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 guideline EXP00-J. Do not ignore values returned by methods. The returned value should be as representative as possible 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 some cases, an error value instead of an exception should be returned; and in other cases an exception should be thrown. A method should not return a value that can hold both valid return data and an error code; see ERR52-JG. 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
As shown in this noncompliant code example,the updateNode() method modifies a node if it can find it in a linked list and does nothing if the node is not in the list.
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. Such methods are prone to misuse; as such a method is likely to modify data in an unexpected fashion.
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 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;
}
Return values for methods can vary depending on the control flow or the information that the developer finds more useful.
A return value that might be null is an in-band error indicator, which is discussed more thoroughly in 52. ERR52-JG. 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.
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