FIO09-J. Do not rely on the write() method to output integers outside the range 0 to 255

The write() method, defined in the class java.io.OutputStream, takes an argument of type int intended to be between 0 and 255. Because a value of type int could be outside this range, failure to range check can result in the truncation of the higher order bits of the input.

The general contract for the write() method says that it writes one byte to the output stream. The byte to be written constitutes the eight lower order bits of the argument b, passed to the write() method; the 24 high-order bits of b are ignored (see [[API 2006]] java.io.OutputStream.write() for more information).

Noncompliant Code Example

This noncompliant code example accepts a value from the user without validating it. Any value that is not in the range of 0 to 255 is truncated. For instance, write(303) prints / on ASCII-based systems because the lower order 8 bits of 303 are used while the 24 high-order bits are ignored (303 % 256 = 47, which is the ASCII code for /). That is, the result is the remainder of the input divided by 256.

class ConsoleWrite {
  public static void main(String[] args) { 
    //Any input value > 255 will result in unexpected output
    System.out.write(Integer.valueOf(args[0]));
    System.out.flush();
  }
}

Compliant Solution (Range-Check Inputs)

Perform range checking to be compliant. This compliant solution prints the corresponding character only if the input integer is in the proper range. If the input is outside the representable range of an int, the Integer.valueOf() method throws a NumberFormatException. If the input can be represented by an int, but is outside the range required by write(), this code throws an ArithmeticException.

class FileWrite {
  public static void main(String[] args) throws NumberFormatException, IOException { 
    FileOutputStream out = new FileOutputStream("output");   

    //Perform range checking  
    int value = Integer.valueOf(args[0]);
    if (value < 0 || value > 255) {
      throw new ArithmeticException("Value is out of range");
    }
  
    out.write(value);
    System.out.flush(); 
  }
}

Compliant Solution (Use writeInt())

This compliant solution uses the writeInt() method of the DataOutputStream class, which is capable of handling an int.

class FileWrite {
  public static void main(String[] args) throws NumberFormatException, IOException { 
    DataOutputStream dos = new DataOutputStream(System.out);
    dos.writeInt(Integer.valueOf(args[0].toString()));
    System.out.flush(); 
  }     
}

Risk Assessment

Using the write() method to output integers writes only the low-order 8 bits of the integers. This truncation can result in unexpected values.

Automated Detection

Automated detection of all uses of the write() method is straightforward. Sound determination of whether the truncating behavior is correct is not feasible in the general case. Heuristic checks could be useful.

Bibliography

      12. Input Output (FIO)      FIO08-J. Use an int to capture the return value of functions that read a character or byte