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Wiki Markup A boxing conversion converts the value of a primitive type to the corresponding value of the reference type. One example is the automatic conversion from {{int}} to {{Integer}} \ [[JLS 2005|AA. References#JLS 05]\]. This is convenient in cases where an object parameter is required, such as with collection classes like {{Map}} and {{List}}. Another use case is for interoperation with methods that require their parameters to be object references rather than primitive types. Automatic conversion to the resulting wrapper types also reduces clutter in code.

Expressions autobox into the intended type when the reference type causing the boxing conversion is one of the specific numeric wrapper types (for example, Boolean, Byte, Character, Short, Integer, Long, Float, or Double). However, autoboxing can produce unexpected results when the reference type causing the boxing conversion is nonspecific (for example, 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 on the basis of the numeric promotion rules governing the expression evaluation. Consequently, programs that use primitive arithmetic expressions as actual arguments passed to method parameters that have nonspecific reference types must cast the expression to the intended primitive numeric type before the boxing conversion takes place (unless the intended type is the resulting type of the expression).

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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);  // tries to remove an Integer
    }
    System.out.println(s.size());
  }
}

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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 \[ [Techtalk 2007|AA. References#Techtalk 07]\].

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 matches the intended boxed type.

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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, although 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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[[Core Java 2004AA. References#Core Java 04]]	Chapter 5	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="ff802502-0fa3-4cad-b8e1-268818cdca29"><ac:plain-text-body><![CDATA[	[[JLS 2005AA. References#JLS 05]]	[§5.1.7, Boxing Conversions	http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.7]	]]></ac:plain-text-body></ac:structured-macro>	<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="05c59749-ad35-4502-a430-f024e6391cde"><ac:plain-text-body><![CDATA[
[[Techtalk 2007AA. References#Techtalk 07]]	The Joy of Sets]]></ac:plain-text-body></ac:structured-macro>

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      02. Expressions (EXP)      EXP05-J. Do not write more than once to the same variable within an expression