Page History

Avoid the use of in-band error indicators. They are much less common in Java than in some other programming languages, but they are used in the read(byte[] b, int off, int len) and read(char[] cbuf, int off, int len) family families of methods in java.io.

In Java, the best way to indicate an exceptional situation is by throwing an exception rather than by returning an error code. Exceptions cannot be ignored in the same way that error codes can, and exceptions are propagated across scopes. When using exceptions, the error-detection and error-handling code is kept separate from the main flow of control. Also, exceptions can be used in situations where error codes cannot be returned (in constructors, for example).

...

This noncompliant code example attempts to read into an array of characters and add an extra character into the buffer immediately after the characters read.:

static final int MAX = 21;
static final int MAX_READ = MAX - 1;
static final char TERMINATOR = '\\';
int read;
char [] chBuff = new char [MAX];
BufferedReader buffRdr;

// setSet up buffRdr

read = buffRdr.read(chBuff, 0, MAX_READ);
chBuff[read] = TERMINATOR;

However, if the input buffer is initially at end-of-file, then the read method will return -1 −1, and the attempt to place the terminator character will throw an ArrayIndexOutOfBoundsException.

...

This compliant solution defines a SafeBufferedReader class and introduces a readSafe method that throws an exception if end-of-file is detected.:

class SafeBufferedReader extends BufferedReader {
   SafeBufferedReader(Reader in) {
      super(in);
   }

   public int readSafe(char[] cbuf, int off, int len) throws EOFException {
      int read = read(cbuf, off, len);
      if (read == -1) {
         throw new EOFException();
      } else {
         return read;
      }
   }
}

// ...

SafeBufferedReader safeBuffRdr;

try {
   read = safeBuffRdr.readSafe(chBuff, 0, MAX_READ);
   chBuff[read] = TERMINATOR;
} catch (EOFException eof) {
   chBuff[0] = TERMINATOR;
}

...

This compliant solution defines a readSafe method that wraps the original read method and throws an exception if end-of-file is detected.:

public static int readSafe(BufferedReader buffer, char[] cbuf, int off, int len) throws IOException {
  int read = buffer.read(cbuf, off, len);
  if (read == -1) {
     throw new EOFException();
  } else {
     return read;
  }
}

// ...

BufferedReader buffRdr;

// set up buffRdr

try {
   read = readSafe(buffRdr, chBuff, 0, MAX_READ);
   chBuff[read] = TERMINATOR;
} catch (EOFException eof) {
   chBuff[0] = TERMINATOR;
}

...

Using in-band error indicators may result in programmers failing to check status codes or using incorrect return values, resulting in leading to undefined behavior.

Given the comparatively rare occurrence of in-band error indicators in Java, it may be possible to compile a list of all methods that use them and automatically detect their use. However, detecting the safe use of in-band error indicators is not feasible.

...

...