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IFormatted Output

O b j e c t i v e s In this appendix you’ll:

■ Use printf formatting.

■ Print with field widths and precisions.

■ Use formatting flags in the printf format string.

■ Print with an argument index.

■ Output literals and escape sequences.

■ Format output with class Formatter.

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I_2 Appendix I Formatted Output

I.1 Introduction In this appendix, we discuss the formatting features of method printf and class Formatter (package java.util). Class Formatter formats and outputs data to a specified destination, such as a string or a file output stream. Many features of printf were dis- cussed earlier in the text. This appendix summarizes those features and introduces others, such as displaying date and time data in various formats, reordering output based on the index of the argument and displaying numbers and strings with various flags.

I.2 Streams Input and output are usually performed with streams, which are sequences of bytes. In input operations, the bytes flow from a device (e.g., a keyboard, a disk drive, a network connection) to main memory. In output operations, bytes flow from main memory to a device (e.g., a display screen, a printer, a disk drive, a network connection).

When program execution begins, three streams are created. The standard input stream typically reads bytes from the keyboard, and the standard output stream typically outputs characters to a command window. A third stream, the standard error stream (System.err), typically outputs characters to a command window and is used to output error messages so they can be viewed immediately. Operating systems typically allow these streams to be redirected to other devices. Streams are discussed in detail in Chapter 15, Files, Streams and Object Serialization.

I.3 Formatting Output with printf Precise output formatting is accomplished with printf. Java borrowed (and enhanced) this feature from the C programming language. Method printf can perform the following formatting capabilities, each of which is discussed in this appendix:

1. Rounding floating-point values to an indicated number of decimal places.

2. Aligning a column of numbers with decimal points appearing one above the other.

3. Right justification and left justification of outputs.

4. Inserting literal characters at precise locations in a line of output.

I.1 Introduction I.2 Streams I.3 Formatting Output with printf

I.4 Printing Integers I.5 Printing Floating-Point Numbers I.6 Printing Strings and Characters I.7 Printing Dates and Times I.8 Other Conversion Characters I.9 Printing with Field Widths and

Precisions

I.10 Using Flags in the printf Format String

I.11 Printing with Argument Indices I.12 Printing Literals and Escape

Sequences I.13 Formatting Output with Class

Formatter

I.14 Wrap-Up

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I.4 Printing Integers I_3

5. Representing floating-point numbers in exponential format.

6. Representing integers in octal and hexadecimal format.

7. Displaying all types of data with fixed-size field widths and precisions.

8. Displaying dates and times in various formats.

Every call to printf supplies as the first argument a format string that describes the output format. The format string may consist of fixed text and format specifiers. Fixed text is output by printf just as it would be output by System.out methods print or println. Each format specifier is a placeholder for a value and specifies the type of data to output. Format specifiers also may include optional formatting information.

In the simplest form, each format specifier begins with a percent sign (%) and is fol- lowed by a conversion character that represents the data type of the value to output. For example, the format specifier %s is a placeholder for a string, and the format specifier %d is a placeholder for an int value. The optional formatting information, such as an argu- ment index, flags, field width and precision, is specified between the percent sign and the conversion character. We demonstrate each of these capabilities.

I.4 Printing Integers Figure I.1 describes the integer conversion characters. (See Appendix J for an overview of the binary, octal, decimal and hexadecimal number systems.) Figure I.2 uses each to print an integer. In lines 9–10, the plus sign is not displayed by default, but the minus sign is. Later in this appendix (Fig. I.14) we’ll see how to force plus signs to print.

Conversion character Description

d Display a decimal (base 10) integer.

o Display an octal (base 8) integer.

x or X Display a hexadecimal (base 16) integer. X uses uppercase letters.

Fig. I.1 | Integer conversion characters.

1 // Fig. I.2: IntegerConversionTest.java

2 // Using the integer conversion characters.

3 4 public class IntegerConversionTest

5 {

6 public static void main(String[] args)

7 {

8 System.out.printf( , 26);

9 System.out.printf( , +26);

10 System.out.printf( , -26);

11 System.out.printf( , 26);

12 System.out.printf( , 26);

Fig. I.2 | Using the integer conversion characters. (Part 1 of 2.)

"%d\n"

"%o\n"

"%x\n"

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I_4 Appendix I Formatted Output

The printf method has the form

where format-string describes the output format, and the optional argument-list contains the values that correspond to each format specifier in format-string. There can be many for- mat specifiers in one format string.

Each format string in lines 8–10 specifies that printf should output a decimal integer (%d) followed by a newline character. At the format specifier’s position, printf substitutes the value of the first argument after the format string. If the format string contains mul- tiple format specifiers, at each subsequent format specifier’s position printf substitutes the value of the next argument in the argument list. The %o format specifier in line 11 out- puts the integer in octal format. The %x format specifier in line 12 outputs the integer in hexadecimal format. The %X format specifier in line 13 outputs the integer in hexadecimal format with capital letters.

I.5 Printing Floating-Point Numbers Figure I.3 describes the floating-point conversions. The conversion characters e and E dis- play floating-point values in computerized scientific notation (also called exponential no- tation). Exponential notation is the computer equivalent of the scientific notation used in mathematics. For example, the value 150.4582 is represented in scientific notation in mathematics as

and is represented in exponential notation as

in Java. This notation indicates that 1.504582 is multiplied by 10 raised to the second power (e+02). The e stands for “exponent.”

Values printed with the conversion characters e, E and f are output with six digits of precision to the right of the decimal point by default (e.g., 1.045921)—other precisions must be specified explicitly. For values printed with the conversion character g, the preci- sion represents the total number of digits displayed, excluding the exponent. The default is six digits (e.g., 12345678.9 is displayed as 1.23457e+07). Conversion character f always prints at least one digit to the left of the decimal point. Conversion characters e and E print

13 System.out.printf( , 26);

14 } // end main

15 } // end class IntegerConversionTest

26 26 -26 32 1a 1A

printf(format-string, argument-list);

1.504582 × 102

1.504582e+02

Fig. I.2 | Using the integer conversion characters. (Part 2 of 2.)

"%X\n"

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I.5 Printing Floating-Point Numbers I_5

lowercase e and uppercase E preceding the exponent and always print exactly one digit to the left of the decimal point. Rounding occurs if the value being formatted has more sig- nificant digits than the precision.

Conversion character g (or G) prints in either e (E) or f format, depending on the floating-point value. For example, the values 0.0000875, 87500000.0, 8.75, 87.50 and 875.0 are printed as 8.750000e-05, 8.750000e+07, 8.750000, 87.500000 and 875.000000

with the conversion character g. The value 0.0000875 uses e notation because the magni- tude is less than 10-3. The value 87500000.0 uses e notation because the magnitude is greater than 107. Figure I.4 demonstrates the floating-point conversion characters.

Conversion character Description

e or E Display a floating-point value in exponential notation. Conver- sion character E displays the output in uppercase letters.

f Display a floating-point value in decimal format.

g or G Display a floating-point value in either the floating-point format f or the exponential format e based on the magnitude of the value. If the magnitude is less than 10–3, or greater than or equal to 107, the floating-point value is printed with e (or E). Otherwise, the value is printed in format f. When conversion character G is used, the output is displayed in uppercase letters.

a or A Display a floating-point number in hexadecimal format. Conver- sion character A displays the output in uppercase letters.

Fig. I.3 | Floating-point conversion characters.

1 // Fig. I.4: FloatingNumberTest.java

2 // Using floating-point conversion characters.

3 4 public class FloatingNumberTest

5 {

6 public static void main(String[] args)

7 {

8 System.out.printf( );

9 System.out.printf( );

10 System.out.printf( );

11 System.out.printf( );

12 System.out.printf( );

13 System.out.printf( );

14 System.out.printf( );

15 } // end main

16 } // end class FloatingNumberTest

1.234568e+07 1.234568e+07 -1.234568e+07

Fig. I.4 | Using floating-point conversion characters. (Part 1 of 2.)

"%e\n", 12345678.9

"%e\n", +12345678.9

"%e\n", -12345678.9

"%E\n", 12345678.9

"%f\n", 12345678.9

"%g\n", 12345678.9

"%G\n", 12345678.9

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I_6 Appendix I Formatted Output

I.6 Printing Strings and Characters The c and s conversion characters print individual characters and strings, respectively. Conversion characters c and C require a char argument. Conversion characters s and S

can take a String or any Object as an argument. When conversion characters C and S are used, the output is displayed in uppercase letters. Figure I.5 displays characters, strings and objects with conversion characters c and s. Autoboxing occurs at line 9 when an int con- stant is assigned to an Integer object. Line 15 outputs an Integer argument with the con- version character s, which implicitly invokes the toString method to get the integer value. You can also output an Integer object using the %d format specifier. In this case, the int

value in the Integer object will be unboxed and output.

1.234568E+07 12345678.900000 1.23457e+07 1.23457E+07

Common Programming Error I.1 Using %c to print a String causes an IllegalFormatConversionException—a String cannot be converted to a character.

1 // Fig. I.5: CharStringConversion.java

2 // Using character and string conversion characters.

3 public class CharStringConversion

4 {

5 public static void main(String[] args)

6 {

7 char character = 'A'; // initialize char

8 String string = "This is also a string"; // String object

9 Integer integer = 1234; // initialize integer (autoboxing)

10 11 System.out.printf( );

12 System.out.printf( );

13 System.out.printf( );

14 System.out.printf( );

15 System.out.printf( ); // implicit call to toString

16 } // end main

17 } // end class CharStringConversion

A This is a string This is also a string THIS IS ALSO A STRING 1234

Fig. I.5 | Using character and string conversion characters.

Fig. I.4 | Using floating-point conversion characters. (Part 2 of 2.)

"%c\n", character

"%s\n", "This is a string"

"%s\n", string

"%S\n", string

"%s\n", integer

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I.7 Printing Dates and Times I_7

I.7 Printing Dates and Times The conversion character t (or T) is used to print dates and times in various formats. It’s always followed by a conversion suffix character that specifies the date and/or time for- mat. When conversion character T is used, the output is displayed in uppercase letters. Figure I.6 lists the common conversion suffix characters for formatting date and time compositions that display both the date and the time. Figure I.7 lists the common con- version suffix characters for formatting dates. Figure I.8 lists the common conversion suf- fix characters for formatting times. For the complete list of conversion suffix characters, visit http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html.

Conversionsuffix character Description

c Display date and time formatted as day month date hour:minute:second time-zone year

with three characters for day and month, two digits for date, hour, min- ute and second and four digits for year—for example, Wed Mar 03

16:30:25 GMT-05:00 2004. The 24-hour clock is used. GMT-05:00 is the time zone.

F Display date formatted as year-month-date with four digits for the year and two digits each for the month and date (e.g., 2004-05-04).

D Display date formatted as month/day/year with two digits each for the month, day and year (e.g., 03/03/04).

r Display time in 12-hour format as hour:minute:second AM|PM with two digits each for the hour, minute and second (e.g., 04:30:25 PM).

R Display time formatted as hour:minute with two digits each for the hour and minute (e.g., 16:30). The 24-hour clock is used.

T Display time as hour:minute:second with two digits for the hour, min- ute and second (e.g., 16:30:25). The 24-hour clock is used.

Fig. I.6 | Date and time composition conversion suffix characters.

Conversion suffix character Description

A Display full name of the day of the week (e.g., Wednesday).

a Display the three-character name of the day of the week (e.g., Wed). B Display full name of the month (e.g., March).

b Display the three-character short name of the month (e.g., Mar).

d Display the day of the month with two digits, padding with leading zeros as necessary (e.g., 03).

m Display the month with two digits, padding with leading zeros as neces- sary (e.g., 07).

Fig. I.7 | Date formatting conversion suffix characters. (Part 1 of 2.)

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I_8 Appendix I Formatted Output

Figure I.9 uses the conversion characters t and T with the conversion suffix characters to display dates and times in various formats. Conversion character t requires the corre- sponding argument to be a date or time of type long, Long, Calendar (package java.util) or Date (package java.util)—objects of each of these classes can represent dates and times. Class Calendar is preferred for this purpose because some constructors and

e Display the day of month without leading zeros (e.g., 3).

Y Display the year with four digits (e.g., 2004). y Display the last two digits of the year with leading zeros (e.g., 04).

j Display the day of the year with three digits, padding with leading zeros as necessary (e.g., 016).

Conversion suffix character Description

H Display hour in 24-hour clock with a leading zero as necessary (e.g., 16).

I Display hour in 12-hour clock with a leading zero as necessary (e.g., 04).

k Display hour in 24-hour clock without leading zeros (e.g., 16).

l Display hour in 12-hour clock without leading zeros (e.g., 4).

M Display minute with a leading zero as necessary (e.g., 06).

S Display second with a leading zero as necessary (e.g., 05).

Z Display the abbreviation for the time zone (e.g., EST, stands for Eastern Standard Time, which is 5 hours behind Greenwich Mean Time).

p Display morning or afternoon marker in lowercase (e.g., pm).

P Display morning or afternoon marker in uppercase (e.g., PM).

Fig. I.8 | Time formatting conversion suffix characters.

1 // Fig. I.9: DateTimeTest.java

2 // Formatting dates and times with conversion characters t and T.

3 import java.util.Calendar;

4 5 public class DateTimeTest

6 {

7 public static void main(String[] args)

8 {

9 // get current date and time

10 Calendar dateTime = Calendar.getInstance();

Fig. I.9 | Formatting dates and times with conversion characters t and T. (Part 1 of 2.)

Conversion suffix character Description

Fig. I.7 | Date formatting conversion suffix characters. (Part 2 of 2.)

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I.8 Other Conversion Characters I_9

methods in class Date are replaced by those in class Calendar. Line 10 invokes static

method getInstance of Calendar to obtain a calendar with the current date and time. Lines 13–17, 20–22 and 25–26 use this Calendar object in printf statements as the value to be formatted with conversion character t. Lines 20–22 and 25–26 use the optional argument index ("1$") to indicate that all format specifiers in the format string use the first argument after the format string in the argument list. You’ll learn more about argu- ment indices in Section I.11. Using the argument index eliminates the need to repeatedly list the same argument.

I.8 Other Conversion Characters The remaining conversion characters are b, B, h, H, % and n. These are described in Fig. I.10. Lines 9–10 of Fig. I.11 use %b to print the value of boolean (or Boolean) values false and true. Line 11 associates a String to %b, which returns true because it’s not null. Line 12 associates a null object to %B, which displays FALSE because test is null. Lines 13–14 use %h to print the string representations of the hash-code values for strings "hello" and "Hello". These values could be used to store or locate the strings in a Hashtable or HashMap (both discussed in Chapter 16, Generic Collections). The hash- code values for these two strings differ, because one string starts with a lowercase letter and

11 12 // printing with conversion characters for date/time compositions

13 System.out.printf( );

14 System.out.printf( );

15 System.out.printf( );

16 System.out.printf( );

17 System.out.printf( );

18 19 // printing with conversion characters for date

20 System.out.printf( );

21 System.out.printf( );

22 System.out.printf( );

23 24 // printing with conversion characters for time

25 System.out.printf( );

26 System.out.printf( );

27 } // end main

28 } // end class DateTimeTest

Wed Feb 25 15:00:22 EST 2009 2009-02-25 02/25/09 03:00:22 PM 15:00:22 Wednesday, February 25, 2009 WEDNESDAY, FEBRUARY 25, 2009 Wed, Feb 25, 09 15:00:22 EST 03:00:22 PM

Fig. I.9 | Formatting dates and times with conversion characters t and T. (Part 2 of 2.)

"%tc\n", dateTime

"%tF\n", dateTime

"%tD\n", dateTime

"%tr\n", dateTime

"%tT\n", dateTime

"%1$tA, %1$tB %1$td, %1$tY\n", dateTime

"%1$TA, %1$TB %1$Td, %1$TY\n", dateTime

"%1$ta, %1$tb %1$te, %1$ty\n", dateTime

"%1$tH:%1$tM:%1$tS\n", dateTime

"%1$tZ %1$tI:%1$tM:%1$tS %tP", dateTime

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I_10 Appendix I Formatted Output

the other with an uppercase letter. Line 15 uses %H to print null in uppercase letters. The last two printf statements (lines 16–17) use %% to print the % character in a string and %n

to print a platform-specific line separator.

Conversion character Description

b or B Print "true" or "false" for the value of a boolean or Bool- ean. These conversion characters can also format the value of any reference. If the reference is non-null, "true" is output; otherwise, "false". When conversion character B is used, the output is displayed in uppercase letters.

h or H Print the string representation of an object’s hash-code value in hexadecimal format. If the corresponding argument is null, "null" is printed. When conversion character H is used, the output is displayed in uppercase letters.

% Print the percent character. n Print the platform-specific line separator (e.g., \r\n on Win-

dows or \n on UNIX/LINUX).

Fig. I.10 | Other conversion characters.

1 // Fig. I.11: OtherConversion.java

2 // Using the b, B, h, H, % and n conversion characters.

3 4 public class OtherConversion

5 {

6 public static void main(String[] args)

7 {

8 Object test = null;

9 System.out.printf(" \n", false);

10 System.out.printf(" \n", true);

11 System.out.printf(" \n", "Test");

12 System.out.printf(" \n", test);

13 System.out.printf("Hashcode of \"hello\" is \n", "hello");

14 System.out.printf("Hashcode of \"Hello\" is \n", "Hello");

15 System.out.printf("Hashcode of null is \n", test);

16 System.out.printf("Printing a in a format string\n");

17 System.out.printf("Printing a new line next line starts here");

18 } // end main

19 } // end class OtherConversion

false true true FALSE Hashcode of "hello" is 5e918d2

Fig. I.11 | Using the b, B, h, H, % and n conversion characters. (Part 1 of 2.)

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I.9 Printing with Field Widths and Precisions I_11

I.9 Printing with Field Widths and Precisions The size of a field in which data is printed is specified by a field width. If the field width is larger than the data being printed, the data is right justified in that field by default. We discuss left justification in Section I.10. You insert an integer representing the field width between the % and the conversion character (e.g., %4d) in the format specifier. Figure I.12 prints two groups of five numbers each, right justifying those numbers that contain fewer digits than the field width. The field width is increased to print values wider than the field and that the minus sign for a negative value uses one character position in the field. Also, if no field width is specified, the data prints in exactly as many positions as it needs. Field widths can be used with all format specifiers except the line separator (%n).

Hashcode of "Hello" is 42628b2 Hashcode of null is NULL Printing a % in a format string Printing a new line next line starts here

Common Programming Error I.2 Trying to print a literal percent character using % rather than %% in the format string might cause a difficult-to-detect logic error. When % appears in a format string, it must be followed by a conversion character in the string. The single percent could accidentally be followed by a legitimate conversion character, thus causing a logic error.

1 // Fig. I.12: FieldWidthTest.java

2 // Right justifying integers in fields.

3 4 public class FieldWidthTest

5 {

6 public static void main(String[] args)

7 {

8 System.out.printf( );

9 System.out.printf( );

10 System.out.printf( );

11 System.out.printf( );

12 System.out.printf( ); // data too large

13 14 System.out.printf( );

15 System.out.printf( );

16 System.out.printf( );

17 System.out.printf( ); // data too large

18 System.out.printf( ); // data too large

19 } // end main

20 } // end class RightJustifyTest

Fig. I.12 | Right justifying integers in fields. (Part 1 of 2.)

Fig. I.11 | Using the b, B, h, H, % and n conversion characters. (Part 2 of 2.)

"%4d\n", 1

"%4d\n", 12

"%4d\n", 123

"%4d\n", 1234

"%4d\n\n", 12345

"%4d\n", -1

"%4d\n", -12

"%4d\n", -123

"%4d\n", -1234

"%4d\n", -12345

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I_12 Appendix I Formatted Output

Method printf also provides the ability to specify the precision with which data is printed. Precision has different meanings for different types. When used with floating- point conversion characters e and f, the precision is the number of digits that appear after the decimal point. When used with conversion characters g, a or A, the precision is the maximum number of significant digits to be printed. When used with conversion character s, the precision is the maximum number of characters to be written from the string. To use precision, place between the percent sign and the conversion specifier a decimal point (.) followed by an integer representing the precision. Figure I.13 demonstrates the use of pre- cision in format strings. When a floating-point value is printed with a precision smaller than the original number of decimal places in the value, the value is rounded. Also, the format specifier %.3g indicates that the total number of digits used to display the floating-point value is 3. Because the value has three digits to the left of the decimal point, the value is rounded to the ones position.

The field width and the precision can be combined by placing the field width, fol- lowed by a decimal point, followed by a precision between the percent sign and the con- version character, as in the statement

which displays 123.457 with three digits to the right of the decimal point right justified in a nine-digit field—this number will be preceded in its field by two blanks.

1 12

123 1234 12345

-1 -12 -123 -1234 -12345

Common Programming Error I.3 Not providing a sufficiently large field width to handle a value to be printed can offset other data being printed and produce confusing outputs. Know your data!

printf("%9.3f", 123.456789);

1 // Fig. I.13: PrecisionTest.java

2 // Using precision for floating-point numbers and strings.

3 public class PrecisionTest

4 {

5 public static void main(String[] args)

6 {

7 double f = 123.94536;

8 String s = "Happy Birthday";

Fig. I.13 | Using precision for floating-point numbers and strings. (Part 1 of 2.)

Fig. I.12 | Right justifying integers in fields. (Part 2 of 2.)

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I.10 Using Flags in the printf Format String I_13

I.10 Using Flags in the printf Format String Various flags may be used with method printf to supplement its output formatting ca- pabilities. Seven flags are available for use in format strings (Fig. I.14).

To use a flag in a format string, place it immediately to the right of the percent sign. Several flags may be used in the same format specifier. Figure I.15 demonstrates right jus- tification and left justification of a string, an integer, a character and a floating-point number. Line 9 serves as a counting mechanism for the screen output.

Figure I.16 prints a positive number and a negative number, each with and without the + flag. The minus sign is displayed in both cases, the plus sign only when the + flag is used.

9 10 System.out.printf("Using precision for floating-point numbers\n");

11 12 13 System.out.printf("Using precision for strings\n");

14 15 } // end main

16 } // end class PrecisionTest

Using precision for floating-point numbers 123.945 1.239e+02 124

Using precision for strings Happy Birth

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 conver- sion character x.

0 (zero) Pad a field with leading zeros.

, (comma) Use the locale-specific thousands separator (i.e., ',' for U.S. locale) to dis- play decimal and floating-point numbers.

( Enclose negative numbers in parentheses.

Fig. I.14 | Format string flags.

Fig. I.13 | Using precision for floating-point numbers and strings. (Part 2 of 2.)

System.out.printf("\t%.3f\n\t%.3e\n\t%.3g\n\n", f, f, f);

System.out.printf("\t%.11s\n", s);

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I_14 Appendix I Formatted Output

Figure I.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. The value -

547 is not preceded by a space in the output because of its minus sign. Figure I.18 uses the # flag to prefix 0 to the octal value and 0x to the hexadecimal value.

1 // Fig. I.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\n");

10 System.out.printf("%10s%10d%10c%10f\n\n", "hello", 7, 'a', 1.23);

11 12 13 } // end main

14 } // end class MinusFlagTest

Columns: 0123456789012345678901234567890123456789

hello 7 a 1.230000

Fig. I.15 | Right justifying and left justifying values.

1 // Fig. I.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\t%d\n", 786, -786);

9 System.out.printf( );

10 } // end main

11 } // end class PlusFlagTest

786 -786 +786 -786

Fig. I.16 | Printing numbers with and without the + flag.

1 // Fig. I.17: SpaceFlagTest.java

2 // Printing a space before non-negative values.

Fig. I.17 | Printing a space before nonnegative values. (Part 1 of 2.)

System.out.printf(

"%-10s%-10d%-10c%-10f\n", "hello", 7, 'a', 1.23);

"%+d\t%+d\n", 786, -786

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I.10 Using Flags in the printf Format String I_15

Figure I.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.

4 public class SpaceFlagTest

5 {

6 public static void main(String[] args)

7 {

8 9 } // end main

10 } // end class SpaceFlagTest

547 -547

1 // Fig. I.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 11 12 } // end main

13 } // end class PoundFlagTest

037 0x1f

Fig. I.18 | Using the # flag with conversion characters o and x.

1 // Fig. I.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( , 452);

9 System.out.printf( , 452);

10 System.out.printf( , 452);

11 } // end main

12 } // end class ZeroFlagTest

Fig. I.19 | Printing with the 0 (zero) flag fills in leading zeros. (Part 1 of 2.)

Fig. I.17 | Printing a space before nonnegative values. (Part 2 of 2.)

System.out.printf("% d\n% d\n", 547, -547);

System.out.printf("%#o\n", c);

System.out.printf("%#x\n", c);

"%+09d\n"

"%09d\n"

"% 9d\n"

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I_16 Appendix I Formatted Output

Figure I.20 uses the comma (,) flag to display a decimal and a floating-point number with the thousands separator. Figure I.21 encloses negative numbers in parentheses using the (flag. The value 50 is not enclosed in parentheses in the output because it’s a positive number.

+00000452 000000452

452

1 // Fig. I.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( );

9 System.out.printf( );

10 System.out.printf( );

11 } // end main

12 } // end class CommaFlagTest

58,625 58,625.21 12,345,678.90

Fig. I.20 | Using the comma (,) flag to display numbers with the thousands separator.

1 // Fig. I.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( );

9 System.out.printf( );

10 System.out.printf( );

11 } // end main

12 } // end class ParenthesesFlagTest

50 (50) (5.0e+01)

Fig. I.21 | Using the (flag to place parentheses around negative numbers.

Fig. I.19 | Printing with the 0 (zero) flag fills in leading zeros. (Part 2 of 2.)

"%,d\n", 58625

"%,.2f", 58625.21

"%,.2f", 12345678.9

"%(d\n", 50

"%(d\n", -50

"%(.1e\n", -50.0

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I.11 Printing with Argument Indices I_17

I.11 Printing with Argument Indices An argument index is an optional integer followed by a $ sign that indicates the argu- ment’s position in the argument list. For example, lines 20–22 and 25–26 in Fig. I.9 use argument index "1$" to indicate that all format specifiers use the first argument in the ar- gument list. Argument indices enable programmers to reorder the output so that the ar- guments in the argument list are not necessarily in the order of their corresponding format specifiers. Argument indices also help avoid duplicating arguments. Figure I.22 prints ar- guments in the argument list in reverse order using the argument index.

I.12 Printing Literals and Escape Sequences Most literal characters to be printed in a printf statement can simply be included in the format string. However, there are several “problem” characters, such as the quotation mark (") that delimits the format string itself. Various control characters, such as newline and tab, must be represented by escape sequences. An escape sequence is represented by a back- slash (\), followed by an escape character. Figure I.23 lists the escape sequences and the actions they cause.

1 // Fig. I.22: ArgumentIndexTest

2 // Reordering output with argument indices.

3 4 public class ArgumentIndexTest

5 {

6 public static void main(String[] args)

7 {

8 System.out.printf(

9 "Parameter list without reordering: %s %s %s %s\n",

10 "first", "second", "third", "fourth");

11 System.out.printf(

12 ,

13 "first", "second", "third", "fourth");

14 } // end main

15 } // end class ArgumentIndexTest

Parameter list without reordering: first second third fourth Parameter list after reordering: fourth third second first

Fig. I.22 | Reordering output with argument indices.

Escape sequence Description

\' (single quote) Output the single quote (') character.

\" (double quote) Output the double quote (") character.

\\ (backslash) Output the backslash (\) character.

\b (backspace) Move the cursor back one position on the current line.

Fig. I.23 | Escape sequences. (Part 1 of 2.)

"Parameter list after reordering: %4$s %3$s %2$s %1$s\n"

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I_18 Appendix I Formatted Output

I.13 Formatting Output with Class Formatter So far, we’ve discussed displaying formatted output to the standard output stream. What should we do if we want to send formatted outputs to other output streams or devices, such as a JTextArea or a file? The solution relies on class Formatter (in package java.util), which provides the same formatting capabilities as printf. Formatter is a utility class that enables programmers to output formatted data to a specified destination, such as a file on disk. By default, a Formatter creates a string in memory. Figure I.24 demonstrates how to use a Formatter to build a formatted string, which is then displayed in a message dialog.

Line 11 creates a Formatter object using the default constructor, so this object will build a string in memory. Other constructors are provided to allow you to specify the des- tination to which the formatted data should be output. For details, see http://

docs.oracle.com/javase/7/docs/api/java/util/Formatter.html.

\f (new page or form feed) Move the cursor to the start of the next logical page.

\n (newline) Move the cursor to the beginning of the next line.

\r (carriage return) Move the cursor to the beginning of the current line.

\t (horizontal tab) Move the cursor to the next horizontal tab position.

Common Programming Error I.4 Attempting to print as literal data in a printf statement a double quote or backslash character without preceding that character with a backslash to form a proper escape se- quence might result in a syntax error.

1 // Fig. I.24: FormatterTest.java

2 // Formatting output with class Formatter.

3 import java.util.Formatter;

4 import javax.swing.JOptionPane;

5 6 public class FormatterTest

7 {

8 public static void main(String[] args)

9 {

10 // create Formatter and format output

11 Formatter formatter = new Formatter();

12 13 14 // display output in JOptionPane

15 JOptionPane.showMessageDialog(null, );

16 } // end main

17 } // end class FormatterTest

Fig. I.24 | Formatting output with class Formatter. (Part 1 of 2.)

Escape sequence Description

Fig. I.23 | Escape sequences. (Part 2 of 2.)

formatter.format("%d = %#o = %#X", 10, 10, 10);

formatter.toString()

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I.14 Wrap-Up I_19

Line 12 invokes method format to format the output. Like printf, method format

takes a format string and an argument list. The difference is that printf sends the for- matted output directly to the standard output stream, while format sends the formatted output to the destination specified by its constructor (a string in memory in this program). Line 15 invokes the Formatter’s toString method to get the formatted data as a string, which is then displayed in a message dialog.

Class String also provides a static convenience method named format that enables you to create a string in memory without the need to first create a Formatter object. Lines 11–12 and line 15 in Fig. I.24 could have been replaced by

I.14 Wrap-Up This appendix summarized how to display formatted output with various format charac- ters and flags. We displayed decimal numbers using format characters d, o, x and X; float- ing-point numbers using format characters e, E, f, g and G; and dates and times in various format using format characters t and T and their conversion suffix characters. You learned how to display output with field widths and precisions. We introduced the flags +, -, space, #, 0, comma and (that are used together with the format characters to produce out- put. We also demonstrated how to format output with class Formatter.

String s = String.format("%d = %#o = %#x", 10, 10, 10);

JOptionPane.showMessageDialog(null, s);

Fig. I.24 | Formatting output with class Formatter. (Part 2 of 2.)

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