Formatting

Introduction

• String formatting can be done using the printf() and the format() methods added in java.io.PrintStream (System.out is a PrintStream)
• Internally they ise the java.util.Formatter class.
• Basic format:

printf("format string", argument(s));

• It can be cleaner to use:

• When we want to use variables and strings in a print, it can get irritating to keep concatenating and keeping the whitespaces and commas in mind:

System.out.println("The sum of " + i + " and " + j + " is " + (i+j));

• A printf, can make things easier:

System.out.printf("The sum of %d and %d is %d", i, j, (i+j));

Formatter Class

• The java.util.Formatter is the class that contains all the string format functionality through it's format() methods.
• System.out.printf()/System.out.format()/String.format() etc all utilize the Formatter class.
• A formatter can be constructed with output sources such as a StringBuffer, a File or an OutputStream.
• Example here is a formatter, writing to a file:

                File piFile = new File("/tmp/pi.txt");
                piFile.createNewFile();
		System.out.println("Created file");
		Formatter f2 = new Formatter(piFile);
		f2.format("The value of PI rounded to %d decimals is %.3f", 3, Math.PI);
		System.out.println("Formatted to file");
		f2.close();

Format String

• Syntax:

%[arg_index$][flags][width][.precision] conversion_char

• Every argument must have a format string in the above syntax, or it wont be printed!

Arguments within [] are optional. Only the conversion_char is mandatory. The following are the basic conversion chars:

• b - boolean
• c - char
• d - integer
• f - floating point
• s - string
• n - line separator

There are many more conversion chars for each data type:

• Integers have o, x for octal/hex.
• Floats have e (scientific), g, A - esoteric usage.

Optional arguments

• arg_index is the number of the argument followed by a $. The no starts from 1.
• example:

System.out.printf("%2$s , %1$s", "Suhrid", "Karthik");
//Prints : Karthik, Suhrid

• flags
  • "-" : Left justify
  • "+" : Include a sign
  • "0" : Pad with zeroes
  • "," : Use locale specific group separators
  • "(" : Enclose -ve numbers in brackets

• examples:

int i = 11;
int j = -22;
out.printf("%+d , %+d", i, j);
//prints: +11 , -22

• Some flags require width to be mandatory - e.g. left-justify and zero-pad.

• width indicates the minimum number of characters to print.

out.printf("%3d", 101231);
//prints 101231.

out.printf("%5d", 17);
//prints "   17"

• Finally the precision - a number prefix with a ".". This is to indicate the number of digits to print after a decimal point. This applies only to floats.

• The format() method is very strict about data types. It will not try to format float as int or int as float etc. A java.util.IllegalFormatConversionException will be thrown.
• There is a whole bunch of runtime exceptions associated with string formating. All are subclasses of IllegalFormatException.

e.g. IllegalFormatFlagsException, IllegalFormatPrecisionException etc. etc.

• Note that Some of the output can be written by format and then the exception can be thrown mid-way.