Page History

The interfaces of the Java Collections Framework [JCF 2014] use generically typed, parameterized methods, such as add(E e) and put(K key, V value), to insert objects into the collection or map, but they have other methods, such as contains(), remove(), and get(), that accept an argument of type Object rather than a parameterized type. Consequently, these methods accept an object of any type. The collections framework interfaces were designed in this manner to maximize backwards compatibility, but this design can also lead to coding errors. Programmers must ensure that arguments passed to methods such as Map<K,V> get(), Collection<E>  contains(), and remove() have the same type as the parameterized type of the corresponding class instance.

Noncompliant Code Example

After adding and removing 10 elements, the HashSet in this noncompliant code example still contains 10 elements and not the expected 0. Java's type checking requires that only values of type Short can be inserted into s. Consequently, the programmer has added a cast to short so that the code will compile. However, the Collections<E>.remove() method accepts an argument of type Object rather than of type E, allowing a programmer to attempt to remove an object of any type. In this noncompliant code example, the programmer has neglected to also cast the variable i before passing it to the remove() method, which is autoboxed into an object of type Integer rather

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 method has no effect.

import java.util.HashSet;
 
public class ShortSet {
  public static void main(String[] args) {
    	HashSet<Short> s = new HashSet<Short>();
    	for (shortint i = 0; i < 10010; i++) {
    	  s.add((short)i);
     // Cast required so that the code compiles
	  s.remove(i - 1);  // triesTries 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]. This noncompliant code example also violates EXP00-J. Do not ignore values returned by methods because the remove() method returns a Boolean value indicating its success.

Compliant Solution

Objects removed from a collection must share the type of the collection 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.

import java.util.HashSet;
 
public class ShortSet {
  public static void main(String[] args) {
    HashSet<Short> s = new HashSet<Short>();
    	for (shortint i = 0; i < 10010; i++) {
	      s.add((short)i);
	  // Remove a Short
	  if (s.remove((short)(i) -== 1)false); {
	 // removes a Short
    }
     System.err.println("Error removing " + i);
	  }
	}
	System.out.println(s.size());
  }
}

Exceptions

EXP04-J-EX1: The collections framework equals() method also takes an argument of type Object, but it is acceptable to pass an object of a different type from that of the underlying collection/map to the equals() method. Doing so cannot cause any confusion because the contract of the equals() method stipulates that objects of different classes will never be equivalent (see MET08-J. Preserve the equality contract when overriding the equals() method for more information).

EXP04-J-EX2: Some Java programs, particularly legacy programs, may iterate through a collection of variously typed objects with the expectation that only those objects with the same type as the collection parameter will be operated on. An exception is allowed when there is no expectation that the operation is not a no-op.

Risk Assessment

Allowing autoboxing to produce objects of an unintended type Passing arguments to certain Java Collection Framework methods that are of a different type from that of the class instance can cause silent failures with some APIs, such as the Collections library. These failures can result , resulting in unintended object retention, memory leaks, or incorrect program operationoperation [Techtalk 2007].

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

[Core Java 2004]	Chapter 5, "Inheritance"
[

JLS 2005

JCF 2014]	The Java Collections Framework
[JLS 2015]	§5.1.7, "Boxing Conversions"
[Seacord 2015]	Image Added IDS17-J. Prevent XML External Entity Attacks LiveLesson
[Techtalk 2007]	"The Joy of Sets"

...

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