28 Formatted Output

OBJECTIVES In this chapter you will learn: To understand input and output streams. To use printf formatting. To print with field widths and precisions. To use formatting flags in the printf format string. To print with an argument index. To output literals and escape sequences. To format output with class Formatter.

28.1   Introduction 28.2   Streams 28.3   Formatting Output with printf 28.4   Printing Integers 28.5   Printing Floating-Point Numbers 28.6   Printing Strings and Characters 28.7   Printing Dates and Times 28.8   Other Conversion Characters 28.9   Printing with Field Widths and Precisions 28.10   Using Flags in the printf Format String 28.11   Printing with Argument Indices 28.12   Printing Literals and Escape Sequences 28.13   Formatting Output with Class Formatter 28.14   Wrap-Up

28.1 Introduction Method printf Class Formatter Formats and outputs data to the standard output stream, System.out Class Formatter Formats and outputs data to a specified destination E.g., a string or a file output stream

28.2 Streams Streams Sequences of bytes Can often be redirected Standard input – keyboard Standard output – screen Standard error – screen More in Chapters 14 and 24

28.3 Formatting Output with printf Precise output formatting Conversion specifications: flags, field widths, precisions, etc. Can perform rounding aligning columns right/left justification inserting literal characters exponential format octal and hexadecimal format fixed width and precision date and time format

28.3 Formatting Output with printf (Cont.) Format String Describe the output format Consist of fixed text and format specifier Format specifier Placeholder for a value Specify the type of data to output Begins with a percent sign (%) and is followed by a conversion character E.g., %s, %d Optional formatting information Argument index, flags, field width, precision Specified between % and conversion character

28.4 Printing Integers Integer Format Whole number (no decimal point): 25, 0, -9 Positive, negative, or zero Only minus sign prints by default (later we shall change this) Format printf( format-string, argument-list ); format-string Describe the output format argument-list Contain the value corresponding to each format specifier

Fig. 28.1 | Integer conversion characters.

Outline Output positive and negative integers IntegerConversionT est.java Lines 8-10 Line 11 Line 12 Line 13 Program output Output positive and negative integers Output the integer in octal format Output the integer in hexadecimal format Output the integer in hexadecimal format with capital letters

28.5 Printing Floating-Point Numbers Have a decimal point (33.5) Computerized scientific notation (exponential notation) 150.4582 is 1.504582 x 10² in scientific 150.4582 is 1.504582e+02 in exponential (e stands for exponent) use e or E f – print floating point with at least one digit to left of decimal g (or G) - prints in f or e (E) Use exponential if the magnitude is less than 10-3, or greater than or equal to 107

Fig. 28.3 | Floating-point conversion characters.

Outline FloatingNumberTest .java Lines 8-10 Line 11 Line 12 Lines 13-14 Program output Output positive and negative floating-point numbers using the e conversion character Output floating-point number with uppercase E preceding the exponent Output floating-point number using the f conversion character Output floating-point number using the g and G conversion character

28.6 Printing Strings and Characters Conversion character c and C Require char C displays the output in uppercase letters Conversion character s and S String Object Implicitly use object’s toString method S displays the output in uppercase letters

Common Programming Error 28.1 Using %c to print a string causes an IllegalFormatConversionException—a string cannot be converted to a character.

Outline Display character with conversion character c CharStringConversi on.java Line 12 Line 13 Lines 14-15 Line 16 Program output Display character with conversion character c Display string with conversion character s Display string with conversion characters s and S Display Integer object with conversion characters s

28.7 Printing Dates and Times Conversion characters t and T Print dates and times in various formats Followed by a conversion suffix character Require the corresponding argument to be of type long, Long, Calendar or Date Conversion suffix characters Specify the date and/or time format Format date and time compositions Format date Format time

Fig. 28.6 | Date and time composition conversion suffix characters.

Fig. 28.7 | Date formatting conversion suffix characters.

Fig. 28.8 | Time formatting conversion suffix characters.

Outline DateTimeTest.java (1 of 2) Line 10 Lines 13-17 Lines 20-22 Obtain a Calendar with the current date and time Use the Calendar object in printf statements as the value to be formatted with conversion character t Use the optional argument index to indicate that all format specifiers in the format string use the first argument

Outline DateTimeTest.java (2 of 2) Program output

28.8 Other Conversion Characters Remaining conversion characters b or B boolean or Boolean value h or H String representation of an object’s hash code in hexadecimal format % Percent character n Platform-specific line separator \r\n on Windows \n on UNIX\Linux

Common Programming Error 28.2 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.

Fig. 28.10 | Other conversion specifiers.

Outline Print the value of boolean values false and true OtherConversion.ja va Lines 9-10 Lines 11-12 Lines 13-14 Line 15 Lines 16-17 Program output Print the value of boolean values false and true Associate a String and a null object to %b and %B Print the string representations of the hash code values for strings “hello” and “Hello” Print null in uppercase letters Print the % character in a string and a platform-specific line separator

28.9 Printing with Field Widths and Precisions Size of field in which data is printed If width larger than data, default right justified If field width too small, increases to fit data Minus sign uses one character position in field Integer width inserted between % and conversion specifier E.g., %4d – field width of 4 Can be used with all format specifiers except the line separator (%n)

28.9 Printing with Field Widths and Precisions (Cont.) Meaning varies depending on data type Floating point Number of digits to appear after decimal (e or E and f) Maximum number of significant digits (g or G) Strings Maximum number of characters to be written from string Format Use a dot (.) then precision number after % e.g., %.3f

28.9 Printing with Field Widths and Precisions (Cont.) Field width and precision Can both be specified %width.precision %5.3f Negative field width – left justified Positive field width – right justified Precision must be positive Example: printf( "%9.3f", 123.456789 );

Common Programming Error 28.3 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!

Outline Print positive numbers with field width FieldWidthTest.jav a Lines 8-12 Lines 14-18 Program output Print positive numbers with field width Print negative numbers with field width

Outline PrecisionTest.java Line 11 Line 14 Program output Print same floating-point number with same precision but different conversion character Print string with precision

28.10 Using Flags in the printf Format String Supplement formatting capabilities Place flag immediately to the right of percent sign Several flags may be combined

Fig. 28.14 | Format string flags.

Outline MinusFlagTest.java Line 10 Lines 11-12 Program output Right justify a string, an integer, a character and a floating-point number left justify a string, an integer, a character and a floating-point number

Outline Print a positive number with a plus sign PlusFlagTest.java Program output Print a positive number with a plus sign

Outline Prefix a space to the positive number with the space flag SpaceFlagTest.java Line 8 Program output Prefix a space to the positive number with the space flag

Outline PoundFlagTest.java Lines 10-11 Program output Use the # flag to prefix 0 to the octal value and 0x to the hexadecimal value

Outline ZeroFlagTest.java Line 8 Line 9 Line 10 Program output Combine the + flag and the 0 flag print 452 in a field of width 9 with a + sign and leading zeros Print 452 in a field of width 9 using only the 0 flag Print 452 in a field of width 9 using only the space flag

Outline CommaFlagTest.java Lines 8-10 Program output Use the comma flag to display a decimal and a floating-point number with the thousands separator

Outline Enclose negative numbers in parentheses using the ( flag ParenthesesFlagTes t.java Lines 8-10 Program output Enclose negative numbers in parentheses using the ( flag

28.11 Printing with Argument Indices Argument index Optional decimal integer followed by a $ sign Indicate the position of the argument in the argument list E.g., 1$ -- first argument Usage Reorder the output Avoid duplicating arguments

Outline ArgumentIndexTest Lines 11-13 Program output Print arguments in the argument list in reverse order using the argument index

28.12 Printing Literals and Escape Sequences Most characters can be printed Certain "problem" characters, such as the quotation mark (") Must be represented by escape sequences Represented by a backslash \ followed by an escape character

Common Programming Error 28.4 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 sequence might result in a syntax error.

Fig. 28.23 | Escape sequences.

28.13 Formatting Output with Class Formatter Provides same formatting capabilities as printf Output formatted data to a specified destination E.g., a file on disk By default, Formatter creates a string in memory String static method format Create a string in memory without Formatter

Outline FormatterTest.java Line 11 Line 12 Line 15 Program output Create a Formatter object using the default constructor, which will build a string in memory Invoke method format to format the output Invoke Formatter’s toString method to get the formatted data as a string