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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 (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).

Noncompliant Code Example

Wiki MarkupThis 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. The HashSet contains only values of type Short; the code attempts to remove objects of type Integer. Consequently, the remove() operation accomplishes nothing.

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());
  }
}

 one of type {{Short}}. 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);  // tries to remove an Integer
    }
    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 matches the intended boxed type.

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