java12-fundamentals-collections-corner-cases-workshop

• references
  • Collections Corner Cases by Stuart Marks

preface

• goals of this workshop: show some corner cases concerning collections
  • connection between array and Arrays.asList(array)
  • connection between map and its collections: entrySet, keySet and values
  • connection between comparator and equals in context of sets
• workshop: workshop package, answers: answers package

project description

• view collections don't contain their own elements - elements are stored elsewhere
• Arrays.asList(array) - proxy to array
  • e.g. if you sort Arrays.asList(array) the array is also sorted
• entrySet, keySet, values are mutable view collections connected to the map
  • note that you can remove element from it (and it affects the map) but you cannot add element (exception)
• comparator-equals contract
  • a comparator C on a set of elements S is said to be consistent with equals if and only if C.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S
  • it is surprisingly easy to create comparators that are inconsistent with equals
    • e.g. comparing strings ignoring case
  • many comparators for sorting are inconsistent with equals (salary)
  • there exists structures in jdk that are internally inconsistent with equals
    • IdentityHashMap (membership determined by ==)