Module java.base
Package java.util

Interface Formattable

public interface Formattable
The Formattable interface must be implemented by any class that needs to perform custom formatting using the 's' conversion specifier of Formatter. This interface allows basic control for formatting arbitrary objects. For example, the following class prints out different representations of a stock's name depending on the flags and length constraints:
   import java.nio.CharBuffer;
   import java.util.Formatter;
   import java.util.Formattable;
   import java.util.Locale;
   import static java.util.FormattableFlags.*;


   public class StockName implements Formattable {
       private String symbol, companyName, frenchCompanyName;
       public StockName(String symbol, String companyName,
                        String frenchCompanyName) {


       public void formatTo(Formatter fmt, int f, int width, int precision) {
           StringBuilder sb = new StringBuilder();

           // decide form of name
           String name = companyName;
           if (fmt.locale().equals(Locale.FRANCE))
               name = frenchCompanyName;
           boolean alternate = (f & ALTERNATE) == ALTERNATE;
           boolean usesymbol = alternate || (precision != -1 && precision < 10);
           String out = (usesymbol ? symbol : name);

           // apply precision
           if (precision == -1 || out.length() < precision) {
               // write it all
           } else {
               sb.append(out.substring(0, precision - 1)).append('*');

           // apply width and justification
           int len = sb.length();
           if (len < width)
               for (int i = 0; i < width - len; i++)
                   if ((f & LEFT_JUSTIFY) == LEFT_JUSTIFY)
                       sb.append(' ');
                       sb.insert(0, ' ');


       public String toString() {
           return String.format("%s - %s", symbol, companyName);

When used in conjunction with the Formatter, the above class produces the following output for various format strings.

   Formatter fmt = new Formatter();
   StockName sn = new StockName("HUGE", "Huge Fruit, Inc.",
                                "Fruit Titanesque, Inc.");
   fmt.format("%s", sn);                   //   -> "Huge Fruit, Inc."
   fmt.format("%s", sn.toString());        //   -> "HUGE - Huge Fruit, Inc."
   fmt.format("%#s", sn);                  //   -> "HUGE"
   fmt.format("%-10.8s", sn);              //   -> "HUGE      "
   fmt.format("%.12s", sn);                //   -> "Huge Fruit,*"
   fmt.format(Locale.FRANCE, "%25s", sn);  //   -> "   Fruit Titanesque, Inc."

Formattables are not necessarily safe for multithreaded access. Thread safety is optional and may be enforced by classes that extend and implement this interface.

Unless otherwise specified, passing a null argument to any method in this interface will cause a NullPointerException to be thrown.

  • Method Summary

    Modifier and Type
    formatTo​(Formatter formatter, int flags, int width, int precision)
    Formats the object using the provided formatter.
  • Method Details

    • formatTo

      void formatTo(Formatter formatter, int flags, int width, int precision)
      Formats the object using the provided formatter.
      formatter - The formatter. Implementing classes may call formatter.out() or formatter.locale() to obtain the Appendable or Locale used by this formatter respectively.
      flags - The flags modify the output format. The value is interpreted as a bitmask. Any combination of the following flags may be set: FormattableFlags.LEFT_JUSTIFY, FormattableFlags.UPPERCASE, and FormattableFlags.ALTERNATE. If no flags are set, the default formatting of the implementing class will apply.
      width - The minimum number of characters to be written to the output. If the length of the converted value is less than the width then the output will be padded by '  ' until the total number of characters equals width. The padding is at the beginning by default. If the FormattableFlags.LEFT_JUSTIFY flag is set then the padding will be at the end. If width is -1 then there is no minimum.
      precision - The maximum number of characters to be written to the output. The precision is applied before the width, thus the output will be truncated to precision characters even if the width is greater than the precision. If precision is -1 then there is no explicit limit on the number of characters.
      IllegalFormatException - If any of the parameters are invalid. For specification of all possible formatting errors, see the Details section of the formatter class specification.