# Class Characters.Filter

Object
Character.Subset
Filter

Enclosing class:
  `Characters`

public static class Characters.Filter extends Character.Subset
Subsets of Unicode characters identified by their general category. The categories are identified by constants defined in the `Character` class, like `LOWERCASE_LETTER`, `UPPERCASE_LETTER`, `DECIMAL_DIGIT_NUMBER` and `SPACE_SEPARATOR`.

An instance of this class can be obtained from an enumeration of character types using the `forTypes(byte[])` method, or using one of the constants predefined in this class. Then, Unicode characters can be tested for inclusion in the subset by calling the `contains(int)` method.

## Relationship with international standards

ISO 19162:2015 §B.5.2 recommends to ignore spaces, case and the following characters when comparing two identified object names: “`_`” (underscore), “`-`” (minus sign), “`/`” (solidus), “`(`” (left parenthesis) and “`)`” (right parenthesis). The same specification also limits the set of valid characters in a name to the following (§6.3.1):

> `A-Z a-z 0-9 _ [ ] ( ) { } < = > . , : ; + - (space) % & ' " * ^ / \ ? | °`

Note: SIS does not enforce this restriction in its programmatic API, but may perform some character substitutions at Well Known Text (WKT) formatting time.
If we take only the characters in the above list which are valid in a Unicode identifier and remove the characters that ISO 19162 recommends to ignore, the only characters left are letters and digits.

Since:
  0.3
See Also:
  - `Character​.Subset`
  - `Character​.getType(int)`

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final Characters​.Filter`
  `LETTERS_AND_DIGITS`
  The subset of all characters for which `Character​.isLetterOrDigit(int)` returns `true`.
  `static final Characters​.Filter`
  `UNICODE_IDENTIFIER`
  The subset of all characters for which `Character​.isUnicodeIdentifierPart(int)` returns `true`, excluding ignorable characters.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `contains(int codePoint)`
  Returns `true` if this subset contains the given Unicode character.
  `static Characters​.Filter`
  `forTypes(byte... types)`
  Returns a subset representing the union of all Unicode characters of the given types.

  ### Methods inherited from class Character.Subset

  `equals, hashCode, toString`

  ### Methods inherited from class Object

  `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- ## Field Details

  - ### LETTERS_AND_DIGITS

    public static final Characters.Filter LETTERS_AND_DIGITS
    The subset of all characters for which `Character​.isLetterOrDigit(int)` returns `true`. This subset includes the following general categories:

    > `Character​.LOWERCASE_LETTER`, `UPPERCASE_LETTER`, `TITLECASE_LETTER`, `MODIFIER_LETTER`, `OTHER_LETTER` and `DECIMAL_DIGIT_NUMBER`.

    SIS uses this filter when comparing two identified object names. See the Relationship with international standards section in this class javadoc for more information.
    See Also:
      - `AbstractIdentifiedObject​.isHeuristicMatchForName(String)`
      - `Citations​.identifierMatches(Citation, String)`

  - ### UNICODE_IDENTIFIER

    public static final Characters.Filter UNICODE_IDENTIFIER
    The subset of all characters for which `Character​.isUnicodeIdentifierPart(int)` returns `true`, excluding ignorable characters. This subset includes all the `LETTERS_AND_DIGITS` categories with the addition of the following ones:

    > `Character​.LETTER_NUMBER`, `CONNECTOR_PUNCTUATION`, `NON_SPACING_MARK` and `COMBINING_SPACING_MARK`.

- ## Method Details

  - ### contains

    public boolean contains(int codePoint)
    Returns `true` if this subset contains the given Unicode character.
    Parameters:
      `codePoint` - the Unicode character, as a code point value.
    Returns:
      `true` if this subset contains the given character.

  - ### forTypes

    public static Characters.Filter forTypes(byte... types)
    Returns a subset representing the union of all Unicode characters of the given types.
    Parameters:
      `types` - the character types, as `Character` constants.
    Returns:
      the subset of Unicode characters of the given type.
    See Also:
      - `Character​.LOWERCASE_LETTER`
      - `Character​.UPPERCASE_LETTER`
      - `Character​.DECIMAL_DIGIT_NUMBER`
      - `Character​.SPACE_SEPARATOR`