Using Flags in the printf Format String

Various flags may be used with method printf to supplement its output formatting capabilities. Seven flags are available for use in format strings (Fig. 28.14).

Figure 28.14. Format string flags.

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Flag	Description
`-` (minus sign)	Left justify the output within the specified field.
`+` (plus sign)	Display a plus sign preceding positive values and a minus sign preceding negative values.
`space`	Print a space before a positive value not printed with the + flag.
`#`	Prefix `0` to the output value when used with the octal conversion character `o`.
	Prefix `0x` to the output value when used with the hexadecimal conversion character `x`.
`0` (zero)	Pad a field with leading zeros.
`,` (comma)	Use the locale-specific thousands separator (i.e., `','` for U.S. locale) to display decimal and floating-point numbers.
`(`	Enclose negative numbers in parentheses.

To use a flag in a format string, place the flag immediately to the right of the percent sign. Several flags may be used in the same format specifier. Figure 28.15 demonstrates right justification and left justification of a string, an integer, a character and a floating-point number. Note that line 9 serves as a counting mechanism for the screen output.

Figure 28.15. Right justifying and left justifying values.

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 1 // Fig. 28.15: MinusFlagTest.java
 2 // Right justifying and left justifying values.
 3
 4 public class MinusFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.println( "Columns:" );
 9 System.out.println( "0123456789012345678901234567890123456789
" );
10 System.out.printf( "%10s%10d%10c%10f

", "hello", 7, 'a', 1.23 );
11 System.out.printf( 
12  "%-10s%-10d%-10c%-10f
", "hello", 7, 'a', 1.23 );
13 } // end main
14 } // end class MinusFlagTest

Columns: 0123456789012345678901234567890123456789 hello 7 a 1.230000 hello 7 a 1.230000

Figure 28.16 prints a positive number and a negative number, each with and without the + flag. Note that the minus sign is displayed in both cases, but the plus sign is displayed only when the + flag is used.

Figure 28.16. Printing numbers with and without the + flag.

 1 // Fig. 28.16: PlusFlagTest.java
 2 // Printing numbers with and without the + flag.
 3
 4 public class PlusFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.printf( "%d	%d
", 786, -786 );
 9 System.out.printf( "%+d	%+d
", 786, -786 );
10 } // end main
11 } // end class PlusFlagTest

786 -786 +786 -786

Figure 28.17 prefixes a space to the positive number with the space flag. This is useful for aligning positive and negative numbers with the same number of digits. Note that the value -547 is not preceded by a space in the output because of its minus sign. Figure 28.18 uses the # flag to prefix 0 to the octal value and 0x to the hexadecimal value.

Figure 28.17. Using the space flag to print a space before non-negative values.

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 1 // Fig. 28.17: SpaceFlagTest.java
 2 // Printing a space before non-negative values.
 3
 4 public class SpaceFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.printf( "% d
% d
", 547, -547 );
 9 } // end main
10 } // end class SpaceFlagTest

547 -547

Figure 28.18. Using the # flag with conversion characters o and x.

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 1 // Fig. 28.18: PoundFlagTest.java
 2 // Using the # flag with conversion characters o and x.
 3 
 4 public class PoundFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 int c = 31; // initialize c
 9
10 System.out.printf( "%#o
", c );
11 System.out.printf( "%#x
", c );
12 } // end main
13 } // end class PoundFlagTest

037 0x1f

Figure 28.19 combines the + flag, the 0 flag and the space flag to print 452 in a field of width 9 with a + sign and leading zeros, next prints 452 in a field of width 9 using only the 0 flag, then prints 452 in a field of width 9 using only the space flag.

Figure 28.19. Printing with the 0 (zero) flag fills in leading zeros.

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 1 // Fig. 28.19: ZeroFlagTest.java
 2 // Printing with the 0 (zero) flag fills in leading zeros.
 3
 4 public class ZeroFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.printf( "%+09d
", 452 );
 9 System.out.printf( "%09d
", 452 );
10 System.out.printf( "% 9d
", 452 );
11 } // end main
12 } // end class ZeroFlagTest

+00000452 000000452 452

Figure 28.20 use the comma (,) flag to display a decimal and a floating-point number with the thousands separator.Figure 28.21 encloses negative numbers in parentheses using the ( flag. Note that the value 50 is not enclosed in parentheses in the output because it is a positive number.

Figure 28.20. Using the comma (,) flag to display number with thousands separator.

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 1 // Fig. 28.20: CommaFlagTest.java
 2 // Using the comma (,) flag to display numbers with thousands separator.
 3
 4 public class CommaFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.printf( "%,d
", 58625 );
 9 System.out.printf( "%,.2f", 58625.21 );
10 System.out.printf( "%,.2f", 12345678.9 );
11 } // end main
12 } // end class CommaFlagTest

58,625 58,625.21 12,345,678.90

Figure 28.21. Using the ( flag to place parentheses around negative numbers.

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 1 // Fig. 28.21: ParenthesesFlagTest.java
 2 // Using the ( flag to place parentheses around negative numbers.
 3
 4 public class ParenthesesFlagTest
 5 {
 6 public static void main( String args[] )
 7 {
 8 System.out.printf( "%(d
", 50 );
 9 System.out.printf( "%(d
", -50 );
10 System.out.printf( "%(.1e
", -50.0 );
11 } // end main
12 } // end class ParenthesesFlagTest