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It is difficult to control how data members declared public or protected are accessed. Attackers can manipulate such members in unexpected ways. As a result data members must be declared private. Use wrapper accessor methods to expose class members that are beyond to be accessed outside of the package in which their class is declared. Using wrapper methods enables appropriate monitoring and control of the modification of data members (for example, by defensive copying, validating input, and logging). The wrapper methods can preserve class invariants.

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As a {{public}} data member, {{total}} can be altered by external code, independentindependently of the {{add()}} and {{remove()}} methods. It is a bad practice to expose fields from a {{public}} class \[[Bloch 2008|AA. Bibliography#Bloch 08]\].

Compliant Solution (

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Private)

This compliant solution declares total as private and provides a public accessor so that the required member can be accessed beyond the current package. The add() and remove() methods modify its value without violating any class invariants.

Note that accessor methods should be careful about care must be taken when providing references to private mutable objects from acessor methods; see rule "OBJ05-J. Defensively copy private mutable class members before returning their references" for details.

public class Widget {
  private int total; // Declared private

  public int getTotal () {
    return total;
  }

  // definitions for add() and remove() remain the same
}

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One example of appropriate public instance variables is the case where the class is essentially a data structure, with no behavior. In other words, if you would have used a struct instead of a class (if Java supported struct), then it's appropriate to make the class's instance variables public.

*OBJ01-EX1:* "If a class is package-private or is a {{private}} nested class, there is nothing inherently wrong with exposing its data fields —-- assuming they do an adequate job of describing the abstraction provided by the class. This approach generates less visual clutter than the accessor-method approach, both in the class definition and in the client code that uses it" \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. This exception applies to both mutable and immutable fields.

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Detection of public and protected data members is trivial; heuristic detection of the presence or absence of getter and setter wrapper 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.

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