OBJ00-J. Declare data members private

If data members are declared public or protected, it is difficult to control how they are accessed. It is possible that they can be manipulated in unintended ways, with undefined consequences. If they need to be exposed beyond the package their class is declared in, acceessor methods may be used. Also, with the use of setter methods, modification of data members can be monitored as appropriate (e.g., by defensive copying, validating input, logging and so on). Methods that are declared public or protected must preserve the invariants of the class and their use should not be abused.

Noncompliant Code Example

In this noncompliant code example, the data member total is meant to keep track of the total number of elements as they are added and removed from a container. However, as a public data member, total can be altered by external code, independent of these actions. This noncompiant example violates the condition that public classes must not expose data members by declaring them public. It is a bad practice to expose both mutable as well as immutable fields from a public class [[Bloch 08]].

public class Widget {
  public int total;
  void add (SomeType someParameter) {
    total++;
    // ...
  }
  void remove (SomeType someParameter) {
    total--;
    // ...
  }
}

Compliant Solution

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

public class Widget {
  private int total;
  void add (someType someParameter) {
    total++;
    // ...
  }
  void remove (someType someParameter) {
    total--;
    // ...
  }
  public int getTotal () {
    return total;
  }
}

Noncompliant Code Example

This noncompliant code example shows a mutable hash map with public accessibility.

public static final HashMap<Integer, String> hm = new HashMap<Integer, String>();

Compliant Solution

Mutable data members that are static must always be declared private.

private static final HashMap<Integer, String> hm = new HashMap<Integer, String>();

Exceptions

EX1: According to Sun's Code Conventions document [[Conventions 09]]:

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.

EX2: "if a class is package-private or is a private nested class, there is
nothing inherently wrong with exposing its data fields - assumng 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 08]]. This exception applies to both mutable as well as immutable fields.

Risk Assessment

Failing to declare data members private can break encapsulation.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as OBJ00-CPP. Declare data members private.

References

[[JLS 06]] Section 6.6, Access Control
[[SCG 07]] Guideline 3-2 Define wrapper methods around modifiable internal state
[[Long 05]] Section 2.2, Public Fields
[[Bloch 08]] Items 13: Minimize the accessibility of classes and members; 14: In public classes, use accessor methods, not public fields

08. Object Orientation (OBJ)      08. Object Orientation (OBJ)      OBJ01-J. Understand how a superclass can affect a subclass