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Methods should be designed to return a value that allows the developer to learn about the current state of the object and/or the result of an operation. This advice is consistent with EXP00-J. Do not ignore values returned by methods. The returned value should be representative of the last known state and should be chosen keeping in mind the perceptions and mental model of the developer.

Feedback can also be provided by throwing either standard or custom exception objects derived from the Exception class. With this approach, the developer can still get precise information about the outcome of the method and proceed to take the necessary actions. To do so, the exception should provide a detailed account of the abnormal condition at the appropriate abstraction level.

APIs should use a combination of these approaches both to help clients distinguish correct results from incorrect ones and to encourage careful handling of any incorrect results. In cases where there is a commonly accepted error value that cannot be misinterpreted as a valid return value for the method, that error value should be returned; and in other cases, an exception should be thrown. A method must not return a value that can hold both valid return data and an error code; see ERR52-J. Avoid in-band error indicators for more details.

Alternatively, an object can provide a state-testing method [Bloch 2008] that checks whether the object is in a consistent state. This approach is useful only in cases where the object's state cannot be modified by external threads. This prevents a time-of-check, time-of-use (TOCTOU) race condition between invocation of the object's state-testing method and the call to a method that depends on the object's state. During this interval, the object's state could change unexpectedly or even maliciously.

Method return values and/or error codes must accurately specify the object's state at an appropriate level of abstraction. Clients must be able to rely on the value for performing critical decisions.

Noncompliant Code Example

The updateNode() method in this noncompliant code example modifies a node if it can find it in a linked list and does nothing if the node is not found. 

public void updateNode(int id, int newValue) {		
  Node current = root;
  while (current != null) {
    if (current.getId() == id) {
      current.setValue(newValue);
      break;
    }
    current = current.next;
  }
}

This method fails to indicate whether it modified any node. Consequently, a caller cannot determine that the method succeeded or failed silently.

Compliant Solution (Boolean)

This compliant solution returns the result of the operation as true if it modified a node and false if it did not.

public boolean updateNode(int id, int newValue) {		
  Node current = root;
  while (current != null) {
    if (current.getId() == id) {
      current.setValue(newValue);
      return true; // Node successfully updated
    }
    current = current.next;
  }
  return false;
}

Compliant Solution (Exception)

This compliant solution returns the modified Node when one is found and throws a NodeNotFoundException when the node is not available in the list.

public Node updateNode(int id, int newValue) 
    throws NodeNotFoundException {
  Node current = root;
  while (current != null) {
    if (current.getId() == id) {
      current.setValue(newValue);
      return current;
    }
    current = current.next;
  }	
  throw new NodeNotFoundException();
}

Using exceptions to indicate failure can be a good design choice, but throwing exceptions is not always appropriate. In general, a method should throw an exception only when it is expected to succeed but an unrecoverable situation occurs or when it expects a method higher up in the call hierarchy to initiate recovery.

Compliant Solution (Null Return Value)

This compliant solution returns the updated Node so that the developer can simply check for a null value if the operation fails. 

public Node updateNode(int id, int newValue) {	
  Node current = root;
  while (current != null) {
    if (current.getId() == id) {
      current.setValue(newValue);
      return current;
    }
    current = current.next;
  }
  return null;
}

A return value that might be null is an in-band error indicator, which is discussed more thoroughly in ERR52-J. Avoid in-band error indicators. This design is permitted but is considered inferior to other designs, such as those shown in the other compliant solutions in this guideline.

Applicability

Failure to provide appropriate feedback through a combination of return values, error codes, and exceptions can lead to inconsistent object state and unexpected program behavior.

Bibliography

[Bloch 2008]Item 59, "Avoid unnecessary use of checked exceptions"
[Ware 2008]Writing Secure Java Code

 


3 Comments

  1. By returning a valid node or null, the 2nd CS violates ERR52-JG. Avoid in-band error indicators, which doesn't give any exception for returning valid objects vs null. How to resolve?

    I would like to see more wisdom regarding when to use a return value indicating failure vs throwing an exception, but I don't have any to provide...nor, I suspect, does anyone else.

    1. Added an exception to ERR52 for null return values (admitting that they are inferior to other designs). And moved the null-return-value CS to the bottom of the rule explaining why the other CSs are better.

  2. We aren't as harsh about null in the C standard:

    ERR02-EX1: Null pointers are another example of an in-band error indicator. Use of null pointers is allowed because it is supported by the language. According to the C Standard, Section 6.3.2.3 [ISO/IEC 9899:2011]:

    If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.