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According to the Java API \[[API 2006|AA. Java References#API 06]\], class {{Character}} documentation (Unicode Character Representations):

The char data type (and consequently the value that a Character object encapsulates) are based on the original Unicode specification, which defined characters as fixed-width 16-bit entities. The Unicode standard has since been changed to allow for characters whose representation requires more than 16 bits. The range of legal code points is now U+0000 to U+10FFFF, known as Unicode scalar value.

The Java 2 platform uses the UTF-16 representation in char arrays and in the String and StringBuffer classes. In this representation, supplementary characters are represented as a pair of char values, the first from the high-surrogates range, (\uD800-\uDBFF), the second from the low-surrogates range (\uDC00-\uDFFF).

An int value represents all Unicode code points, including supplementary code points. The lower (least significant) 21 bits of int are used to represent Unicode code points and the upper (most significant) 11 bits must be zero. Unless otherwise specified, the behavior with respect to supplementary characters and surrogate char values is as follows:

• The methods that only accept a char value cannot support supplementary characters. They treat char values from the surrogate ranges as undefined characters. For example, Character.isLetter('\uD840') returns false, even though this specific value if followed by any low-surrogate value in a string would represent a letter.
• The methods that accept an int value support all Unicode characters, including supplementary characters. For example, Character.isLetter(0x2F81A) returns true because the code point value represents a letter (a CJK ideograph).

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This noncompliant code example attempts to trim leading letters from the {{string}}. It fails to accomplish this task because {{Character.isLetter()}} does not work for supplementary and combining characters. \[[Hornig 2007|AA. Java References#Hornig 07]\] (sic).

// Fails for supplementary or combining characters
public static String trim_bad1(String string) {
  char ch;
  for (int i = 0; i < string.length(); i += 1) {
    ch = string.charAt(i);
    if (!Character.isLetter(ch)) {
      break;
    }
  }
  return string.substring(i);
}

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This noncompliant code example ameliorates the problem by using the {{String.codePointAt()}} method, which accepts an {{int}} argument. This works for supplementary characters but not for combining characters. \[[Hornig 2007|AA. Java References#Hornig 07]\] (sic).

// Fails for combining characters
public static String trim_bad2(String string) {
  int ch;
  for (int i = 0; i < string.length(); i += Character.charCount(ch)) {
    int ch = string.codePointAt(i);
    if (!Character.isLetter(ch)) {
      break;
    }
  }
  
  return string.substring(i);
}

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This compliant solution works for both supplementary and combining characters \[[Hornig 2007|AA. Java References#Hornig 07]\] (sic). According to the Java API \[[API 2006|AA. Java References#API 06]\], class {{java.text.BreakIterator}} documentation:

The BreakIterator class implements methods for finding the location of boundaries in text. Instances of BreakIterator maintain a current position and scan over text returning the index of characters where boundaries occur.

The boundaries returned may be those of supplementary characters, combining character sequences, or ligature clusters. For example, an accented character might be stored as a base character and a diacritical mark.

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Failing to account for supplementary and combining characters can lead to unexpected behavior.

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