Java String Formatting

Printing or producing a simple String message has been really trivial in Java. Most of the time, we use concatenation to create String instance we want:

int a = 3;
String str = "Integer `a` has value `" + a + "`";

Though modern Java compiler uses StringBuilder to optimize statements like these, using concatention to construct String has its limitations, which include:

• The pattern of the String is not reusable;
• The statements can be unacceptably long if we try to construct a complicated message;
• It is impossible to designate the precision of a floating point number.

Fortunately, Java SE 5.0 brought back the venerable printf method from the C library, which also come with the basic feature of string formatting.

Elements of Formatting Patterns

printf method in Java is much like its counterpart in C. For example, the call

System.out.pinrlnt("%8.2f", x);

prints x with a field width of 8 characters and a precision of 2 characters.

You can supply multiple parameters to printf. For example:

System.out.printf("Hello, %s. Next year, you'll be %d.", name, age);

The formatting patterns is written in a String constant, where format specifiers that start with a % character is replaced with the corresponding argument. A format specifier is composed of flag and/or conversion character. Flags are used to control the apperance of the formatted string, while the conversion character that ends a format specifier indicates the type of the value to be formatted: f is a floating-point number, s a string, and d a decimal integer.

Conversion Character	Type	Example
d	Decimal integer	159
x	Hexadecimal integer	9f
o	Octal integer	237
f	Fixed-point floating-point	15.9
e	Exponential floating-point	1.59e+01
g	Genral floating-point (the shorter of e and f)
a	Hexadecimal floating-point	0x1.fccdp3
s	String	Hello
c	Character	H
b	boolean	true
h	Hash code	42628b2
%	The percent symbol
n	The platform-dependent line seperator

| Flag | Purpose | Example |
| + | Prints sign for positive and negative numbers. | +3333.33 |
| space | Adds a space before positive numbers. | | 3333.33| |
| 0 | Adds leading zeroes. | 003333.33 |
| - | Left-justifies field. | |3333.33 | |
| ( | Encloses negative numbers in parentheses. | (3333.33) |
| , | Adds group separators. | 3,333.33 |
| # (for f format) | Always includes a decimal point. | 3,333. |
| # (for x or o format) | Adds 0x or 0 prefix. | 0xcafe |
| $ | Specifies the index of the argument to be formatted; for example,
%1$d %1$x prints the first argument in deximal and hexadecimal. | 159 9F |
| < | Formats the same value as the previous secification; for example,
%d %<x prints the same number in decimal and hexadecimal. | 159 9F |

You can use the static String.format method to create a formatted string without printing it:

String message = String.format("Hello, %s. Next year, you'll be %d.", name, age);

Conversion characters for Date formatting are also available, which start with t:

System.out.printf("%tc", new Date());
// Mon Fex 09 18:05:19 PST 2004

Conversion Character	Type	Example
c	Complete date and time	Mon Feb 09 18:05:19 PST 2004
F	ISO 8601 date	2004-02-09
D	U.S. formatted date (month/day/year)	02/09/2004
T	24-hour time	18:05:19
r	12-hour time	06:05:19 pm
R	24-hour time, no seconds	18:05
Y	Four-digit year (with leading zeroes)	2004
y	Last two digits of the year (with leading zeroes)	04
C	First two digits of the year (with leading zeroes)	20
B	Full month name	February
b or h	Abbreviated month name	Feb
m	Two-digit month (with leading zeroes)	02
d	Two-digit day (with leading zeroes)	09
e	Two-digit day (without leading zeroes)	9
A	Full weekday name	Monday
a	Abbreviated weekday name	Mon
j	Three-digit day of year (with leading zeroes), between 001 and 366	069
H	Two-digit hour (with leading zeroes), between 00 and 23	18
k	Two-digit hour (without leading zeroes), between 0 and 23	18
I	Two-digit hour (with leading zeroes), between 01 and 12	06
l	Two-digit hour (without leading zeroes), between 1 and 12	6
M	Two-digit minutes (with leading zeroes)	05
S	Two-digit seconds (with leading zeroes)	19
L	Three-digit milliseconds (with leading zeroes)	047
N	Nine-digit nanoseconds (with leading zeroes)	047000000
P	Uppercase morning or afternoon marker	PM
p	Lowercase morning or afternoon marker	pm
z	RFC 822 numeric offset from GMT	-0800
Z	Time zone	PST
s	Seconds since 1970-01-01 00:00:00 GMT	1078884319
Q	Milliseconds since 1970-01-01 00:00:00 GMT	1078884319047

As you can see in the preceding table, some of the formates yield only a part of a given date – for example, just the day or just the month.
It would be a bit silly if you had to supply the day multiple times for format each part. For that reason, a format string can indicate
the index of the argument to be formatted. The index must immediately follow the %, and it must be terminated by a $. For example:

System.out.println("%1$s %2$tB %2$te, %2$tY", "Due date:", new Date());
// Due date: February 9, 2004

Alternatively, you can use the < flag. It indicates that the same argument as in the preceding format specification should be used again.
That is, the statement

System.out.println("%s %tB %<te, %<tY", "Due date:", new Date());

yields the same output as the preceding statement.