java - different generic type-erasure behaviour for Hashmap and EnumMap -


i noticed surprising(for me) difference between hashmap , enummap in 'generics behaviour'.

what mean?

consider following 2 code snippets:

snippet 1

enum types { a, b, c }  enum wrong { }  public class test {      public static void main(string... args) {          enummap<types, integer> m = new enummap<types, integer>(types.class);          enummap m1 = m;          m1.put(wrong.a, 1);      }  }

out:

exception in thread "main" java.lang.classcastexception: class wrong != class types

snippet 2

enum types { a, b, c }  enum wrong { }   public class test {      public static void main(string... args) {          hashmap<types, integer> m = new hashmap<types, integer>();          hashmap m1 = m;          m1.put(wrong.a, 1);                 }  }

out:

successful compilation!

conclusion:

therefore hashmap erasure fullfils same code enummap - not. why?

your conclusion incorrect: erasure occurs in both cases, , in same way. difference enummap object keeps reference class object key type in field, , uses class object perform reflective cast. causing exception, can tell stacktrace.

in contrast, ordinary map uses unchecked cast, succeeds if object of incorrect type (giving rise heap pollution).


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