FieldPosition in formatting a number with NumberFormat

Question

Anybody able to explain the following taken from this API in plain English?

You can also use forms of the parse and format methods with ParsePosition and FieldPosition to allow you to:

progressively parse through pieces of a string

align the decimal point and other areas

For example, you can align numbers in two ways: If you are using a monospaced font with spacing for alignment, you can pass the FieldPosition in your format call, with field = INTEGER_FIELD. On output, getEndIndex will be set to the offset between the last character of the integer and the decimal. Add (desiredSpaceCount - getEndIndex) spaces at the front of the string.

I have not got what is the use of FieldPosition and that API above posted does not help (at least to me). A simple example showing the output would be super! Thanks in advance!

Answer 1

NumberFormat is the abstract base class for all number formats. This class provides the interface for formatting and parsing numbers.

To use number format, first you have to get a locale instance.

Then you can set many attributes of the formatting. Like, you have the option to show comma, restrict the number of decimal places, and set minimum and maximum integer length. If you want to display ‘%’ with respect to locale then you have to use NumberFormat. Just don’t append ‘%’ as a string to your result. Do you want to show paranthesis like (3745) in place of “-” to denote a negative numer, then use NumberFormat. Like these, there are numerous uses.

You can check JavaDoc for more methods

This tells you how to do it..!!

    NumberFormat numberFormat = NumberFormat.getInstance();

    // setting number of decimal places
    numberFormat.setMinimumFractionDigits(2);
    numberFormat.setMaximumFractionDigits(2);

    // you can also define the length of integer
    // that is the count of digits before the decimal point
    numberFormat.setMinimumIntegerDigits(1);
    numberFormat.setMaximumIntegerDigits(10);

    // if you want the number format to have commas
    // to separate the decimals the set as true
    numberFormat.setGroupingUsed(true);

    // convert from integer to String
    String formattedNr = numberFormat.format(12345678L);
    // note that the output will have 00 in decimal place


    // convert from decimal to String
    numberFormat.format(12345.671D);

    // format a String to number
    Number n1 = null;
    Number n2 = null;

      n1 = numberFormat.parse("1,234");
      n2 = numberFormat.parse("1.234");

    // show percentage
    numberFormat = NumberFormat.getPercentInstance();
    numberFormat.format(0.98);
    // answer will be 98%

This is how you use field positions with number format.

// Get a default NumberFormat instance.
        NumberFormat numForm = NumberFormat.getInstance();

        // Format some decimals using the pattern supplied above.
        StringBuffer dest1 = new StringBuffer(24);
        StringBuffer dest2 = new StringBuffer(24);
        FieldPosition pos = new FieldPosition(NumberFormat.FRACTION_FIELD);

        dest1 = numForm.format(22.3423D, dest1, pos);
        System.out.println("dest1 = " + dest1);
        System.out.println("FRACTION is at: " + pos.getBeginIndex() +
            ", " + pos.getEndIndex());

        dest2 = numForm.format(64000D, dest2, pos);
        System.out.println("dest2 = " + dest2);
        System.out.println("FRACTION is at: " + pos.getBeginIndex() +
            ", " + pos.getEndIndex());
/*
Output:
dest1 = 22.342
FRACTION is at: 3, 6
dest2 = 64,000
FRACTION is at: 6, 6
*/

Answer 2

The java doc of the FieldPosition and ParsePosition classes give some more hints.

Basically you can use FieldPosition if you do not want to format a whole Date or Number, but only a part of it (e.g. if your UI splits an amount into several parts (like giving dollars and cents in two output fields). If you need something like that, you can use the FieldPosition to just retrieve the part which you are interested in. For ParsePosition I do not have a good use case in my mind right now, maybe someone else can help out here.