OBJ01-J. Be aware that a final reference may not always refer to immutable data

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 changed after initialization (other than through the use of the unsupported sun.misc.Unsafe class).

However, if the variable is a reference to a mutable object, the object's members that appear to be immutable, may in fact be mutable. For example, a final field that stores a reference to an object does not imply immutability of the object itself. Similarly, a final method parameter obtains a copy of the object reference through pass-by-value, but the referenced data remains mutable.

According to the Java Language Specification [[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.

Noncompliant Code Example (mutable class, final reference)

In this noncompliant code example, the values of instance fields a and b 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 be altered.

class FinalClass {
  private int a;
  private int b;

  FinalClass(int a, int b){
    this.a = a;
    this.b = b;
  }
  void set_ab(int a, int b){
    this.a = a;
    this.b = b;
  }
  void print_ab(){
    System.out.println("the value a is: " + this.a);
    System.out.println("the value b is: " + this.b);
  }
}

public class FinalCaller {
  public static void main(String[] args) {
    final FinalClass fc = new FinalClass(1, 2);
    fc.print_ab();
    // change the value of a,b.
    fc.set_ab(5, 6);
    fc.print_ab();
  }
}

Compliant Solution (final fields)

If a and b must remain immutable after their initialization, they should be declared as final. However, this requires the elimination of the setter method set_ab().

class FinalClass {
  private final int a;
  private final int b;
  // ...
}

Compliant Solution (provide copy functionality)

This compliant solution provides a clone() method in the final class and does not require the elimination of the setter method.

final public class FinalClass implements Cloneable {
  private int a;
  private int b;

  FinalClass(int a, int b){
    this.a = a;
    this.b = b;
  }
  void print_ab(){
    System.out.println("the value a is: "+ this.a);
    System.out.println("the value b is: "+ this.b);
  }
  void set_ab(int a, int b){
    this.a = a;
    this.b = b;
  }
  public FinalClass clone() throws CloneNotSupportedException{
    FinalClass cloned = (FinalClass) super.clone();
    return cloned;
  }
}

public class FinalCaller {
  public static void main(String[] args) throws CloneNotSupportedException {
    final FinalClass nf = new FinalClass(1, 2);
    nf.print_ab();
    
    // Get the copy of original object
  
    FinalClass nf2 = nf.clone();
    // Change the value of a,b of the copy.
    nf2.set_ab(5, 6);
    // Original value will not be changed
    nf.print_ab();
  }
}

The clone() method returns a copy of the original object. This new object can be freely used without exposing the original object. Using the clone() method allows the class to remain mutable. (OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely)

The FinalClass 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. This compliant solution complies with OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely.

Noncompliant Code Example (arrays)

This noncompliant code example uses a public static final array. Clients can trivially modify the contents of the array (although they are unable to change the array reference, 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 (unmodifiable wrappers)

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. For more details about unmodifiable wrappers, refer to SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers.

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]] Sections 4.12.4 "final Variables" and 6.6, "Access Control"
[[Bloch 08]] Item 13: Minimize the accessibility of classes and members
[[Core Java 04]] Chapter 6
[[MITRE 09]] CWE ID 607 "Public Static Final Field References Mutable Object"

OBJ00-J. Declare data members as private and provide accessible wrapper methods      08. Object Orientation (OBJ)      OBJ02-J. Do not ignore return values of methods that operate on immutable objects