The ABC's of Emoji

Unicode, character encoding, etc.

Posted by S.L on September 10, 2018



  • Which language can use US-ASCII to encode all its characters?
  • How many characters can char represent in Java?
  • Can we use char to represent ‘😀’ in Java?
  • What will return if you call “😀”.length() and “😀”.getBytes() in Java?
  • Can we get the emoji calling “😀c”.substring(0,1)?
  • Can we execute insert into tb(‘name’) values(‘😀’) in MySQL?

How to Define Character

  • Character set - defines all readable characters
  • Coded character set - use a code point to delegate a character in character repertoire
  • Character encoding - the transformation between code point and its own code units

Code Point

  • A unique number assigned to each Unicode character
  • Usually expressed in Hexadecimal as U+xxxx, e.g. code point for A is U+0041


  • 17 Planes
  • 136755 characters defined
  • U+0000~U+10FFFF, 21 bit
  • Support over 1.1M possible characters


  • Basic Multilingual Plane, U+0000~U+FFFF, 65535 in total

Supplementary Characters

  • Code points between U+10000 and U+10FFFF are the supplementary characters
  • Can not be described as a single 16-bit entity

Character Encoding

  • A mapping from the numbers of one ore more coded character sets to sequences of one or more fixed-width code units
  • The most commonly used code units are bytes, but 16-bit, 32-bit integers can also be used for internal processing
  • UTF-32, UTF-16 and UTF-8 are character encoding schemas for the Unicode standard


  • UTF-32 encodes each Unicode character as one 32-bit code units, e.g. 00 00 00 41 represent A
  • It’s the most convenient representation for internal processing
  • But it’s memory-wasting
  • Try “a”.getBytes(“utf-32”) in Java, you’ll get byte[4]{0, 0, 0, 97}


  • UTF-16 encodes each Unicode character as one or two 16-bit code units, U+0000~U+FFFF, 0~65535
  • Each character is encoded using 2 or 4 bytes
  • The internal Java encoding
  • Code points between U+0000 and U+FFFF are represented as a 16-bit Java char value
  • e.g. U+4E2D - 中, 2 bytes, char c = ‘中’
  • Code points between U+10000 and U+10FFFF are the supplementary characters which char in Java can not hold
  • Try “HELLO”.getBytes() in Java, you’ll see it’s encoded using UTF-16


  • Big Endian, e.g. “a”.getBytes(“utf-16be”)->byte[2]{0, 97}
  • Little Endian, e.g. “a”.getBytes(“utf-16be”)->byte[2]{97, 0}


  • BOM = Byte Order Mark
  • Big Endian starts with U+FEFF, e.g. “a”.getBytes(“utf-16”)->byte[4]{0xFE, 0xFF, 0, 97}
  • Little Endian starts with U+FFFE
  • UTF-16 and UTF-32 have to deal with the issue of BE and LE, because they use multi-byte code units

Java and Supplementary Characters

  • Unicode was originally designed as a fixed-width 16-bit character encoding
  • Java used to hold all Unicode characters using char
  • But later Unicode 3.1 has been extended up to 1,114,112, 21-bit character encoding
  • Since J2SE5.0, JDK supports version 4.0 of Unicode standard


  • 8-bit, variable-width encoding
  • Encodes each Unicode character using 1 to 4 bytes
  • .class files is encode using UTF-8
  • No BOM needed, try “a”.getBytes(“utf-8be”), you’ll get UnsupportedEncodingException

UTF-8 to UTF-16

  • For those bytes:
    • starts with 0
      • e.g. 0xxxxxxx ==> 00000000 0xxxxxxxx
    • starts with 110,
      • e.g. 110xxxxx 10yyyyyy ==> 00000xxx xxyyyyyy
    • starts with 1110
      • e.g. 1110xxxx 10yyyyyy 10zzzzzz ==> xxxxyyyy yyzzzzzz
  • e.g. “中”
    • Unicode U+4E2D: 01001110 00101101
    • UTF-8 4E B8 AD : 11100100 10111000 10101101 

UTF-16 to UTF-8

  • For those bytes:
    • Less than 0x007F(00000000 01111111)
      • e.g. 0x00000000 xxxxxx ==> 0xxxxxxxx
    • Less than 0x07FF(00000111 11111111),
      • e.g. 00000aaa bbbbbbbb ==> 110aaabb 10bbbbbb
    • Others
      • e.g. aaaaaaaa bbbbbbbb ==> 1110aaaa 10aaaabb 10bbcccccc 
  • e.g. “中”
    • Unicode U+4E2D: 01001110 00101101
    • UTF-8 4E B8 AD : 11100100 10111000 10101101 

Supplementary Encoding in UTF-16

  • UTF-16 covers U+0000~U+FFFF using 2 bytes
  • For Unicode U(U+10000~U+10FFFF):
    • Minus 0x10000, get U’(0x00000~0xFFFFF), 20 bits
      • e.g. U’ = yyyyyyyyyyxxxxxxxxxx
    • Using W1 to represent the first 10 bits,
      • e.g. W1 = 110110yyyyyyyyyy, W1 in D800~DBFF
    • Using W2 to represent the second 10 bits,
      • e.g. W2 = 110111xxxxxxxxxx, W2 in DC00~DFFF


Emoji History

  • In 1999, Shigetaka Kurita created the first 180 emoji collection for a Japanese mobile web platform
  • Sounds /ɪˈmoʊdʒi/ from Japanese
  • “e”(picture), “moji”(character)

Text&Color Shape

Emoji character can have two main kinds of presentation:

  • An emoji presentation, with colorful and perhaps whimsical shapes, even animated
  • A text presentation, such as black & white

Emoji Modifiers

  • Emoji modifier - A character that can be used to modify the appearance of a preceding emoji in an emoji modifier sequence
  • Emoji modifier base - A character whose appearance can be modified by a subsequent emoji modifier in an emoji modifier sequence
  • Emoji modifier sequence - A sequence of the following form: emoji_modifier_sequence := emoji_modifier_base emoji_modifier

Fitzpatrick Modifiers

  • When one of these characters follows certain characters, then a font should show the sequence as a single glyph with the specified skin tone
  • If the font doesn’t show the combined character, the user can still see that a skin tone was intended

Variation Selectors

  • VS is a Unicode block containing 16 Variation Selector format characters(designated VS1 through VS16)
  • They are used to specify a specific glyph variant for a Unicode character
  • At present only standardized variation sequences with VS1, VS15 and VS16 have been defined


An invisible code point which specifies that the preceding character should be rendered in a textual fashion


  • An invisible code point which specifies that the preceding character should be displayed with emoji presentation
  • Only required if the preceding character defaults to text presentation
  • Often used in Emoji ZWJ Sequences, where one or more characters in the sequence have text and Emoji presentation

Emoji ZWJ Sequences

  • Joining characters as a single glyph if available
  • Behave like single emoji character, even though internally they are sequences

Surrogate Pair

  • It is possible to combine two code points defined in the BMP to express another code point that lies outside of the first 65635 code points. This combination is called surrogate pair.
  • Leading Surrogate: U+D800~U+DB7F
  • Trailing Surrogate: U+DC00~U+DFFF
  • The values from U+D800 to U+DFFF are reserved for used in UTF-16, no characters are assigned to them as code points

Java API

  • String.codePointAt(int index):int
  • Character.highSurrogate(int codePoint):char
  • Character.lowSurrogate(int codePoint):char
  • Character.charCount(int codePoint):int
  • Character.isSupplementaryCodePoint(int codePoint):boolean
  • Character.isSurrogate(char):boolean
  • Character.isSurrogatePair(char, char):boolean



本文首次发布于 S.L’s Blog, 作者 @stuartlau ,转载请保留原文链接.