# Class IDN

java.lang.Object
java.net.IDN

public final class IDN extends Object
Provides methods to convert internationalized domain names (IDNs) between a normal Unicode representation and an ASCII Compatible Encoding (ACE) representation. Internationalized domain names can use characters from the entire range of Unicode, while traditional domain names are restricted to ASCII characters. ACE is an encoding of Unicode strings that uses only ASCII characters and can be used with software (such as the Domain Name System) that only understands traditional domain names.

Internationalized domain names are defined in RFC 3490. RFC 3490 defines two operations: ToASCII and ToUnicode. These 2 operations employ Nameprep algorithm, which is a profile of Stringprep, and Punycode algorithm to convert domain name string back and forth.

The behavior of aforementioned conversion process can be adjusted by various flags:

- If the ALLOW_UNASSIGNED flag is used, the domain name string to be converted can contain code points that are unassigned in Unicode 3.2, which is the Unicode version on which IDN conversion is based. If the flag is not used, the presence of such unassigned code points is treated as an error.
- If the USE_STD3_ASCII_RULES flag is used, ASCII strings are checked against RFC 1122 and RFC 1123. It is an error if they don't meet the requirements.

These flags can be logically OR'ed together.

The security consideration is important with respect to internationalization domain name support. For example, English domain names may be homographed - maliciously misspelled by substitution of non-Latin letters. Unicode Technical Report #36 discusses security issues of IDN support as well as possible solutions. Applications are responsible for taking adequate security measures when using international domain names.

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

Since:
  1.6
External Specifications
  - RFC 1122: Requirements for Internet Hosts - Communication Layers
  - RFC 1123: Requirements for Internet Hosts - Application and Support
  - RFC 3454: Preparation of Internationalized Strings ("stringprep")
  - RFC 3490: Internationalizing Domain Names in Applications (IDNA)
  - RFC 3491: Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)
  - RFC 3492: Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)
  - Unicode Security Considerations

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final int`
  `ALLOW_UNASSIGNED`
  Flag to allow processing of unassigned code points
  `static final int`
  `USE_STD3_ASCII_RULES`
  Flag to turn on the check against STD-3 ASCII rules

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static String`
  `toASCII(String input)`
  Translates a string from Unicode to ASCII Compatible Encoding (ACE), as defined by the ToASCII operation of RFC 3490.
  `static String`
  `toASCII(String input, int flag)`
  Translates a string from Unicode to ASCII Compatible Encoding (ACE), as defined by the ToASCII operation of RFC 3490.
  `static String`
  `toUnicode(String input)`
  Translates a string from ASCII Compatible Encoding (ACE) to Unicode, as defined by the ToUnicode operation of RFC 3490.
  `static String`
  `toUnicode(String input, int flag)`
  Translates a string from ASCII Compatible Encoding (ACE) to Unicode, as defined by the ToUnicode operation of RFC 3490.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Field Details

  - ### ALLOW_UNASSIGNED

    public static final int ALLOW_UNASSIGNED
    Flag to allow processing of unassigned code points
    See Also:
      - Constant Field Values

  - ### USE_STD3_ASCII_RULES

    public static final int USE_STD3_ASCII_RULES
    Flag to turn on the check against STD-3 ASCII rules
    See Also:
      - Constant Field Values

- ## Method Details

  - ### toASCII

    public static String toASCII(String input, int flag)
    Translates a string from Unicode to ASCII Compatible Encoding (ACE), as defined by the ToASCII operation of RFC 3490.

    ToASCII operation can fail. ToASCII fails if any step of it fails. If ToASCII operation fails, an IllegalArgumentException will be thrown. In this case, the input string should not be used in an internationalized domain name.

    A label is an individual part of a domain name. The original ToASCII operation, as defined in RFC 3490, only operates on a single label. This method can handle both label and entire domain name, by assuming that labels in a domain name are always separated by dots. The following characters are recognized as dots: \u002E (full stop), \u3002 (ideographic full stop), \uFF0E (fullwidth full stop), and \uFF61 (halfwidth ideographic full stop). if dots are used as label separators, this method also changes all of them to \u002E (full stop) in output translated string.

    Parameters:
      `input` - the string to be processed
      `flag` - process flag; can be 0 or any logical OR of possible flags
    Returns:
      the translated `String`
    Throws:
      `IllegalArgumentException` - if the input string doesn't conform to RFC 3490 specification
    External Specifications
      - RFC 3490: Internationalizing Domain Names in Applications (IDNA)

  - ### toASCII

    public static String toASCII(String input)
    Translates a string from Unicode to ASCII Compatible Encoding (ACE), as defined by the ToASCII operation of RFC 3490.

    This convenience method works as if by invoking the two-argument counterpart as follows:

    > `toASCII`(input, 0);

    Parameters:
      `input` - the string to be processed
    Returns:
      the translated `String`
    Throws:
      `IllegalArgumentException` - if the input string doesn't conform to RFC 3490 specification
    External Specifications
      - RFC 3490: Internationalizing Domain Names in Applications (IDNA)

  - ### toUnicode

    public static String toUnicode(String input, int flag)
    Translates a string from ASCII Compatible Encoding (ACE) to Unicode, as defined by the ToUnicode operation of RFC 3490.

    ToUnicode never fails. In case of any error, the input string is returned unmodified.

    A label is an individual part of a domain name. The original ToUnicode operation, as defined in RFC 3490, only operates on a single label. This method can handle both label and entire domain name, by assuming that labels in a domain name are always separated by dots. The following characters are recognized as dots: \u002E (full stop), \u3002 (ideographic full stop), \uFF0E (fullwidth full stop), and \uFF61 (halfwidth ideographic full stop).

    Parameters:
      `input` - the string to be processed
      `flag` - process flag; can be 0 or any logical OR of possible flags
    Returns:
      the translated `String`
    External Specifications
      - RFC 3490: Internationalizing Domain Names in Applications (IDNA)

  - ### toUnicode

    public static String toUnicode(String input)
    Translates a string from ASCII Compatible Encoding (ACE) to Unicode, as defined by the ToUnicode operation of RFC 3490.

    This convenience method works as if by invoking the two-argument counterpart as follows:

    > `toUnicode`(input, 0);

    Parameters:
      `input` - the string to be processed
    Returns:
      the translated `String`
    External Specifications
      - RFC 3490: Internationalizing Domain Names in Applications (IDNA)