An in-band error indicator is a value returned by a method that indicates either a legitimate return value or an illegitimate value that indicates an error. Some common examples of in-band error indicators include

In-band error indicators require checking for the error; however, this checking is often overlooked. Failure to check for such error conditions not only violates EXP00-J. Do not ignore values returned by methods but also has the unfortunate effect of propagating invalid values that may subsequently be treated as valid in later computations.

Avoid the use of in-band error indicators. They are much less common in Java's core library than in some other programming languages; nevertheless, they are used in the read(byte[] b, int off, int len) and read(char[] cbuf, int off, int len) families of methods in

In Java, the best way to indicate an exceptional situation is by throwing an exception rather than by returning an error code. Exceptions are propagated across scopes and cannot be ignored as easily as error codes can. When using exceptions, the error-detection and error-handling code is kept separate from the main flow of control.

Noncompliant Code Example

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

// Set up buffRdr

read =, 0, MAX_READ);
chBuff[read] = TERMINATOR;

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

Compliant Solution (Wrapping)

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 =, 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 either failing to check status codes or using incorrect return values, leading to unexpected behavior.

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

Returning an object that might be null on failure or a valid object on success is a common example of in-band error indicator. Although better method designs are often available, returning an object that may be null can be acceptable under some circumstances. See MET54-J. Always provide feedback about the resulting value of a method for an example.




    • What are in-band error indicators - need a definition in the intro
    • Are there any other methods that use in-band error indicators?
    • s/?1/-1/ 
    • There's probably an exception if someone uses an 'if condition' to check if read() is returning -1 and then determines whether 'read' variable should be used as an index or not


    1. Fixed the ?1 issue.

      I think this guideline would make more sense as advice for those creating a new function/interface/API. We already have a rule somewhere about ignoring return values, and should just cross-ref that for the unfortunate users of APIs that violate this rule. 

      • Definition added
      • Probably. added some examples of in-band error indicators
      • Thanks, Dean!
      • I think the NCCE/CS's do a good job of showing how read() behaves.
  1. Math.abs(int) can return negative numbers as well. See Pg 6 of

    Not sure if that qualifies as in-band error.

    1. IMO Math.abs(Integer.MIN_VALUE) < 0 is not an in-band error indicator; as Math.abs() "thinks" that it is returning a legit value. An in-band error indicator is a design decision, not just an odd corner case.

  2. You should not throw exceptions from ctors. So I removed the following text:

    Also, exceptions can be used in situations where error codes cannot be returned (in constructors, for example).