EXP05-J. Be careful of autoboxing when removing elements from a Collection

A boxing conversion converts the value of a primitive type to the corresponding value of the reference type, for instance, from int to the type Integer [[JLS 05]]. It can be convenient in many cases where an object parameter is desired, such as with collection classes like Map and List. Another use case is to pass object references to methods, as opposed to primitive types that are always passed by value. The resulting wrapper types also help reduce clutter in code.

Noncompliant Code Example

This noncompliant code snippet prints 100 as the size of the HashSet while it is expected to print 1. The combination of values of types short and int in the operation i-1 leads to autoboxing into an object of type Integer. The HashSet contains only values of type Short whereas the code attempts to remove objects of the (different) type Integer. As a result, the remove operation is equivalent to a No Operation (NOP). The compiler enforces type checking so that only Short values are inserted, however, a programmer is free to remove an object of any type without triggering any exceptions since Collections<E>.remove() takes a parameter of type Object and not a parameter of type E. Such behavior can result in unintended object retention or memory leaks. [[Techtalk 07]]

public class ShortSet {
  public static void main(String[] args) {
    HashSet<Short> s = new HashSet<Short>();
      for(short i=0; i<100;i++) {
        s.add(i);
        s.remove(i - 1);
      }
    System.out.println(s.size());
  }
}

Compliant Solution

Avoid mixing the different boxed integer types. To remedy the issue described in the noncompliant code, if an arithmetic operation is going to produce a primitive type which will be autoboxed to the wrong type, cast the primitive type correctly before allowing the autoboxing to work.

public class ShortSet {
  public static void main(String[] args) {
    HashSet<Short> s = new HashSet<Short>();
      for(short i=0; i<100;i++) {
        s.add(i);
        s.remove((short)(i-1));  //cast to short 
      }
    System.out.println(s.size());
  }
}

Risk Assessment

Numeric promotion and autoboxing while removing elements from a Collection, can make operations on the Collection fail silently.

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

[[Core Java 04]] Chapter 5
[[JLS 05]] Section 5.1.7
[[Techtalk 07]] "The Joy of Sets"

EXP04-J. Be wary of invisible implicit casts when using compound assignment operators      03. Expressions (EXP)      EXP06-J. Be aware of the short-circuit behavior of the conditional AND and OR operators