collections - Java excplicit cast of nested maps -

why cast work?

import java.util.hashmap; import java.util.map;  public class testmap {     public static void main(string[] args) {         map<string, map<string, map<string, map<string,integer>>>> resultmap = new hashmap<>();         map<string, object> amap = new hashmap<string, object>();         map<string, integer> hiddenmap = new hashmap<string, integer>();         hiddenmap.put("fortytwo", 42);         amap.put("key", hiddenmap);         resultmap =  (map<string, map<string, map<string, map<string, integer>>>>) amap.get("key");         system.out.println(resultmap);     } } 

also this:

map<string, map<string, map<string, map<string,map<string,integer>>>>> resultmap = new hashmap<>(); ... resultmap =  (map<string, map<string, map<string, map<string,map<string,integer>>>>>) amap.get("key"); 

and on...

how happen hidden map map<string, integer> gets cast map<string, map<string, map<string, map<string,integer>>>> resultmap?

always prints:

{fortytwo=42}

also works (map instead of map):

public static void main(string[] args) {          map<string, map<string, map<string, map<string,map<string,integer>>>>> resultmap = new hashmap<>();         map<string, map> amap = new hashmap<string, map>();         map<string, integer> hiddenmap = new hashmap<string, integer>();         hiddenmap.put("fortytwo", 42);         amap.put("key", hiddenmap);         resultmap =  (map<string, map<string, map<string, map<string,map<string,integer>>>>>) amap.get("key");         system.out.println(resultmap);      } 

edit: @shizhz says, because of type erasure of course! code above equivalent to:

map resultmap = new hashmap(); map amap = new hashmap(); map hiddenmap = new hashmap(); hiddenmap.put("fortytwo", 42); amap.put("key", hiddenmap); resultmap = (map) amap.get("key"); 

which works

because java generics used @ compile time provide tighter type checks, type parameter erased compiler according type erasure rules:

• replace type parameters in generic types bounds or object if type parameters unbounded. produced bytecode, therefore, contains ordinary classes, interfaces, , methods.
• insert type casts if necessary preserve type safety.
• generate bridge methods preserve polymorphism in extended generic types.

in code map<string, map> amap = new hashmap<string, map>();, value in amap raw type map, means compiler has no idea what's type contains, when try cast raw type of map generics type of map map<string, integer>, best compiler can giving warning. generic type erased @ compile time , type cast generated when value generic map, can runtime classcastexception exception if type mismatchs.

let's have @ following example:

public static void main(string[] args) {     map map = new hashmap();      map.put("hello", "world");     map.put(new integer(1), 1);     map.put(new object(), lists.newarraylist("hello"));      map<string, integer> m =  (map<string, integer>) map;     system.out.println(m);      integer = m.get("hello");// classcastexception happens @ here @ runtime } 

i'm trying convert map containing kinds of keys , values map<string, integer> there's no compile error, after type erasure, above code equivalent to:

public static void main(string[] args) {     map map = new hashmap();      map.put("hello", "world");     map.put(new integer(1), 1);     map.put(new object(), lists.newarraylist("hello"));      map m = (map) map;     system.out.println(m);      integer = (integer)m.get("hello"); } 

now can tell why last line caused classcastexception.