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

A Boxing conversion converts the values of a primitive type to the corresponding values of the reference type, for instance, from int to the type Integer [JLS 5.1.7 Boxing Conversion]. 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.

Autoboxing can automatically wrap the primitive type to the corresponding wrapper object. Some care should be taken during this process, especially when performing comparisons. The Java Language Specification [[JLS 05]] explains this point clearly:

If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

Noncompliant Code Example

This noncompliant code snippet [[Techtalk 07]] prints 100 as the size of the HashSet while it is expected to print 1. The combination of a short and an integer value in the operation i-1 leads to autoboxing into an Integer object. The HashSet contains only short values whereas (distinctly typed) Integer objects are being removed successively. The remove operation is as a result 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 an Object parameter and not E. Such behavior can result in unintended object retention or memory leaks.

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 different types together with an integer type. If inevitable, cast the autoboxed Integer object to a Short to remedy the issue described in the noncompliant code.

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