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Similarly, a final method parameter obtains an immutable copy of the object reference. Again, this has no effect on the mutability of the referenced data.

Noncompliant Code Example (Mutable Class, final Reference)

Sometimes, when a variable is declared final, it is believed to be immutable. If the variable is a primitive type, declaring it final means that its value cannot be subsequently changed. However, if the variable is a reference to a mutable object, the object's contained data that appears to be immutable, may actually be mutable. For example, a final method parameter that is a reference to an object. The argument to this method uses pass-by-value to copy the reference but the referenced data remains mutable.

According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\], section 4.12.4 "{{final}} Variables":

... if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.

...

In this noncompliant code example, the programmer has declared the reference to the point instance to be final under the incorrect assumption that doing so prevents modification of the values of the instance fields a and b x and y. The values of the instance fields can be changed even after their initialization . When an object reference is declared final, it only signifies that the reference cannot be changed, whereas the referenced contents can still bebecause the final clause applies only to the reference to the point instance and not to the referenced object.

Code Block
bgColor #ffcccc
class Point FinalClass{ private int ax; private int by; FinalClassPoint(int ax, int by) { this.ax = ax; this.by = by; } void set_abxy(int ax, int by) { this.ax = ax; this.by = by; } void print_abxy() { System.out.println("the value ax is: " + this.ax); System.out.println("the value by is: " + this.by); } } public class FinalCallerPointCaller { public static void main(String[] args) { final FinalClassPoint fcpoint = new FinalClassPoint(1, 2); fcpoint.print_abxy(); // changeChange the value of ax,b. y fcpoint.set_abxy(5, 6); fcpoint.print_abxy(); } }

Compliant Solution

...

(final Fields)

When the values of the x and y instance variables must remain If a and b have to be kept immutable after their initialization, the simplest approach is to declare them as they should be declared final. However, this requires the elimination of the setter method invalidates a set_abxy(). method because it can no longer change the values of x and y:

class Point {
  private final int ax;
  private final int b;

Compliant Solution

y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void print_xy() {
    System.out.println("the value x is: " + this.x);
    System.out.println("the value y is: " + this.y);
  }

  // set_xy(int x, int y) no longer possible
}

With this modification, the values of the instance variables become immutable and consequently match the programmer's intended usage model.

Compliant Solution (Provide Copy Functionality)

If the class must remain mutable, another compliant solution is to provide copy functionality. This compliant solution provides a clone() method in the class Point, avoiding the elimination of the setter method:This compliant solution provides a clone() method in the class and does not require the elimination of the setter method. The clone() method returns a copy of the original object. This new object can be freely used without affecting the original object. Using the clone() method allows the class to remain mutable. (OBJ36-J. Provide mutable classes with a clone method to allow passing instances to untrusted code safely)

final public class NewFinalPoint implements Cloneable {
  private int ax;
  private int b;

  NewFinal(int ay;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void set_xy(int x, int by) {
    this.ax = ax;
    this.by = by;
  }
  void print_abxy() {
    System.out.println("the value ax is: "+ this.ax);
    System.out.println("the value by is: "+ this.by);
  }
  public voidPoint set_abclone(int) a, int b)throws CloneNotSupportedException{
    Point this.acloned = a;
    this.b = b(Point) super.clone();
  }
  public// NewFinalNo clone()need throwsto CloneNotSupportedException{
clone x and y NewFinalas clonedthey = (NewFinal) super.clone();are primitives
    return cloned;
  }
}

public class NewFinalCallerPointCaller {
  public static void main(String[] args) 
      throws CloneNotSupportedException {
    finalPoint NewFinalpoint nf = new NewFinalPoint(1, 2);    // Is not changed in main()
    nfpoint.print_abxy();
    
    //get Get the copy of original object
    Point pointCopy try {
      NewFinal nf2 = nf.clone();= point.clone();
    // pointCopy now holds a unique reference to the 
    // newly cloned Point instance

    // changeChange the value of ax,by of the copy.
      nf2pointCopy.set_abxy(5, 6);
  
    // originalOriginal value will not be changedremains unchanged
      nfpoint.print_abxy();
    } catch (CloneNotSupportedException e) { /* Forward to handler */ }
  }
}

The class is declared final to prevent subclasses from overriding the clone() method . This enables the class to be accessed and used, while preventing the fields from being modified, and complies with OBJ36returns a copy of the original object that reflects the state of the original object at the moment of cloning. This new object can be used without exposing the original object. Because the caller holds the only reference to the newly cloned instance, the instance fields cannot be changed without the caller's cooperation. This use of the clone() method allows the class to remain securely mutable. (See OBJ04-J. Provide mutable classes with a clone method copy functionality to safely allow passing instances to untrusted code safely.

Noncompliant Code Example

Another common mistake is to use a public static final array. Clients can trivially modify the contents of the array (although they are unable to change the array itself, as it is final).

In this noncompliant code example, the elements of the {{items\[\]}} array, are modifiable.

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

Compliant Solution

This compliant solution defines a private array and a public method that returns a copy of the array.

private static final String[] items = { ... };
public static final String[] somethings() {
  return items.clone();
}

As a result, the original array values cannot be modified by a client.

Compliant Solution

This compliant solution declares a private array from which a public immutable list is constructed.

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.

Risk Assessment

Using final to declare the reference to a mutable object is potentially misleading because the contents of the object can still be changed.

Related Vulnerabilities

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

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] Sections 4.12.4 "final Variables" and 6.6, "Access Control"
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 13: Minimize the accessibility of classes and members
\[[Core Java 04|AA. Java References#Core Java 04]\] Chapter 6
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 607|http://cwe.mitre.org/data/definitions/607.html] "Public Static Final Field References Mutable Object"

)

The Point class is declared final to prevent subclasses from overriding the clone() method. This enables the class to be suitably used without any inadvertent modifications of the original object.

Applicability

Incorrectly assuming that final references cause the contents of the referenced object to remain mutable can result in an attacker modifying an object believed to be immutable.

Bibliography

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Image Added Image Added Image AddedOBJ02-J. Avoid using finalizers      08. Object Orientation (OBJ)      OBJ04-J. Encapsulate the absence of an object by using a Null Object