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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).

Noncompliant Code Example (

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Read)

This noncompliant code example tries to read up to 1024 bytes from a socket and build a String from them. It does this by reading the bytes in a while loop, as recommended by FIO10-J. Ensure the array is filled when using read() to fill an array. If it ever detects that the socket has more than 1024 available, it throws an exception. This prevents untrusted input from potentially exhausting the program's memory.

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This code fails to consider the interaction between characters represented with a multi-byte encoding and the boundaries between the loop iterations. If the last byte read from the data stream in one read() operation is the leading byte of a multibyte character, the trailing bytes are not encountered until the next iteration of the while loop. However, multi-byte encoding is resolved during construction of the new String within the loop. Consequently, the multibyte encoding is interpreted incorrectly.

Compliant Solution (

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Read)

This compliant solution does not actually create the string until all the data is available.

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public final int MAX_SIZE = 1024;

public String readBytes(Socket socket) throws IOException {
  InputStream in = socket.getInputStream();
  Reader r = new InputStreamReader(in, "UTF-8");
  char[] data = new char[MAX_SIZE+1];
  int offset = 0;
  int charsRead = 0;
  String str = new String(data);
  while ((charsRead = r.read(data, offset, data.length - offset)) != -1) {
    offset += charsRead;
    str += new String(data, offset, data.length - offset);
    if (offset >= data.length) {
      throw new IOException("Too much input");
    }
  }
  in.close();
  return str;
}

Noncompliant Code Example (

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Substring)

This noncompliant code example attempts to trim leading letters from the {{string}}. It fails to accomplish this task because {{Character.isLetter()}} lacks support for supplementary and combining characters \[[Hornig 2007|AA. Bibliography#Hornig 07]\].

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

Noncompliant Code Example (substring)

This noncompliant code example attempts to fix the problem by using the {{String.codePointAt()}} method, which accepts an {{int}} argument. This works for supplementary characters but fails for combining characters \[[Hornig 2007|AA. Bibliography#Hornig 07]\].

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

Compliant Solution (

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Substring)

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

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