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This noncompliant code example attempts to convert the byte array representing a BigInteger into a String. Because some of the bytes do not denote valid characters, the resulting String representation loses information. Converting the String back to a BigInteger produces a different value.

BigInteger x = new BigInteger("530500452766");
// convert x to a String
byte[] byteArray = x.toByteArray();
String s = new String(byteArray);
// convert s back to a BigInteger
byteArray = s.getBytes();
x = new BigInteger(byteArray);

When this program was run on a Linux platform where the default character encoding is US-ASCII, the string s got the value {?J??, because some of the characters were unprintable. When converted back to a BigInteger, x got the value 149830058370101340468658109.

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Do not try to convert the String object to a byte array to obtain the original BigInteger. Character encoded data may yield a byte array that, when converted to a BigInteger, results in a completely different value.

BigInteger x = new BigInteger("530500452766");
String s = x.toString();  // valid character data
try {
  byte[] byteArray = s.getBytes("UTF8");
  // ns prints as "530500452766"
  String ns = new String(byteArray, "UTF8");  
  // construct the original BigInteger
  BigInteger x1 = new BigInteger(ns); 
} catch (UnsupportedEncodingException ex) {
  // handle error
}

Noncompliant Code Example

This noncompliant code example corrupts the data when string contains characters that are not representable in the specified charset.

// Corrupts data on errors
public static byte[] toCodePage(String charset, String string)
  throws UnsupportedEncodingException {
  return string.getBytes(charset);
}
 
// Fails to detect corrupt data
public static String fromCodePage(String charset, byte[] bytes)
  throws UnsupportedEncodingException {
  return new String(bytes, charset);
}

Compliant Solution

The java.nio.charset.CharsetEncoder class can transform a sequence of 16-bit Unicode characters into a sequence of bytes in a specific charset, while the java.nio.charset.CharacterDecoder class can reverse the procedure [API 2006].

This compliant solution uses the CharsetEncoder and CharsetDecoder classes to handle encoding conversions.

public static byte[] toCodePage(String charset, String string)
  throws IOException {
   
  Charset cs = Charset.forName(charset);
  CharsetEncoder coder = cs.newEncoder();
  ByteBuffer bytebuf = coder.encode(CharBuffer.wrap(string));
  byte[] bytes = new byte[bytebuf.limit()];
  bytebuf.get(bytes);
  return bytes;
}

Noncompliant Code Example

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// Corrupts data on errors
public static void toFile(String charset, String filename,
                        String string) throws IOException {
   
  FileOutputStream stream = new FileOutputStream(filename, true);
  OutputStreamWriter writer = new OutputStreamWriter(stream, charset);
  writer.write(string, 0, string.length());
  writer.close();
}

Compliant Solution

This compliant solution uses the CharsetEncoder class to perform the required function.

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