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A boxing conversion converts the value of a primitive type to the corresponding value of the reference type, for example, from {{int}} to {{Integer}} \[[JLS 2005|AA. Bibliography#JLS 05]\]. 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. |
Noncompliant Code Example
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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 \[[Techtalk 2007|AA. Bibliography#Techtalk 07]\]. |
| Code Block | ||
|---|---|---|
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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);
}
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.
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|---|---|---|
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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((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.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
EXP10-J | low | probable | low | P6 | L2 |
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.
Related Guidelines
Bibliography
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="f0593038c6f82eeb-a5d61286-4f634b75-a8a2bb63-ead7e2a15fd18f381724cd7d"><ac:plain-text-body><![CDATA[ | [[Core Java 2004 | AA. Bibliography#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="83329943d2383990-64d1cddd-423e45d8-9d85bb5d-f27182f580d7945d99d4c85d"><ac:plain-text-body><![CDATA[ | [[JLS 2005 | AA. Bibliography#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="1632a22e4acb28ee-c2f61ed1-466d48d1-a99d9e32-4d08db426f86508b6a9bad9d"><ac:plain-text-body><![CDATA[ | [[Techtalk 2007 | AA. Bibliography#Techtalk 07]] | "The Joy of Sets" | ]]></ac:plain-text-body></ac:structured-macro> |
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