student:java:formatting [JMU CS Wiki]

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===== Formatting Strings and Output in Java =====



==== Overview ====


It is often the case that a product's requirements include  detailed specifications about the format(s) used when presenting  information to users. Java has many different ways to satisfy these kinds  of requirements. This document discusses the most common.


==== Creating a Formatted String ====


It is possible in Java to create complicated ''%%String%%''  objects using the concatenation operator however it is difficult to  closely control the format of the result. For example,  the width of the ''%%String%%'' that is returned by  ''%%"CS" + course%%'' can't be controlled. On the other hand,  the ''%%String%%'' that is returned by  ''%%String.format("CS%3d", course)%%'' will contain  exactly 5 characters.


The static ''%%format()%%'' method in the ''%%String%%'' class is  passed one parameter called a //format string// and then any number  of other parameters. In the above example, the format string is  ''%%"CS%3d"%%'' and the one other parameter  is ''%%course%%''. A format string consists  of ''%%String%%'' literals and //format specifiers//. In the above  example, the format specifier is ''%%%3d%%''.

Format specifiers have the following syntax.

|  %[//flags//][//width//][.//precision//]//conversion//  |

Common values of //conversion// include  ''%%b%%'' for ''%%boolean%%'',  ''%%c%%'' for ''%%char%%'',  ''%%d%%'' for ''%%int%%'',  ''%%e%%'' for ''%%double%%'' in scientific notation,  ''%%f%%'' for ''%%double%%'', and  ''%%s%%'' for ''%%String%%''.

Common values of the optional //flag// include  ''%%-%%'' for left-justified,  ''%%+%%'' to always include the sign,  ''%%0%%'' to pad with zeros,  ''%%,%%'' to use grouping separators,  ''%%(%%'' to put negative numbers in parentheses, and  a space character to use a space for the sign of a postive number  (as opposed to omitting it).

The optional //width// determines the total number of characters  that this portion of the ''%%String%%'' will contain. The optional  //precision// determines the nnumbenr of digits to the right of the  decimal point for ''%%double%%'' values.

So, for example the format specifier ''%%%8d%%'' indicates an  ''%%int%%'' that will be right-justified in a field of width 8.  Similarly, the format specifier ''%%-%6.2f%%'' indicates a  ''%%double%%'' that will be left-justified in a field of width 6  with 2 places to the right of the decimal point.

The format string in the invocation of the ''%%format()%%''  method should have the same number of format specifiers as there are  other parameters in the invocation. This is illustrated in the following  example.

<code java>
 int  year = 2017;
 double sales = 1050987.00;
 String s;

 s = String.format("Year: %4d, Sales: $%,14.2f", year, sales);
</code>

After this code is executed, ''%%s%%'' will contain  ''%%%%Year: 2017, Sales: $ 1,050,987.00%%%%''

==== Printing a Formatted String ====


Obviously, once you have a formatted ''%%String%%'' there are many  things that you can do with it, including printing it (e.g., with either  the ''%%print()%%'' or ''%%println()%%'' methods, both of which  have a single ''%%String%%'' parameter. For example, the previous  example could easily be extended as follows.


<code java>
 int  year = 2017;
 double sales = 1050987.00;
 String s;

 s = String.format("Year: %4d, Sales: $%,14.2f", year, sales);
 System.out.println(s);
</code>

However, when all you want to do is print a formatted ''%%String%%'',  it is easier to do it in one step, rather than two (i.e., first calling  ''%%format()%%'' and then calling ''%%println()%%''. To accomodate  this, all objects that have a ''%%println()%%'' method also have  a ''%%printf()%%'' method that has the same syntax as the  ''%%format()%%'' method. Hence, the previous example could also be  implemented as follows.

<code java>
 int  year = 2017;
 double sales = 1050987.00;
 String s;

 System.out.printf("Year: %4d, Sales: $%,14.2f\n", year, sales);
</code>

Note the addition of the newline character to the end of the format string.
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