03d", 4, 5555); 0000004, 5555
# Alternate form. (Octal and hex only.) format("%o %#o", 010, 010);
format("%x %#x", 0x12, 0x12);		 10 010
12 0x12

Width. The width is a decimal integer specifying the minimum number of characters to be used to represent the argument. If the result would otherwise be shorter than the width, padding will be added (the exact details of which depend on the flags). Note that you can't use width to truncate a field, only to make it wider: see precision for control over the maximum width.

Precision. The precision is a . followed by a decimal integer, giving the minimum number of digits for d, o, x, or X; the minimum number of digits after the decimal point for a, A, e, E, f, or F; the maximum number of significant digits for g or G; or the maximum number of characters for s or S.

Conversion type. One or two characters describing how to interpret the argument. Most conversions are a single character, but date/time conversions all start with t and have a single extra character describing the desired output.

Many conversion types have a corresponding uppercase variant that converts its result to uppercase using the rules of the relevant locale (either the default or the locale set for this formatter).

This table shows the available single-character (non-date/time) conversion types:

String conversions
All types are acceptable arguments. Values of type Formattable have their formatTo method invoked; all other types use toString.
s String. format("%s %s", "hello", "Hello"); hello Hello
S Uppercase string. format("%S %S", "hello", "Hello"); HELLO HELLO
Character conversions
Byte, Character, Short, and Integer (and primitives that box to those types) are all acceptable as character arguments. Any other type is an error.
c Character. format("%c %c", 'd', 'E'); d E
C Uppercase character. format("%C %C", 'd', 'E'); D E
Integer conversions
Byte, Short, Integer, Long, and BigInteger (and primitives that box to those types) are all acceptable as integer arguments. Any other type is an error.
d Decimal. format("%d", 26); 26
o Octal. format("%o", 032); 32
x, X Hexadecimal. format("%x %X", 0x1a, 0x1a); 1a 1A
Floating-point conversions
Float, Double, and BigDecimal (and primitives that box to those types) are all acceptable as floating-point arguments. Any other type is an error.
f Decimal floating point. 
format("%f", 123.456f);
format("%.1f", 123.456f);
format("%1.5f", 123.456f);
format("%10f", 123.456f);
format("%6.0f", 123.456f);
 
123.456001
123.5
123.45600
123.456001
   123
e, E Engineering/exponential floating point. 
format("%e", 123.456f);
format("%.1e", 123.456f);
format("%1.5E", 123.456f);
format("%10E", 123.456f);
format("%6.0E", 123.456f);
 
1.234560e+02
1.2e+02
1.23456E+02
1.234560E+02
 1E+02
g, G Decimal or engineering, depending on the magnitude of the value. format("%g %g", 0.123, 0.0000123); 0.123000 1.23000e-05
a, A Hexadecimal floating point. format("%a", 123.456f); 0x1.edd2f2p6
Boolean conversion
Accepts Boolean values. null is considered false, and instances of all other types are considered true.
b, B Boolean. format("%b %b", true, false);
format("%B %B", true, false);
format("%b", null);
format("%b", "hello");		 true false
TRUE FALSE
false
true
Hash code conversion
Invokes hashCode on its argument, which may be of any type.
h, H Hexadecimal hash code. format("%h", this);
format("%H", this);
format("%h", null);		 190d11
190D11
null
Zero-argument conversions
% A literal % character. format("%d%%", 50); 50%
n Newline. (The value of the system property "line.separator".) format("first%nsecond"); first\nsecond

It's also possible to format dates and times with Formatter, though you should seriously consider using SimpleDateFormat via the factory methods in DateFormat instead. The facilities offered by Formatter are low-level and place the burden of localization on the developer. Using getDateInstance(), getTimeInstance(), and getDateTimeInstance() is preferable for dates and times that will be presented to a human. Those methods will select the best format strings for the user's locale.

The best non-localized form is ISO 8601, which you can get with "%tF" (2010-01-22), "%tF %tR" (2010-01-22 13:39), "%tF %tT" (2010-01-22 13:39:15), or "%tF %tT%z" (2010-01-22 13:39:15-0800).

As with the other conversions, date/time conversion has an uppercase format. Replacing %t with %T will uppercase the field according to the rules of the formatter's locale.

This table shows the date/time conversions:

Date/time conversions
Calendar, Date, and Long (representing milliseconds past the epoch) are all acceptable as date/time arguments. Any other type is an error. The epoch is 1970-01-01 00:00:00 UTC.
ta Localized weekday name (abbreviated). format("%ta", cal, cal); Tue
tA Localized weekday name (full). format("%tA", cal, cal); Tuesday
tb Localized month name (abbreviated). format("%tb", cal); Apr
tB Localized month name (full). format("%tB", cal); April
tc Locale-preferred date and time representation. (See DateFormat for more variations.) format("%tc", cal); Tue Apr 01 16:19:17 CEST 2008
tC 2-digit century. format("%tC", cal); 20
td 2-digit day of month (01-31). format("%td", cal); 01
tD Ambiguous US date format (MM/DD/YY). Do not use. format("%tD", cal); 04/01/08
te Day of month (1-31). format("%te", cal); 1
tF Full date in ISO 8601 format (YYYY-MM-DD). format("%tF", cal); 2008-04-01
th Synonym for %tb.
tH 24-hour hour of day (00-23). format("%tH", cal); 16
tI 12-hour hour of day (01-12). format("%tH", cal); 04
tj 3-digit day of year (001-366). format("%tj", cal); 092
tk 24-hour hour of day (0-23). format("%tH", cal); 16
tl 12-hour hour of day (1-12). format("%tH", cal); 4
tL Milliseconds. format("%tL", cal); 359
tm 2-digit month of year (01-12). format("%tm", cal); 04
tM 2-digit minute. format("%tM", cal); 08
tN Nanoseconds. format("%tN", cal); 359000000
tp a.m. or p.m. format("%tp %Tp", cal, cal); pm PM
tQ Milliseconds since the epoch. format("%tQ", cal); 1207059412656
tr Full 12-hour time (%tI:%tM:%tS %Tp). format("%tr", cal); 04:15:32 PM
tR Short 24-hour time (%tH:%tM). format("%tR", cal); 16:15
ts Seconds since the epoch. format("%ts", cal); 1207059412
tS 2-digit seconds (00-60). format("%tS", cal); 17
tT Full 24-hour time (%tH:%tM:%tS). format("%tT", cal); 16:15:32
ty 2-digit year (00-99). format("%ty", cal); 08
tY 4-digit year. format("%tY", cal); 2008
tz Time zone GMT offset. format("%tz", cal); +0100
tZ Localized time zone abbreviation. format("%tZ", cal); CEST

Number localization. Some conversions use localized decimal digits rather than the usual ASCII digits. So formatting 123 with %d will give 123 in English locales but ١٢٣ in appropriate Arabic locales, for example. This number localization occurs for the decimal integer conversion %d, the floating point conversions %e, %f, and %g, and all date/time %t or %T conversions, but no other conversions.

Thread safety. Formatter is not thread-safe.

See Also

Summary

Nested Classes
enum Formatter.BigDecimalLayoutForm The enumeration giving the available styles for formatting very large decimal numbers. 
Public Constructors
Formatter()
Constructs a Formatter.
Formatter(Appendable a)
Constructs a Formatter whose output will be written to the specified Appendable.
Formatter(Locale l)
Constructs a Formatter with the specified Locale.
Formatter(Appendable a, Locale l)
Constructs a Formatter with the specified Locale and whose output will be written to the specified Appendable.
Formatter(String fileName)
Constructs a Formatter whose output is written to the specified file.
Formatter(String fileName, String csn)
Constructs a Formatter whose output is written to the specified file.
Formatter(String fileName, String csn, Locale l)
Constructs a Formatter with the given Locale and charset, and whose output is written to the specified file.
Formatter(File file)
Constructs a Formatter whose output is written to the specified File.
Formatter(File file, String csn)
Constructs a Formatter with the given charset, and whose output is written to the specified File.
Formatter(File file, String csn, Locale l)
Constructs a Formatter with the given Locale and charset, and whose output is written to the specified File.
Formatter(OutputStream os)
Constructs a Formatter whose output is written to the specified OutputStream.
Formatter(OutputStream os, String csn)
Constructs a Formatter with the given charset, and whose output is written to the specified OutputStream.
Formatter(OutputStream os, String csn, Locale l)
Constructs a Formatter with the given Locale and charset, and whose output is written to the specified OutputStream.
Formatter(PrintStream ps)
Constructs a Formatter whose output is written to the specified PrintStream.
Public Methods
void close()
Closes the Formatter.
void flush()
Flushes the Formatter.
Formatter format(String format, Object... args)
Writes a formatted string to the output destination of the Formatter.
Formatter format(Locale l, String format, Object... args)
Writes a formatted string to the output destination of the Formatter.
IOException ioException()
Returns the last IOException thrown by the Formatter's output destination.
Locale locale()
Returns the Locale of the Formatter.
Appendable out()
Returns the output destination of the Formatter.
String toString()
Returns the content by calling the toString() method of the output destination.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.io.Closeable
From interface java.io.Flushable

Public Constructors

public Formatter ()

Since: API Level 1

Constructs a Formatter.

The output is written to a StringBuilder which can be acquired by invoking out() and whose content can be obtained by calling toString.

The Locale used is the user's default locale. See "Be wary of the default locale".

public Formatter (Appendable a)

Since: API Level 1

Constructs a Formatter whose output will be written to the specified Appendable.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
a the output destination of the Formatter. If a is null, then a StringBuilder will be used.

public Formatter (Locale l)

Since: API Level 1

Constructs a Formatter with the specified Locale.

The output is written to a StringBuilder which can be acquired by invoking out() and whose content can be obtained by calling toString.

Parameters
l the Locale of the Formatter. If l is null, then no localization will be used.

public Formatter (Appendable a, Locale l)

Since: API Level 1

Constructs a Formatter with the specified Locale and whose output will be written to the specified Appendable.

Parameters
a the output destination of the Formatter. If a is null, then a StringBuilder will be used.
l the Locale of the Formatter. If l is null, then no localization will be used.

public Formatter (String fileName)

Since: API Level 1

Constructs a Formatter whose output is written to the specified file.

The charset of the Formatter is the default charset.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
fileName the filename of the file that is used as the output destination for the Formatter. The file will be truncated to zero size if the file exists, or else a new file will be created. The output of the Formatter is buffered.
Throws
FileNotFoundException if the filename does not denote a normal and writable file, or if a new file cannot be created, or if any error arises when opening or creating the file.
SecurityException if there is a SecurityManager in place which denies permission to write to the file in checkWrite(file.getPath()).

public Formatter (String fileName, String csn)

Since: API Level 1

Constructs a Formatter whose output is written to the specified file.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
fileName the filename of the file that is used as the output destination for the Formatter. The file will be truncated to zero size if the file exists, or else a new file will be created. The output of the Formatter is buffered.
csn the name of the charset for the Formatter.
Throws
FileNotFoundException if the filename does not denote a normal and writable file, or if a new file cannot be created, or if any error arises when opening or creating the file.
SecurityException if there is a SecurityManager in place which denies permission to write to the file in checkWrite(file.getPath()).
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (String fileName, String csn, Locale l)

Since: API Level 1

Constructs a Formatter with the given Locale and charset, and whose output is written to the specified file.

Parameters
fileName the filename of the file that is used as the output destination for the Formatter. The file will be truncated to zero size if the file exists, or else a new file will be created. The output of the Formatter is buffered.
csn the name of the charset for the Formatter.
l the Locale of the Formatter. If l is null, then no localization will be used.
Throws
FileNotFoundException if the filename does not denote a normal and writable file, or if a new file cannot be created, or if any error arises when opening or creating the file.
SecurityException if there is a SecurityManager in place which denies permission to write to the file in checkWrite(file.getPath()).
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (File file)

Since: API Level 1

Constructs a Formatter whose output is written to the specified File. The charset of the Formatter is the default charset.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
file the File that is used as the output destination for the Formatter. The File will be truncated to zero size if the File exists, or else a new File will be created. The output of the Formatter is buffered.
Throws
FileNotFoundException if the File is not a normal and writable File, or if a new File cannot be created, or if any error rises when opening or creating the File.
SecurityException if there is a SecurityManager in place which denies permission to write to the File in checkWrite(file.getPath()).

public Formatter (File file, String csn)

Since: API Level 1

Constructs a Formatter with the given charset, and whose output is written to the specified File.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
file the File that is used as the output destination for the Formatter. The File will be truncated to zero size if the File exists, or else a new File will be created. The output of the Formatter is buffered.
csn the name of the charset for the Formatter.
Throws
FileNotFoundException if the File is not a normal and writable File, or if a new File cannot be created, or if any error rises when opening or creating the File.
SecurityException if there is a SecurityManager in place which denies permission to write to the File in checkWrite(file.getPath()).
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (File file, String csn, Locale l)

Since: API Level 1

Constructs a Formatter with the given Locale and charset, and whose output is written to the specified File.

Parameters
file the File that is used as the output destination for the Formatter. The File will be truncated to zero size if the File exists, or else a new File will be created. The output of the Formatter is buffered.
csn the name of the charset for the Formatter.
l the Locale of the Formatter. If l is null, then no localization will be used.
Throws
FileNotFoundException if the File is not a normal and writable File, or if a new File cannot be created, or if any error rises when opening or creating the File.
SecurityException if there is a SecurityManager in place which denies permission to write to the File in checkWrite(file.getPath()).
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (OutputStream os)

Since: API Level 1

Constructs a Formatter whose output is written to the specified OutputStream.

The charset of the Formatter is the default charset.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
os the stream to be used as the destination of the Formatter.

public Formatter (OutputStream os, String csn)

Since: API Level 1

Constructs a Formatter with the given charset, and whose output is written to the specified OutputStream.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
os the stream to be used as the destination of the Formatter.
csn the name of the charset for the Formatter.
Throws
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (OutputStream os, String csn, Locale l)

Since: API Level 1

Constructs a Formatter with the given Locale and charset, and whose output is written to the specified OutputStream.

Parameters
os the stream to be used as the destination of the Formatter.
csn the name of the charset for the Formatter.
l the Locale of the Formatter. If l is null, then no localization will be used.
Throws
UnsupportedEncodingException if the charset with the specified name is not supported.

public Formatter (PrintStream ps)

Since: API Level 1

Constructs a Formatter whose output is written to the specified PrintStream.

The charset of the Formatter is the default charset.

The Locale used is the user's default locale. See "Be wary of the default locale".

Parameters
ps the PrintStream used as destination of the Formatter. If ps is null, then a NullPointerException will be raised.

Public Methods

public void close ()

Since: API Level 1

Closes the Formatter. If the output destination is Closeable, then the method close() will be called on that destination. If the Formatter has been closed, then calling the this method will have no effect. Any method but the ioException() that is called after the Formatter has been closed will raise a FormatterClosedException.

public void flush ()

Since: API Level 1

Flushes the Formatter. If the output destination is Flushable, then the method flush() will be called on that destination.

Throws
FormatterClosedException if the Formatter has been closed.

public Formatter format (String format, Object... args)

Since: API Level 1

Writes a formatted string to the output destination of the Formatter.

Parameters
format a format string.
args the arguments list used in the format() method. If there are more arguments than those specified by the format string, then the additional arguments are ignored.
Returns
  • this Formatter.
Throws
IllegalFormatException if the format string is illegal or incompatible with the arguments, or if fewer arguments are sent than those required by the format string, or any other illegal situation.
FormatterClosedException if the Formatter has been closed.

public Formatter format (Locale l, String format, Object... args)

Since: API Level 1

Writes a formatted string to the output destination of the Formatter.

Parameters
l the Locale used in the method. If locale is null, then no localization will be applied. This parameter does not change this Formatter's default Locale as specified during construction, and only applies for the duration of this call.
format a format string.
args the arguments list used in the format() method. If there are more arguments than those specified by the format string, then the additional arguments are ignored.
Returns
  • this Formatter.
Throws
IllegalFormatException if the format string is illegal or incompatible with the arguments, or if fewer arguments are sent than those required by the format string, or any other illegal situation.
FormatterClosedException if the Formatter has been closed.

public IOException ioException ()

Since: API Level 1

Returns the last IOException thrown by the Formatter's output destination. If the append() method of the destination does not throw IOExceptions, the ioException() method will always return null.

Returns
  • the last IOException thrown by the Formatter's output destination.

public Locale locale ()

Since: API Level 1

Returns the Locale of the Formatter.

Returns
  • the Locale for the Formatter or null for no Locale.
Throws
FormatterClosedException if the Formatter has been closed.

public Appendable out ()

Since: API Level 1

Returns the output destination of the Formatter.

Returns
  • the output destination of the Formatter.
Throws
FormatterClosedException if the Formatter has been closed.

public String toString ()

Since: API Level 1

Returns the content by calling the toString() method of the output destination.

Returns
  • the content by calling the toString() method of the output destination.
Throws
FormatterClosedException if the Formatter has been closed.