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Invariants cannot be enforced for public nonfinal fields or for final fields that reference a mutable object. A protected member of an exported (non-final) class represents a public commitment to an implementation detail. Attackers can manipulate such fields to violate class invariants, or they may be corrupted by multiple threads accessing them concurrently [Bloch 2008]. As a result, fields must be declared private or package-private.

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Because a copy of the array is returned, the original array values (the references to the String objects) cannot be modified by a client. Note that a manual deep copy could be required when dealing with arrays of objects. This generally happens when the objects do not export a clone() method (see OBJ06-J. Defensively copy mutable inputs and mutable internal components for more information).

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This compliant solution constructs a public immutable list from the private array. It is safe to share immutable objects without risk that the recipient can modify them [Mettler 2010]. This example is safe because String is immutable.

private static final String[] items = { ... };

public static final List<String> itemsList =
  Collections.unmodifiableList(Arrays.asList(items));

Neither the original array values nor the public list can be modified by a client. For more details about unmodifiable wrappers, refer to OBJ56-J. Provide sensitive mutable classes with unmodifiable wrappers. This solution can also be used when the array contains mutable objects.

Exceptions

OBJ01-J-EX0: Fields with no associated behavior or invariants can be public. According to Sun's Code Conventions document [Conventions 2009]:

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Failing to limit field accessibility can defeat encapsulation, allow attackers to manipulate fields to violate class invariants, or allow these fields to be corrupted as the result of concurrent accesses from multiple threads.

Automated Detection

Detection of public and protected fields is trivial; heuristic detection of the presence or absence of accessor methods is straightforward. However, simply reporting all detected cases without suppressing those cases covered by the exceptions to this rule would produce excessive false positives. Sound detection and application of the exceptions to this rule is infeasible; however, heuristic techniques may be useful.

Related Guidelines

Bibliography

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