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teaching:labs:mdse:2013:bug_descriptions:jt-bug-cor3 [2013/11/02 09:27]
127.0.0.1 external edit
teaching:labs:mdse:2013:bug_descriptions:jt-bug-cor3 [2018/05/09 01:59] (current)
Line 1: Line 1:
 +====== Bug Cor 3 ======
  
 +===== Inspired by =====
 +
 +  * [[http://​findbugs.sourceforge.net/​bugDescriptions.html#​BC_IMPOSSIBLE_DOWNCAST_OF_TOARRAY | BC: Impossible downcast of toArray() result (BC_IMPOSSIBLE_DOWNCAST_OF_TOARRAY)]]
 +
 +===== Original Description =====
 +
 +"This code is casting the result of calling toArray() on a collection to a type more specific than Object[], as in:
 +
 +<code Java>
 +String[] getAsArray(Collection<​String>​ c) {
 + return (String[]) c.toArray();​
 +}
 +</​Code>​
 +
 +This will usually fail by throwing a ClassCastException. The toArray() of almost all collections return an Object[]. They can't really do anything else, since the Collection object has no reference to the declared generic type of the collection.
 +
 +The correct way to do get an array of a specific type from a collection is to use c.toArray(new String[]); or c.toArray(new String[c.size()]);​ (the latter is slightly more efficient).
 +
 +There is one common/​known exception exception to this. The toArray() method of lists returned by Arrays.asList(...) will return a covariantly typed array. For example, Arrays.asArray(new String[] { "​a"​ }).toArray() will return a String []. FindBugs attempts to detect and suppress such cases, but may miss some." -- 2013-10-13 ​
 +
 +===== Detailed Description =====
 +
 +The [[http://​docs.oracle.com/​javase/​6/​docs/​api/​java/​util/​List.html|List class]] of the Java util package contains the toArray() method, which returns an array containing all the elements of the list. However, the returned array is of type Object[] (since the List collection is capable of storing objects of any class) and upon any attempt to cast this value to an array of a more specific type, a ClassCastException will be thrown.
 +
 +The former limitation is quite inconvenient,​ as prior to operating on any of the elements of the array it would have to be cast independently. That could have a negative impact on performance,​ as an unboxing operation would be executed for each element of the array. Moreover, it is very likely to cause errors that may go undetected during development to arise upon execution.
 +
 +Therefore, the parameterless version of the toArray method should be avoided in favor of its parameterized overload: ​ [[http://​docs.oracle.com/​javase/​6/​docs/​api/​java/​util/​List.html|toArray(T[] a)]]. This version returns an array of the same type as that of the provided parameter.
 +
 +===== Sample Problem Scenario =====
 +
 +<code Java>
 +public class DowncastArray {
 +
 + public static void main(String[] args) {
 +
 + List<​Integer>​ lst = new ArrayList<​Integer>​();​
 + Integer[] arr = (Integer[])lst.toArray();​
 + }
 +}
 +</​Code>​
 +
 +===== Sample False-Positive Scenario =====
 +
 +Be aware that Lists created by means of the [[http://​docs.oracle.com/​javase/​6/​docs/​api/​java/​util/​Arrays.html|Arrays]] utility class are able to produce arrays of the specific type they contain. Therefore, the following code works perfectly and would correspond to a false positive if reported as error:
 +
 +<code Java>
 +public class ValidDowncastArray {
 +
 + public static void main(String[] args) {
 +
 + List<​Integer>​ lst = Arrays.asList(new Integer[] {3, 4, 5, 5});
 + Integer[] arr = (Integer[])lst.toArray();​
 + }
 +
 +}
 +</​Code>​
 +
 +===== Sample Fix =====
 +
 +<code Java>
 +public class DowncastArray {
 +
 + public static void main(String[] args) {
 +
 + List<​Integer>​ lst = new ArrayList<​Integer>​();​
 + Integer[] arr = (Integer[])lst.toArray(new Integer[3]);​
 + }
 +}
 +</​Code>​
 +
 +===== Evaluation Results =====
 +
 +^  Benchmark project ​ ^  Precision ​             ^^^  Recall ​               ^^^
 +|                     ​| ​ FB    |  JT    |  Delta  |  FB    |  JT    |  Delta |
 +|  Project ...        |  ...%  |  ...%  |  ...%   ​| ​ ...%  |  ...%  |  ...%  |  ​
 +|  Project ...        |  ...%  |  ...%  |  ...%   ​| ​ ...%  |  ...%  |  ...%  |   
 +FB = FindBugs, JT = JTransformer,​ Delta = JTransformer - FindBugs
 + 
teaching/labs/mdse/2013/bug_descriptions/jt-bug-cor3.txt · Last modified: 2018/05/09 01:59 (external edit)

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