EXP04-J. Ensure that autoboxed values have the intended type

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]. 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).

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. 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());
  }
}

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].

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.

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));  // removes a 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, 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

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