Conversions and Java Print Streams:-

(1) Date and time conversions.....

Date and time conversions can be applied to java.util.Calendar and java.util.Date objects. They can also be applied to long andLongvalues, in which case the value is assumed to be the number of milliseconds since midnight, January 1, 1970. All date and time conversions begin with atfor lowercase or aTfor uppercase. These conversions are:

%tH/ %TH

Two-digit hour using a 24-hour clock, ranging from 00

to 23

%tI/%TI

Two-digit hour using a 12-hour clock, ranging from 01

to 12

%tk/ %Tk

One- or two-digit hour using a 24-hour clock, ranging

from 0 to 23

%tl/ %Tl

One- or two-digit hour using a 12-hour clock, ranging

from 1 to 12

%tM/ %TM

Two-digit minutes, ranging from 00 to 59

%tS/ %TS

Two-digit seconds, ranging from 00 to 60 (60 is used

for leap seconds)

%tL/ %TL

Three-digit milliseconds, ranging from 000 to 999

%tN/ %TN

Nine-digit nanoseconds, ranging from 000000000 to

999999999

%tp/ %Tp

Locale-specific morning/afternoon indicator, such as

am or PM

%tz/ %Tz

RFC 822 numeric time zone indicator as an offset

from UMT (for instance, Eastern Standard Time is–

0500)

%tZ/ %TZ

An abbreviation for the time zone, such as edt or EST

%ts/ %Ts

Seconds elapsed since midnight, January 1, 1970,

Greenwich Mean Time

%TQ

Milliseconds elapsed since midnight, January 1, 1970,

Greenwich Mean Time

%tB/ %TB

Localized month, such as “January” or “JANVIER”

%tb/ %Tb

Localized, abbreviated month, such as “Jan” or “JAN”

%th/ %Th

Localized, abbreviated month, such as “Jan” or

“JAN” (yes,%tband%thare synonyms; I have no

idea why)

%tA/ %TA

Localized day name, such as “Tuesday” or “MARDI”

%ta/ %Ta

Localized, abbreviated day, such as “Tue” or “TUE”

%tC/ %TC

Two-digit century, ranging from 00 to 99

%tY/ %TY

Year with at least four digits, ranging from 0001 to

the indefinite future

%ty/ %Ty

Two-digit year, ranging from 00 to 99

%tj/ %Tj

Three-digit day of the year, ranging from 001 to 366

%tm/ %Tm

Two-digit month, ranging from 01 to 13 (13 is used in

some non-Gregorian lunar calendars)

%td/ %Td

Two-digit day of the month, ranging from 01 to 31

%te/ %Te

One- or two-digit day of the month, ranging from 1 to

31

%tR/ %TR

Hours and minutes on a 24-hour clock, such as 03:23

or 14:07

%tT/ %TT

Hours, minutes, and seconds on a 24-hour clock,

such as 03:23:17 or 14:07:00

%tr/ %Tr

Hours, minutes, and seconds on a 12-hour clock,

such as 03:23:17 am or 02:07:00 PM

%tD/ %TD

Date in the form month/day/year, such as 05/12/06

%tF/ %TF

ISO 8601 standard date in the form year-month-day,

such as 2006-05-12

%tc/ %Tc

Date and time formatted like so: “Fri May 12

12:27:30 EDT 2006”

Example :- prints the current date and time in all of these formats.

Example :- Date format specifiers

import java.util.Date;

public class DateFormatExample {

public static void main(String[] args) {

Date now = new Date();

System.out.printf("two-digit hour on a 24-hour clock: %tH/%TH\n", now, now);

System.out.printf("two-digit hour on a 12-hour clock: %tI/%TI\n", now, now);

System.out.printf("one- or two-digit hour on a 24-hour clock: %tk/%Tk\n",

now, now);

System.out.printf("one- or two-digit hour on a 12-hour clock: %tl/%Tl\n", now,

now);

System.out.printf("two-digit minutes ranging from 00 to 59: %tH/%TH\n",

now, now);

System.out.printf("two-digit seconds ranging from 00 to 60 : %tS/%TS\n",

now, now);

System.out.printf("milliseconds: %tL/%TL\n", now, now);

System.out.printf("nanoseconds: %tN/%TN\n", now, now);

System.out.printf("locale-specific morning/afternoon indicator: %tp/%Tp\n",

now, now);

System.out.printf("RFC 822 numeric time zone indicator: %tz/%Tz\n", now, now);

System.out.printf("time zone abbreviation: %tZ/%TZ\n", now, now);

System.out.printf("seconds since the epoch: %ts/%Ts\n", now, now);

System.out.printf("milliseconds since the epoch: %TQ\n", now);

System.out.printf("localized month name: %tB/%TB\n", now, now);

System.out.printf("localized, abbreviated month: %tb/%Tb\n", now, now);

System.out.printf("localized, abbreviated month: %th/%Th\n", now, now);

System.out.printf("localized day name: %tA/%TA\n", now, now);

System.out.printf("localized, abbreviated day: %ta/%Ta\n", now, now);

System.out.printf("two-digit century:

%tC/%TC\n", now, now);

System.out.printf("four-digit year:

%tY/%TY\n", now, now);

System.out.printf("two-digit year:

%ty/%Ty\n", now, now);

System.out.printf("three-digit day of the year: %tj/%Tj\n", now, now);

System.out.printf("two-digit month:

%tm/%Tm\n", now, now);

System.out.printf("two-digit day of the month: %td/%Td\n", now, now);

System.out.printf("one- or two-digit day of the month: %te/%Te\n", now, now);

System.out.printf("hours and minutes on a 24-hour clock: %tR/%TR\n", now, now);

System.out.printf("hours, minutes, and seconds on a 24-hour clock: %tT/%TT\n",

now, now);

System.out.printf("hours, minutes, and seconds on a 12-hour clock: %tr/%Tr\n",

now, now);

System.out.printf("month/day/year:

%tD/%TD\n", now, now);

System.out.printf("ISO 8601 standard date: %tF/%TF\n", now, now);

System.out.printf("Unix date format:

%tc/%Tc\n", now, now);

}

}

Here’s the output when this was run on Friday, June 24, 2005 at 6:43 PM EDT:

two-digit hour on a 24-hour clock: 18/18

two-digit hour on a 12-hour clock: 06/06

one- or two-digit hour on a 24-hour clock: 18/18

one- or two-digit hour on a 12-hour clock: 6/6

two-digit minutes ranging from 00 to 59: 18/18

two-digit seconds ranging from 00 to 60 : 50/50

milliseconds: 859/859

nanoseconds: 859000000/859000000

locale-specific morning/afternoon indicator: pm/PM

RFC 822 numeric time zone indicator:

-0500/-0500

time zone abbreviation: EDT/EDT

seconds since the epoch: 1119653030/1119653030

milliseconds since the epoch: 1119653030859

localized month name: June/JUNE

localized, abbreviated month: Jun/JUN

localized, abbreviated month: Jun/JUN

localized day name: Friday/FRIDAY

localized, abbreviated day: Fri/FRI

two-digit century: 20/20

four-digit year: 2005/2005

two-digit year: 05/05

three-digit day of the year: 175/175

two-digit month: 06/06

two-digit day of the month: 24/24

one- or two-digit day of the month: 24/24

hours and minutes on a 24-hour clock: 18:43/18:43

hours, minutes, and seconds on a 24-hour clock: 18:43:50/18:43:50

hours, minutes, and seconds on a 12-hour clock: 06:43:50 PM/06:43:50 PM

month/day/year: 06/24/05/06/24/05

ISO 8601 standard date: 2005-06-24/2005-06-24

Unix date format: Fri Jun 24 18:43:50 EDT 2005/FRI JUN 24 18:43:50 EDT 2005

(2) Conversions and Java Print Streams - Character conversions

Character conversions can be applied to char and java.lang.Character objects. They can also be applied to byte, short,int, and the equivalent type-wrapper objects if the integer falls into the range of Unicode code points (0 to 0x10FFFF). These

conversions are:

%c

A lowercase Unicode character

%C

An uppercase Unicode character

Boolean conversions

Boolean conversions can be applied to boolean primitive values and java.lang.Boolean objects. They can also be applied to all object types, in which case they’re considered to be true if the object is nonnull and false if it is null. All other primitive types are considered to be true, regardless of value. These conversions are:

%b

“true” or “false”

%B

“TRUE” or “FALSE”

These conversions are not localized. Even in France, you’ll see “true” and “false” instead of “vrai” and “faux.”

General conversions

There are two more conversions that can be applied to any object and also to primitive types after autoboxing. These are:

%h/%H

The lowercase/uppercase hexadecimal form of the

object’shashCode, or “null” or “NULL” if the object is

null.

%s/%S

The result of invoking the object’sformatTo()

method if it implements Formattable; otherwise,

the result of invoking its toString()method, or

“null” if the object is null. With%S, this value is then

converted to uppercase.

Example :- General format specifiers

import java.net.*;

public class GeneralFormatExample {

public static void main(String[] args) throws MalformedURLException{

URL u = new URL(http://www.example.com/Article.html);

System.out.printf("boolean: %b\n", u);

System.out.printf("BOOLEAN: %B\n", u);

System.out.printf("hashcode: %h\n", u);

System.out.printf("HASHCODE: %H\n", u);

System.out.printf("string: %s\n", u);

System.out.printf("STRING: %S\n", u);

}

}

Here’s the output from running this on a U.S.-localized system:

boolean: true

BOOLEAN: TRUE

hashcode: 79d2cef0

HASHCODE: 79D2CEF0

string: http://www.example.com/ Article.html

STRING: HTTP://WWW.EXAMPLE.COM/ ARTICLE.HTML

(3). Conversions and Java Print Streams - Format Modifiers

In addition to a conversion code, the format string can also specify a width, a precision, the argument it’s replaced with, and any of several special-purpose flags. The most general format follows this pattern:

%[argument_index$][flags][width][.precision]conversion

Here is a quick definition of those parameters. More detail on each will follow:

argument_index

The number of the argument with which to replace this tag

flags

Indicators of various formatting options

width

The minimum number of characters with which to format the replacement value

precision

The number of characters after the decimal point; alternately, the maximum number of characters in the formatted string

These four options control exactly how a string is formatted as tags are replaced by values.

Argument index

The argument index is specified when the order of the arguments does not match the order of the tags. For example:

out.printf("There are %2$f centimeters in %1$f feet.", feet, 2.54 * feet * 12);

In this case, indexes start with 1 rather than 0, which is unusual for Java. (The format string counts as argument 0.) If you reference a nonexistent argument,printf()throws aMissingFormatArgumentException.

The argument index is particularly useful when you want to repeat the same value more than once in a string, perhaps formatted differently each time. For example:

System.out.printf("Hexadecimal: %1$H Decimal: %1$f", Math.PI);

You can also repeat the previous argument by specifying a less than sigh (<) rather than an integer and a dollar sign ($). For instance, this statement is equivalent to the previous statement:

System.out.printf("Hexadecimal: %1$H Decimal: %

(4). Conversions and Java Print Streams - Flags

Flags indicate a variety of formatting options. Not all flags apply to all types. Using a flag to format a type that does not apply throws a FormatFlagsConversionMismatchException, a subclass of IllegalFormatException. However, you can combine multiple flags that do apply.

Flag Signifies Applies to

- Left-justify. All

# Alternate form. General, integer, floating point

+ Include a sign even if positive.

(Normally, only negative numbers have signs.) Integer, floating point

space Add a leading space to positive numbers. Integer, floating point

(This is where the sign would be and helps line up

positive and negative numbers.)

0 Pad with zeros instead of spaces. Integer, floating point

, Use the locale-specific grouping separator instead Integer, floating point

of a period.

( Use instead of a minus sign to indicate negative Integer, floating point

numbers.

For example, this statement prints adoubleformatted as a 20-character decimal padded with zeros, using a locale-specific grouping separator and parentheses for negative numbers:

System.out.printf("%(+0,20f", -Math.PI);

The result is(00000000003.141593).

The relative order of the flags does not matter. This statement prints the same thing:

System.out.printf("%,0+(20f", -Math.PI);

Width

Each of the various conversions may be preceded by an optional width. This sets the minimum number of characters to print. For example, if you format an integer using the code %5d, it will always be printed at least five characters wide. If the integer has fewer than five digits, extra spaces are added on the left-hand side to make up five characters. If it has five or more digits, no extra spaces are added.

The entire number is always printed. If the argument is larger than can fit in five places, all of it will be printed anyway, and subsequent columns may no longer line up properly.

For example, this statement prints five mathematical constants, each 12 places wide:

System.out.printf("%12f %12f %12f %12f

%12f",

Math.PI, Math.E, 1.0, 0.0, Math.sqrt(2));

By default, extra places are filled with space characters to right-justify the numbers. However, flags can be used to fill extra places with zeros or to left-justify the numbers instead.

This is the output:

3.141593 2.718282 1.000000 0.000000 1.414214

Width is not limited to numeric types. You can specify a width for any format tag: date, time, boolean, and so on.

(5). Conversions and Java Print Streams - Precision

Floating-point types (%e, %E, %f, %g, and%G) may also specify a precision in the form.3or.5. The precision comes after the width but before the conversion code. This indicates how many places are used after the decimal point. For example, this statement formats the same five constants 15 places wide, with 3 places after the decimal point:

System.out.printf("%15.3f %15.3f %15.3f

%15.3f %15.3f",

Math.PI, Math.E, 1.0, 0.0, Math.sqrt(2));

This is the output:

3.142 2.718 1.000 0.000 1.414

A precision can also be applied to strings and other nonnumeric, nondate types. In these cases, it specifies the maximum number of characters to write to the output.

Precision cannot be set for integral types, however. Attempting to do so throws anIllegalFormatPrecisionException. As usual, this is a subclass ofIllegalFormatExceptionand a runtime exception.

Formattable:-

You can format your own custom classes by implementing the Formattable interface. In format strings, you use %s and %S to indicate where you want your custom class instances placed. The default formatting for objects matched to %s and %S is simply whatever is returned by toString(). However, more often than not, this is just a debugging string not really meant for display to end users. If your class implements the java.util.Formattable interface, Java will use the return value of the object’s formatTo() method instead. That method has this signature:

public void formatTo(Formatter formatter, int flags, int width, int precision)

The four arguments are:

formatter

TheFormatterthat calledformatTo. More importantly, this is the object onto which the output will be written. Your method will invoke this object’sformat() methods to write to the underlying stream.

flags

A bitmasked constant providing the values of various flags set for this operation: ^,-,#, etc. You interpret these with theFormattableFlagsclass.

width

The minimum number of characters your method must return. If the returned value has fewer characters than the specified minimum, it will be padded with spaces.

precision

The maximum number of characters your method should return. Earlier, I complained that the uppercasing in the URL class was too naïve because, when formatted, it changed the case of case-sensitive parts such as the path and the query string as well as case-insensitive parts such as the scheme and the hostname. There’s another problem with the naïve uppercasing: the scheme and host names are defined in ASCII, but uppercasing isn’t always. In particular, uppercasing the letter i in Turkey produces the capital dotted I rather than the usual undotted capital I. For instance, WWW.microsoft.com uppercases as WWW.MICROSOFT.COM, which will not resolve.

Example:- demonstrates aFormattableURLclass that uppercases only those parts of a URL that can be uppercased without changing its meaning. Ideally this would be a subclass of java.net.URL, but sinceURL is final, delegation is used instead. In essence,FormattableURLis a wrapper around a URL object that just provides theformatTo()method.

Example . Implementing Formattable

import java.util.*;

import java.net.*;

public class FormattableURL implements Formattable {

private URL delegate;

public FormattableURL(URL url){

this.delegate = url;

}

public void formatTo(Formatter formatter, int flags, int width,

int precision) {

if (precision < -1) {

throw new IllegalFormatPrecisionException(precision);

}

if (width < -1) {

throw new IllegalFormatWidthException(width);

}

if (precision > width) {

throw new IllegalFormatPrecisionException(precision);

}

// Check to see if we've been asked to use any flags we don't interpret

int recognizedFlags

= FormattableFlags.UPPERCASE | FormattableFlags.LEFT_JUSTIFY;

boolean unsupportedFlags = ((~recognizedFlags) & flags) != 0;

if (unsupportedFlags) {

// We should really pass the flags to this constructor.

// However, Java doesn't offer any reasonable way to get these.

throw new IllegalFormatFlagsException("#");

}

boolean upperCase = (flags & FormattableFlags.UPPERCASE) != 0;

StringBuffer sb = new StringBuffer();

String scheme = delegate.getProtocol();

if (upperCase && scheme != null) {

scheme = scheme.toUpperCase(Locale.ENGLISH);

}

String hostname = delegate.getHost();

if (upperCase && hostname != null) {

hostname = hostname.toUpperCase(Locale.ENGLISH);

}

String userInfo = delegate.getUserInfo();

int port = delegate.getPort();

String path = delegate.getPath();

String query = delegate.getQuery(); String fragment = delegate.getRef();

if (scheme != null) {

sb.append(scheme);

sb.append("://");

}

if (userInfo != null) {

sb.append(userInfo);

sb.append("@");

}

if (hostname != null) {

sb.append(hostname);

}

if (port != -1) {

sb.append(':');

sb.append(port);

}

if (path != null) {

sb.append(path);

}

if (query != null) {

sb.append('?');

sb.append(query);

}

if (fragment != null) {

sb.append('#');

sb.append(fragment);

}

boolean leftJustify = (flags & FormattableFlags.LEFT_JUSTIFY) != 0;

// Truncate on the right if necessary

if (precision <>

sb.setLength(precision);

}

else {// Pad with spaces if necessary

while (sb.length() <>

if (leftJustify) sb.append(' ');

else sb.insert(0, ' ');

}

}

formatter.format(sb.toString());

}

}

The formatTo()method first checks to see if the values passed make sense—that is, that the width is greater than or equal to the precision, and both are greater than or equal to –1. (–1 simply indicates that these values weren’t set.) Assuming these checks pass, it splits the delegate URL into its component parts and uppercases the two case-insensitive parts (the scheme and the hostname) if the uppercase flag is set. It then appends all the other parts without changing their cases at all. Finally, if the precision is less than the string’s length, the formatted string is truncated on the right. If the string’s length is less than the specified width, the string is padded with spaces: on the right by default but on the left if the left-justified flag is set. If any other flags are present, anIllegalFormatFlagsExceptionis thrown. Thus, it becomes possible to format a URL like this:

URL url = new URL(http://www.example.org/index.html?name=value#Fred);

System.out.printf("%60.40S\n", new FormattableURL(url));

Formatters and Java Print Streams

Constructors

Exactly where the output from a Formatter ends up depends on what argument you pass to the constructor. You’ve already seen the constructor that takes a filename:

public Formatter(String fileName) throws FileNotFoundException

If the named file does not exist in the current working directory, this constructor attempts to create it. If that fails for any reason other than a security violation, the constructor throws aFileNotFoundException. Security problems are reported with aSecurityExceptioninstead. If the file does exist, its contents are overwritten.

Instead of a filename, you can pass in aFileobject:

public Formatter(File file) throws FileNotFoundException

You can also use aFormatterto write onto aPrintStreamor another kind ofOutputStream:

public Formatter(PrintStream out)

public Formatter(OutputStream out)

or onto anyAppendableobject:

public Formatter(Appendable out)

TheAppendableinterface is a new Java 5 interface for anything onto whichchars can be appended. This includesStringBuffers andStringBuilders. It also includes a number of classes we’ll talk about later, such asCharBuffer andWriter.

Finally, the no-args constructor creates aFormatterwith no specified destination for output:

public Formatter()

In this case, theFormatterwrites everything onto a newStringBuilderobject. You can retrieve this object using theout()method at any time before theFormatteris closed:

public Appendable out() throws FormatterClosedException

You might need to use this method if you want to write unformatted output onto the sameStringBuilder, but more commonly you’ll just use thetoString()method to get the final result. For example:

Formatter formatter = new Formatter();

for (double degrees = 0.0; degrees <>

double radians = Math.PI * degrees / 180.0;

double grads = 400 * degrees / 360;

formatter.format("%5.1f %5.1f %5.1f\n", degrees , radians, grads);

}

String table = formatter.toString();

Formatters and Java Print Streams - Character Sets

stick to the ASCII character set, a single computer, and System.out, character sets aren’t likely to be a problem. However, as data begins to move between different systems, it becomes important to consider what happens when the other systems use different character sets. For example, suppose I use a Formatteror aPrintStream on a typical U.S. or Western European PC to write the sentence “Au cours des dernières années, XML a été adapte dans des domaines aussi diverse que l’aéronautique, le multimédia, la gestion de hôpitaux, les télécommunications, la théologie, la vente au détail, et la littérature médiévale” in a file. Say I then send this file to a Macintosh user, who opens it up and sees “Au cours des derniËres annÈes, XML a ÈtÈ adapte dans des domaines aussi diverse que l’aÈronautique, le multimÈdia, la gestion de hÙpitaux, les tÈlÈcommunications, la thÈologie, la vente au dÈtail, et la littÈrature mÈdiÈvale.” This is not the same thing at all! The confusion is even worse if you go in the other direction.

If you’re writing to the console (i.e.,System.out), you don’t really need to worry about character set issues. The default character set Java writes in is usually the same one the console uses.

Actually, you may need to worry a little. On Windows, the console encoding is usually not the same as the system encoding found in thefile.encodingsystem property. In particular, the console uses a DOS character set such as Cp850 that includes box drawing characters such as L and +, while the rest of the system uses an encoding such as Cp1252 that maps these same code points to alphabetic characters like È and Î. To be honest, the console is reliable enough for ASCII, but anything beyond that requires a GUI.

However, there’s more than one character set, and when transmitting files between systems and programs, it pays to be specific. In the previous example, if we knew the file was going to be read on a Macintosh, we might have specified that it be written with the MacRoman encoding:

Formatter formatter = new Formatter("data.txt", "MacRoman");

More likely, we’d just agree on both the sending and receiving ends to use some neutral format such as ISO-8859-1 or UTF-8. In some cases, encoding details can be embedded in the file you write (HTML, XML) or sent as out-of-band metadata (HTTP, SMTP). However, you do need some way of specifying and communicating the character set in which any given document is written. When you’re writing to anything other than the console or a string, you should almost always specify an encoding explicitly. Three of theFormatterconstructors take character set names as their second argument:

public Formatter(String fileName, String characterSet)

throws FileNotFoundException

public Formatter(File file , String characterSet)

throws FileNotFoundException

public Formatter(OutputStream out, String characterSet)

Formatters and Java Print Streams - Locales

Character sets are not the only localization issue in the Formatter class. For instance, in France, a decimal comma is used instead of a decimal point. Thus, a French user running the earlier degree table example would want to see this:

0,0 0,0 0,0

1,0 0,0 1,1

2,0 0,0 2,2

3,0 0,1 3,3

4,0 0,1 4,4

...

Sometimes Java adapts the format to the local conventions automatically, and sometimes it doesn’t. For instance, if you want decimal commas, you have to write%,5.1finstead of%5.1f. The comma after the percent sign is a flag that tells the formatter to use the local conventions. (It does not actually say to use commas.) Java will now use commas only if the local conventions say to use commas. On a typical U.S. English system, the local convention is a decimal point, and that’s what you’ll get even if you format numbers as%,5.1f.

Of course, sometimes you don’t want a program to adapt to the local conventions. For instance, many companies use PCs adapted to local languages and customs but still need to produce English documents that use American formats. Thus, as an optional third argument to the constructor, you can pass ajava.util.Localeobject:

public Formatter(String fileName, String characterSet, Locale locale)

throws FileNotFoundException

public Formatter(File file, String characterSet, Locale locale)

throws FileNotFoundException

public Formatter(OutputStream out, String characterSet, Locale locale)

For example, to force the use of American conventions regardless of where a program is run, you’d construct aFormatterlike this:

Formatter formatter = new Formatter("data.txt", "ISO-8859-1", Locale.US);

You can also specify a locale when writing to anAppendableobject or aStringBuilder:

public Formatter(Appendable out, Locale locale)

public Formatter(Locale locale)

Character encodings don’t matter for these two cases because bothAppendableandStringBuilderare defined in terms of characters rather than bytes—there’s no conversion to be done. However, locales can change formatting even when the character set stays the same.

On occasion, you might wish to change the locale for one string you write but not for other strings (in a mixed English/French document, perhaps). In that case, you can pass a locale as the first argument to theformat()method before the format string:

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

You can do the same thing with theprintf()andformat()methods in thePrintStreamclass:

public PrintStream printf(Locale locale, String format, Object... args)

Finally, I’ll note that there’s a getter method that returns theFormatter’s current locale:

public Locale locale()

Error Handling

The Formatter class handles errors in much the same way PrintStream does. That is, it sweeps them under the rug and pretends they didn’t happen. Notice how none of the methods mentioned so far threwIOException?

To find out if theFormatterhas encountered an error, invoke itsioException()method:

public IOException ioException()

This returns the las tIOExceptionthrown by the underlying output stream. If there was more than one, only the last one is available.

This is marginally better than PrintStream’s booleancheckError()method. At leastFormatter will tell you what the problem was. However, it still won’t tell you unless you ask. For simple cases in which you don’t have to write a lot of data before closing theFormatterand checking for any errors, this may be adequate. However, programs that need to write for an extended period of time should probably create strings using a Formatter but write them using a regularOutputStream. That way, if an I/O error does happen, you’ll find out soon enough to do something about it.

Formatters and Java Print Streams - Format Specifiers

The Formatter class and the printf() method inPrintStreamthat depends on it support several dozen format specifiers. In addition to integer and floating-point numbers,Formatter offers a wide range of date and time formats. It also has a few general formatters that can display absolutely any object or primitive data type.

All format specifiers begin with percent signs. The minimum format specifier is a percent sign followed by an alphabetic conversion code. This code identifies what the corresponding argument is to be formatted as. For instance,%fformats a number with a decimal point,%dformats it as a decimal (base-10) integer,%oformats it as an octal integer, and%xformats it as a hexadecimal integer. None of these specifiers changes what the number actually is; they’re just different ways of creating a string that represents the number.

To use a literal percent character in a format string, just double escape it. That is,%%is formatted as%in the output.

To get the platform default line separator, use%n.(\nis always a linefeed regardless of platform.%nmay be a carriage return, a linefeed, or a carriage return linefeed pair, depending on the platform.)

Integer conversions

Integer conversions can be applied to all integral types (specifically, byte, short,int, andlong, as well as the type-wrapper classesByte,Short,Integer,Long, and also thejava.math.BigIntegerclass). These conversions are:

%d

A regular base-10 integer, such as 987

%o

A base-8 octal integer, such as 1733

%x

A base-16 lowercase hexadecimal integer, such as

3db

%X

A base-16 uppercase hexadecimal integer, such as

3DB

Example 7-1 prints the number 1023 in all four formats.

Example 7-1. Integer format specifiers

public class IntegerFormatExample {

public static void main(String[] args) {

int n = 1023;

System.out.printf("Decimal: %d\n", n);

System.out.printf("Octal: %o\n", n);

System.out.printf("Lowercase hexadecimal: %x\n", n);

System.out.printf("Uppercase hexadecimal: %X\n", n);

}

}

Here’s the output:

Decimal: 1023

Octal: 1777

Lowercase hexadecimal: 3ff

Uppercase hexadecimal: 3FF

Formatters and Java Print Streams - Floating-point conversions :-\

Floating-point conversions can be applied to all floating-point types: float and double, the type-wrapper classesFloatandDouble, andjava.math.BigDecimal. These conversions are:

%f

A regular base-10 decimal number, such as 3.141593

%e

A decimal number in scientific notation with a

lowercase e, such as 3.141593e+00

%E

A decimal number in scientific notation with an

uppercase E, such as 3.141593E+00

%g

A decimal number formatted in either regular or

scientific notation, depending on its size and

precision, with a lowercase e if scientific notation is

used

%G

A decimal number formatted in either regular or

scientific notation, depending on its size and

precision, with an uppercase E if scientific notation is

used

%a

A lowercase hexadecimal floating-point number, such

as 0x1.921fb54442d18p1

%A

An uppercase hexadecimal floating-point number,

such as 0X1.921FB54442D18P1

Surprisingly, you cannot use these conversions on integer types such asintorBigDecimal. Java will not automatically promote the integer type to a floating-point type when formatting. If you try to use them, it throws anIllegalFormatConversionException.

Example 7-2 prints π in all of these formats.

Example 7-2. Floating-point format specifiers

public class FloatingPointFormatExample {

public static void main(String[] args){

System.out.printf("Decimal: %f\n", Math.PI);

System.out.printf("Scientific notation: %e\n", Math.PI);

System.out.printf("Scientific notation: %E\n", Math.PI);

System.out.printf("Decimal/Scientific: %g\n", Math.PI);

System.out.printf("Decimal/Scientific: %G\n", Math.PI);

System.out.printf("Lowercase Hexadecimal: %a\n", Math.PI);

System.out.printf("Uppercase Hexadecimal: %A\n", Math.PI);

}

}

Here’s the output:

Decimal: 3.141593

Scientific notation: 3.141593e+00

Scientific notation: 3.141593E+00

Decimal/Scientific: 3.14159

Decimal/Scientific: 3.14159

Lowercase Hexadecimal: 0x1.921fb54442d18p1

Uppercase Hexadecimal: 0X1.921FB54442D18P1