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Expressions autobox into the intended type when the reference type causing the boxing conversion is one of the specific numeric wrapper types (e.g., Byte, Short, Integer, Long, Float, or Double). However, autoboxing can produce unexpected results when the reference type causing the boxing conversion is nonspecific (e.g., Number or Object) and the value being converted is the result of an expression that mixes primitive numeric types. In this latter case, the specific wrapper type that results from the boxing conversion is chosen based on the numeric promotion rules governing the expression evaluation. See rule "NUM18-J. Be aware of numeric promotion behavior" for additional explanation of the details of the promotion rules. Consequently, programs that use primitive arithmetic expressions as actual arguments passed to method parameters that have non-specific reference types must cast the expression to the intended primitive numeric type before the boxing conversion takes place.

Noncompliant Code Example

This noncompliant code example prints {{100}} as the size of the {{HashSet}} rather than the expected result ({{1}}). The combination of values of types {{short}} and {{int}} in the operation {{i-1}} causes the result to be autoboxed into an object of type {{Integer}}, rather than one of type {{Short}}. See rule "[NUM18-J. Be aware of numeric promotion behavior]" for additional explanation of the details of the promotion rules. The {{HashSet}} contains only values of type {{Short}}; the code attempts to remove objects of type {{Integer}}. Consequently, the {{remove()}} operation accomplishes nothing. The language's type checking guarantees that only values of type {{Short}} can be inserted into the {{HashSet}}.  Nevertheless, programmers are free to attempt to remove an object of _any_ type because {{Collections<E>.remove()}} accepts an argument of type {{Object}} rather than of type {{E}}. Such behavior can result in unintended object retention or memory leaks \[[Techtalk 2007|AA. Bibliography#Techtalk 07]\]. 

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

Objects removed from a collection must share the type of the elements of the collection. Numeric promotion and autoboxing can produce unexpected object types. This compliant solution uses an explicit cast to short that parallels the intended boxed type.

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

Allowing autoboxing to produce objects of an unintended type can cause silent failures with some APIs, such as the Collections library. These failures can result in unintended object retention, memory leaks, or incorrect program operation.

Automated Detection

Detection of invocations of Collection.remove() whose operand fails to match the type of the elements of the underlying collection is straightforward. It is possible, albeit unlikely, that some of these invocations could be intended. The remainder are heuristically likely to be in error. Automated detection for other APIs could be possible.

Bibliography

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