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The interfaces of the Java Collections Framework [JCF 2014] use generically typed, parameterized methods such methods, such as add(E e) or and put(K key, V value), to insert objects into the collection or map, but they have other methods, such as contains(), remove(), or and get(), that accept an argument of type Object rather than a parameterized type, Theoretically this enables a programmer to attempt to remove . Consequently, these methods accept an object of any type.   The Collections Framework Interfaces 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 the Map<K,V> get() method or the , Collection<E>  contains(), or and remove() methods have the same type as the parameterized type of the corresponding class instance.

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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.  HoweverHowever, 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;
 
public class ShortSet {
  public static void main(String[] args) {
	HashSet<Short> s = new HashSet<Short>();
	for (int i = 0; i < 10; i++) {
	  s.add((short)i); // castCast required so that the code compiles
	  s.remove(i); // triesTries 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 Boolean value indicating its success.

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import java.util.HashSet;
 
public class ShortSet {
  public static void main(String[] args) {
    HashSet<Short> s = new HashSet<Short>();
	for (int i = 0; i < 10; i++) {
	  s.add((short)i);
	  // removeRemove a Short
	  if (s.remove((short)i) == false) {
	    System.err.println("Error removing " + i);
	  }
	}
	System.out.println(s.size());
  }
}

Exceptions

EXP04-EX0J-EX1: The Collections Framework 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. This Doing so cannot cause any confusion because the contract of the equals() method stipulates that objects of different classes will never be equivalent . See (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].

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
[

Core Java 2004]

Chapter 5

[JLS 2005

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