How to cast List Object to List MyClass

Question

This does not compile  any suggestion appreciated          List lt Object gt  list   getList      return  List lt Customer gt   list    Compiler says  cannot cast List lt Object gt  to List lt Customer gt

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List lt Object   gt  testNovedads   crudService createNativeQuery               quot SELECT ID NOVEDAD PK  OBSERVACIONES  ID SOLICITUD PAGO FK FROM DBSEGUIMIENTO SC NOVEDADES WHERE ID NOVEDAD PK  lt  2000 quot     Convertir lt TestNovedad gt  convertir   new Convertir lt TestNovedad gt     Collection lt TestNovedad gt  novedads   convertir toList testNovedads  TestNovedad class   for  TestNovedad testNovedad   novedads        System out println testNovedad toString        public Collection lt T gt  toList List lt Object   gt  objects  Class lt T gt  type        Gson gson   new Gson        JSONObject jsonObject   new JSONObject        Collection lt T gt  collection   new ArrayList lt  gt         Field   fields   TestNovedad class getDeclaredFields        for  Object   object   objects            int pos   0          for  Field field   fields                jsonObject put field getName    object pos                        collection add gson fromJson jsonObject toString    type              return collection

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Another approach would be using a java 8 stream       List lt Customer gt  customer   myObjects stream                                      filter Customer class  isInstance                                     map Customer class  cast                                     collect toList

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Depending on your other code the best answer may vary  Try   List lt   extends Object gt  list   getList    return  List lt Customer gt   list    or   List list   getList    return  List lt Customer gt   list    But have in mind it is not recommended to do such unchecked casts

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Note that I am no java programmer  but in  NET and C   this feature is called contravariance or covariance  I haven t delved into those things yet  since they are new in  NET 4 0  which I m not using since it s only beta  so I don t know which of the two terms describe your problem  but let me describe the technical issue with this   Let s assume you were allowed to cast  Note  I say cast  since that s what you said  but there are two operations that could be possible  casting and converting   Converting would mean that you get a new list object  but you say casting  which means you want to temporarily treat one object as another type   Here s the problem with that   What would happen if the following was allowed  note  I m assuming that before the cast  the list of objects actually only contain Customer objects  otherwise the cast wouldn t work even in this hypothetical version of java    List lt Object gt  list   getList    List lt Customer gt  customers    List lt Customer gt  list  list Insert 0  new someOtherObjectNotACustomer     Customer c   customers 0     In this case  this would attempt to treat an object  that isn t a customer  as a customer  and you would get a runtime error at one point  either form inside the list  or from the assignment   Generics  however  is supposed to give you type-safe data types  like collections  and since they like to throw the word  guaranteed  around  this sort of cast  with the problems that follow  is not allowed   In  NET 4 0  I know  your question was about java   this will be allowed in some very specific cases  where the compiler can guarantee that the operations you do are safe  but in the general sense  this type of cast will not be allowed  The same holds for java  although I m unsure about any plans to introduce co- and contravariance to the java language   Hopefully  someone with better java knowledge than me can tell you the specifics for the java future or implementation

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As others have pointed out  you cannot savely cast them  since a List lt Object gt  isn t a List lt Customer gt   What you could do  is to define a view on the list that does in-place type checking  Using Google Collections that would be   return Lists transform list  new Function lt Object  Customer gt        public Customer apply Object from        if  from instanceof Customer          return  Customer from            return null     or throw an exception  or do something else that makes sense

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You can do something like this  List lt Customer gt  cusList   new ArrayList lt Customer gt      for Object o  list               cusList add  Customer o              return cusList     Or the Java 8 way  list stream   forEach x- gt cusList add  Customer x    return cuslist

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With Java 8 Streams   Sometimes brute force casting is fine   List lt MyClass gt  mythings    List lt MyClass gt    Object  objects   But here s a more versatile solution   List lt Object gt  objects   Arrays asList  String1    String2     List lt String gt  strings   objects stream                           map element- gt  String  element                          collect Collectors toList       There s a ton of benefits  but one is that you can cast your list more elegantly if you can t be sure what it contains   objects stream        filter element- gt element instanceof String       map element- gt  String element       collect Collectors toList

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You can use a double cast   return  List lt Customer gt    List  getList

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Depending on what you want to do with the list  you may not even need to cast it to a List lt Customer gt    If you only want to add Customer objects to the list  you could declare it as follows       List lt Object gt  list   getList    return  List lt   super Customer gt   list    This is legal  well  not just legal  but correct - the list is of  some supertype to Customer    and if you re going to be passing it into a method that will merely be adding objects to the list then the above generic bounds are sufficient for this   On the other hand  if you want to retrieve objects from the list and have them strongly typed as Customers - then you re out of luck  and rightly so   Because the list is a List lt Object gt  there s no guarantee that the contents are customers  so you ll have to provide your own casting on retrieval    Or be really  absolutely  doubly sure that the list will only contain Customers and use a double-cast from one of the other answers  but do realise that you re completely circumventing the compile-time type-safety you get from generics in this case    Broadly speaking it s always good to consider the broadest possible generic bounds that would be acceptable when writing a method  doubly so if it s going to be used as a library method   If you re only going to read from a list  use List lt   extends T gt  instead of List lt T gt   for example - this gives your callers much more scope in the arguments they can pass in and means they are less likely to run into avoidable issues similar to the one you re having here

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You can t because List lt Object gt  and List lt Customer gt  are not in the same inheritance tree     You could add a new constructor to your List lt Customer gt  class that takes a List lt Object gt  and then iterate through the list casting each Object to a Customer and adding it to your collection   Be aware that an invalid cast exception can occur if the caller s List lt Object gt  contains something that isn t a Customer   The point of generic lists is to constrain them to certain types   You re trying to take a list that can have anything in it  Orders  Products  etc   and squeeze it into a list that can only take Customers

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That s because although a Customer is an Object  a List of Customers is not a List of Objects  If it was  then you could put any object in a list of Customers

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You can create a new List and add the elements to it    For example    List lt A gt  a   getListOfA    List lt Object gt  newList   new ArrayList lt  gt     newList addAll a

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You should just iterate over the list and cast all Objects one by one

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Similar with Bozho above  You can do some workaround here  although i myself don t like it  through this method    public  lt T gt  List lt T gt  convert List list  T t       return list      Yes  It will cast your list into your demanded generic type   In the given case above  you can do some code like this        List lt Object gt  list   getList        return convert list  new Customer

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you can always cast any object to any type by up-casting it to Object first  in your case     List lt Customer gt   Object list     you must be sure that at runtime the list contains nothing but Customer objects   Critics say that such casting indicates something wrong with your code  you should be able to tweak your type declarations to avoid it  But Java generics is too complicated  and it is not perfect  Sometimes you just don t know if there is a pretty solution to satisfy the compiler  even though you know very well the runtime types and you know what you are trying to do is safe  In that case  just do the crude casting as needed  so you can leave work for home

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Your best bet is to create a new List lt Customer gt   iterate through the List lt Object gt   add each item to the new list  and return that

[java] How to cast List<Object> to List<MyClass>

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