Java I/O

Specifying Width with FieldPosition

The Java core API does not include any classes that pad numbers with spaces like the traditional I/O APIs in Fortran, C, and other languages. Part of the reason is that it’s no longer a valid assumption that all output is written in a monospaced font on a VT-100 terminal. Therefore, spaces are insufficient to line up numbers in tables. Ideally, if you’re writing tabular data in a GUI, you can use a real table component like JTable in the Java foundation classes. If that’s not possible, you can measure the width of the string using a FontMetrics object and offset the position at which you draw the string. And if you are outputting to a terminal or a monospaced font, then you can manually prefix the string with the right number of spaces.

The java.text.FieldPosition class separates strings into their component parts, called fields. (This is another unfortunate example of an overloaded term. These fields have nothing to do with the fields of a Java class.) For example, a typical date string can be separated into 18 fields including era, year, month, day, date, hour, minute, second, and so on. Of course, not all of these may be present in any given string. For example, 1999 CE includes only a year and an era field. The different fields that can be parsed are represented as public final static int fields (there’s that annoying overloading again) in the corresponding format class. The java.text.DateFormat class defines these kinds of fields as mnemonic ...