• Background:
  • You have experience with using format specifiers to create formatted String objects and generate formatted output
• A Limitation:
  • You've always created the format specifier at "compile time"
• Overcoming this Limitation:
  • Create the format specifier at "run time" using various String operations/methods

javaexamples/programmingpatterns/DynamicFormatting.java (Fragment: hardcodedsimple)

        for (int i = 0; i < data.length; i++) {
            System.out.printf("%10d\n", data[i]);
        }
        

However, suppose, instead, that you want the field to be as narrow as possible.

• Finding the Largest Element:
  • Use an accumulator
• Finding the Number of Digits in the Largest Element:
  • Use digit counting
• What's Left?
  • Creating a format specifier

• Some Observations:
  • The format specifier is a String
  • Though we commonly use a literal, a variable could be used
• Returning to the Example:
  • javaexamples/programmingpatterns/DynamicFormatting.java (Fragment: hardcodedvariable)

            fs = "%10d\n";
            
            for (int i = 0; i < data.length; i++) {
                System.out.printf(fs, data[i]);
            }
            

• Recall the Syntax of Format Specifiers:
  • %[flags][width][.precision]conversion
• Dynamic Construction:
  • javaexamples/programmingpatterns/DynamicFormatting.java (Fragment: pattern)

            fs = "%";
            if (flags != null) fs += flags;
            if (width > 0)     fs += width;
            if (precision > 0) fs += "." + precision;
            fs += conversion;
            

javaexamples/programmingpatterns/DynamicFormatting.java (Fragment: pi)

        for (int digits = 1; digits <= 10; digits++) {
            fs = "%" + (digits + 2) + "." + digits + "f\n";
            System.out.printf(fs, Math.PI);
        }
        

javaexamples/programmingpatterns/DynamicFormatting.java (Fragment: center)

    /**
     * Use dynamic formatting to center a String in a field of a given width.
     * 
     * @param source The String to center
     * @param width  The width of the resulting String
     * @return       The centered String
     */
    public static String center(String source, int width) {
        int      field, n, append;
        String   fs, result;
        
        // Calculate the number of spaces in the resulting String
        n = width - source.length();        
        if (n <= 0) return source;
        
        // Calculate the width of the field for source (it will be
        // right-justified in this field)
        field = (width + source.length()) / 2;
        
        // Calculate the number of spaces to append to the right
        append = width - field;
        
        // Build the format specifier
        fs = "%" + field + "s%" + append + "s";
        
        result = String.format(fs, source, " ");
        return result;
    }