Methods should always try 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 with the perceptions and mental model of the developer in mind.
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 from incorrect results and to encourage careful handling of any incorrect results. At the same time, in some cases, an error value instead of an exception should be returned, and vice versa. A method should not return a value that could both indicate valid return data or an error indicator; 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 combination with external synchronization. Lack of appropriate synchronization leads to 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, a method modifies a node if it can find it in a linked list and does nothing if the node is not in the list. But this method fails to indicate whether it modified any node. A method can silently corrupt the state of the object if it provides little communication describing what it did.
public void updateNode(int id, int newValue) {
Node current = root;
while (current != null) {
if (current.getId() == id) {
current.setValue(newValue);
break;
}
current = current.next;
}
}
Compliant Solution (boolean)
This compliant solution returns the result of the operation: true if it modified a node and false otherwise:
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 solution returns the modified Node when one is found and throws a NodeNotFoundException when the node is not available in the list.
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 always expected to succeed—an exception is appropriate here only if the developer believes the method will always succeed.
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();
}
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. Return values for methods can vary depending on the control flow or the information that the developer finds more useful.
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-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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