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A boxing conversion converts the value of a primitive type to the corresponding value of the reference type,. forOne example, is the automatic conversion 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. |
Expressions autobox into the intended type when the reference type causing the boxing conversion is one of the specific numeric wrapper types (e.g. 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 (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.
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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 \[[Techtalk 2007|AA. Bibliography#Techtalk 07]\]. |
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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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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)); // castremoves toa short Short
}
System.out.println(s.size());
}
}
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="2d216d00684a30f4-613b57d5-4f754b54-9af8b939-58638089630d7854f164a1d1"><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="97704fd936fbd53c-66bee70a-4a00490a-9d6bac8b-2d1c01ead5499363221ec28f"><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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