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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 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() method has no effect.

import java.util.HashSet;
 

A boxing conversion converts the value of a primitive type to the corresponding value of the reference type, for instance, from {{int}} to the type {{Integer}} \[[JLS 2005|AA. Bibliography#JLS 05]\]. It can be convenient in many cases where an object parameter is desired, such as with collection classes like {{Map}} and {{List}}. Another use case is to pass object references to methods, as opposed to primitive types that are always passed by value. The resulting wrapper types also help reduce clutter in code.

Noncompliant Code Example

This noncompliant code example prints {{100}} as the size of the {{HashSet}} while it is expected to print {{1}}. The combination of values of types {{short}} and {{int}} in the operation {{i-1}} leads to autoboxing of the result into an object of type {{Integer}}. (See guideline [EXP05-J. Be aware of integer promotions in binary operators].) The {{HashSet}} contains values of only one type {{Short}} whereas the code attempts to remove objects of the (different) type {{Integer}}. As a result, the remove operation is equivalent to a _No Operation_ (NOP). The compiler enforces type checking so that only {{Short}} values are inserted; however, a programmer is free to remove an object of any type without triggering any exceptions because {{Collections<E>.remove()}} accepts an argument of type {{Object}} and not {{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 (int i = 0; i for(short i=0< 10; i<100;i++) {
     	   s.add((short)i);
 // Cast required so that the code compiles
	  s.remove(i - 1);
 // Tries to remove an Integer
	}
    	System.out.println(s.size());
  }
}

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

Avoid mixing the different boxed integer types. If an arithmetic operation is expected to produce a primitive type which may get autoboxed to a wrong type, add explicit casts to the primitive type before allowing autoboxing to take overObjects removed from a collection must share the type of the collection elements. 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 (int i = 0; i for(short i=0< 10; i<100;i++) {
      	  s.add((short)i);
	  // Remove a Short
	  if (s.remove((short)(i-1)); //cast to short 
 == false) {
	    System.err.println("Error removing }
" + i);
	  }
	}
	System.out.println(s.size());
  }
}

Risk Assessment

Numeric promotion and autoboxing while removing elements from a Collection, can make operations on the Collection fail silently.

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

Passing arguments to certain Java Collection Framework methods that are of a different type from that of the class instance can cause silent failures, resulting in unintended object retention, memory leaks, or incorrect program operation [Techtalk 2007].

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

\[[Core Java 2004|AA. Bibliography#Core Java 04]\] Chapter 5 
\[[JLS 2005|AA. Bibliography#JLS 05]\] Section 5.1.7
\[[Techtalk 2007|AA. Bibliography#Techtalk 07]\] "The Joy of Sets"

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"
[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"

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Image Added Image Added Image AddedEXP10-J. Avoid side effects in assertions      04. Expressions (EXP)      EXP12-J. Ensure a null pointer is not dereferenced