EXP11-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 2005]]. 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 example 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 of the result into an object of type Integer. (See guideline [EXP05-J. Be aware of integer promotions in binary operators].) The HashSet contains values of only one 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 because Collections<E>.remove() accepts an argument of type Object and not E. Such behavior can result in unintended object retention or memory leaks. [[Techtalk 2007]]

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. If an arithmetic operation is expected to produce a primitive type which may get autoboxed to a wrong type, add explicit casts to the primitive type before allowing autoboxing to take over.

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 2004]] Chapter 5
[[JLS 2005]] Section 5.1.7
[[Techtalk 2007]] "The Joy of Sets"

EXP10-J. Avoid side effects in assertions      04. Expressions (EXP)      EXP12-J. Ensure a null pointer is not dereferenced