Java Unicode

What is Java Unicode Handling?

Java's approach to Unicode reflects the historical evolution of the Unicode Standard. Java was designed in the mid-1990s when Unicode was a 16-bit encoding, and this assumption was baked into the language's char type and String class. When Unicode later expanded to 21 bits to accommodate scripts like Egyptian Hieroglyphs and emoji, Java had to retrofit support for code points beyond U+FFFF — the supplementary characters.

The char Type: 16-bit BMP Only

Java's char is a 16-bit unsigned integer representing a UTF-16 code unit, not a full Unicode code point. This means char can represent only characters in the Basic Multilingual Plane (U+0000 to U+FFFF). Characters outside this range — everything from U+10000 onward — require two char values called a surrogate pair.

char c = '\u0041';  // 'A' — works fine, BMP character
// char cannot hold U+1F600 (😀) — it requires a surrogate pair

String as UTF-16

Java's String class stores its content as a UTF-16 sequence. For most Western text this is transparent, but supplementary characters produce a String where the visual character count differs from length():

String emoji = "😀";          // U+1F600 GRINNING FACE
emoji.length();               // 2 — two UTF-16 code units (surrogate pair)
emoji.codePointCount(0, emoji.length());  // 1 — one Unicode code point

codePointAt() vs charAt()

The distinction between code-unit-based and code-point-based iteration is the central Java Unicode challenge:

String s = "A😀B";

// charAt() — returns UTF-16 code units
s.charAt(0);   // 'A'
s.charAt(1);   // '\uD83D' — high surrogate, NOT a printable character
s.charAt(2);   // '\uDE00' — low surrogate

// codePointAt() — returns full Unicode code points
s.codePointAt(0);  // 65 (A)
s.codePointAt(1);  // 128512 (😀, U+1F600)
s.codePointAt(3);  // 66 (B)

For correct supplementary-character-aware iteration, use String.codePoints() in Java 8+:

s.codePoints().forEach(cp ->
    System.out.println(new String(Character.toChars(cp))));

The Character Class

Character provides static utility methods for Unicode properties. Since Java 5, many methods have overloaded versions that accept int code points (not just char):

Character.isLetter('A');                   // true
Character.isLetter(0x1F600);               // false (emoji, not a letter)
Character.getType('A');                    // Character.UPPERCASE_LETTER
Character.toUpperCase(0x0073);             // 0x0053 ('S')
Character.isSupplementaryCodePoint(0x1F600); // true

Pattern Matching and Unicode

Regular expressions in Java require the Pattern.UNICODE_CHARACTER_CLASS flag to make \w, \d, \s match Unicode categories rather than just ASCII:

Pattern p = Pattern.compile("\\w+", Pattern.UNICODE_CHARACTER_CLASS);
p.matcher("Héllo").matches();  // true — accented letter is a word char

Quick Facts