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Functional Java is an open source library that supports closures for the Java programming language
There is a newer version: 5.0
package fj;

import fj.data.Option;
import static fj.data.Option.some;
import static fj.data.Option.none;

/**
 * The digits zero to nine.
 *
 * @version %build.number%
 */
public enum Digit {
  /**
   * Zero.
   */
  _0,

  /**
   * One.
   */
  _1,

  /**
   * Two.
   */
  _2,

  /**
   * Three.
   */
  _3,

  /**
   * Four.
   */
  _4,

  /**
   * Five.
   */
  _5,

  /**
   * Six.
   */
  _6,

  /**
   * Seven.
   */
  _7,

  /**
   * Eight.
   */
  _8,

  /**
   * Nine.
   */
  _9;

  /**
   * Converts this digit to a long.
   *
   * @return A long for this digit.
   */
  public long toLong() {
    switch (this) {
      case _0:
        return 0L;
      case _1:
        return 1L;
      case _2:
        return 2L;
      case _3:
        return 3L;
      case _4:
        return 4L;
      case _5:
        return 5L;
      case _6:
        return 6L;
      case _7:
        return 7L;
      case _8:
        return 8L;
      default:
        return 9L;
    }
  }

  /**
   * Converts this digit to a character.
   *
   * @return A character for this digit.
   */
  public char toChar() {
    switch (this) {
      case _0:
        return '0';
      case _1:
        return '1';
      case _2:
        return '2';
      case _3:
        return '3';
      case _4:
        return '4';
      case _5:
        return '5';
      case _6:
        return '6';
      case _7:
        return '7';
      case _8:
        return '8';
      default:
        return '9';
    }
  }

  /**
   * Converts the right-most digit in the given long value to a digit.
   *
   * @param i The long to convert.
   * @return The right-most digit in the given long value as a digit.
   */
  public static Digit fromLong(final long i) {
    final long x = Math.abs(i) % 10L;
    return x == 0L ? _0 :
        x == 1L ? _1 :
            x == 2L ? _2 :
                x == 3L ? _3 :
                    x == 4L ? _4 :
                        x == 5L ? _5 :
                            x == 6L ? _6 :
                                x == 7L ? _7 :
                                    x == 8L ? _8 :
                                        _9;
  }

  /**
   * Converts the given character in the given long value to a digit.
   *
   * @param c The character to convert.
   * @return The character in the given long value as a digit.
   */
  public static Option fromChar(final char c) {
    switch (c) {
      case '0':
        return some(_0);
      case '1':
        return some(_1);
      case '2':
        return some(_2);
      case '3':
        return some(_3);
      case '4':
        return some(_4);
      case '5':
        return some(_5);
      case '6':
        return some(_6);
      case '7':
        return some(_7);
      case '8':
        return some(_8);
      case '9':
        return some(_9);
      default:
        return none();
    }
  }

  /**
   * First-class conversion from digit to a long.
   */
  public static final F toLong = d -> d.toLong();

  /**
   * First-class conversion from a long to a digit.
   */
  public static final F fromLong = i -> fromLong(i);

  /**
   * First-class conversion from a digit to a character.
   */
  public static final F toChar = d -> d.toChar();

  /**
   * First-class conversion from a character to a digit.
   */
  public static final F> fromChar = c -> fromChar(c);
}