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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(), or 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.

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Code Block

bgColor	#FFCCCC

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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Code Block

bgColor	#CCCCFF

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-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 (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 in cases where when there is no expectation that the operation is not a no-op.

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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].

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
EXP04-J	Low	Probable	Low	P6	L2

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.

Tool

Version

Checker

Description

PVS-Studio

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