HashMap key String value ArrayList returns an Object instead of ArrayList

Question

I m storing data in a HashMap with  key  String  value  ArrayList   The part I m having trouble with declares a new ArrayList  current   searches the HashMap for the String  dictCode   and if found sets current as the returned value ArrayList   ArrayList current   new ArrayList          if dictMap containsKey dictCode         current   dictMap get dictCode          The  current       line returns a compiler error of   Error  incompatible types found     java lang Object required  java util ArrayList   I don t understand this    does that HashMap return an Object instead of the ArrayList I stored in it as the value  How do I convert this object into an ArrayList   Thank you

User · Answer

The get method of the HashMap is returning an Object  but the variable current is expected to take a ArrayList   ArrayList current   new ArrayList           current   dictMap get dictCode     For the above code to work  the Object must be cast to an ArrayList   ArrayList current   new ArrayList           current    ArrayList dictMap get dictCode     However  probably the better way would be to use generic collection objects in the first place   HashMap lt String  ArrayList lt Object gt  gt  dictMap       new HashMap lt String  ArrayList lt Object gt  gt         Populate the HashMap   ArrayList lt Object gt  current   new ArrayList lt Object gt           if dictMap containsKey dictCode         current   dictMap get dictCode          The above code is assuming that the ArrayList has a list of Objects  and that should be changed as necessary   For more information on generics  The Java Tutorials has a lesson on generics

User · Answer

I suppose your dictMap is of type HashMap  which makes it default to HashMap lt Object  Object gt   If you want it to be more specific  declare it as HashMap lt String  ArrayList gt   or even better  as HashMap lt String  ArrayList lt T gt  gt

User · Answer

Using generics  as in the above answers  is your best bet here  I ve just double checked and   test put  test   arraylistone    ArrayList current   new ArrayList    current    ArrayList  test get  test      will work as well  through I wouldn t recommend it as the generics ensure that only the correct data is added  rather than trying to do the handling at retrieval time

User · Answer

How is the HashMap declaration expressed in that scope  It should be   HashMap lt String  ArrayList gt  dictMap   If not  it is assumed to be Objects   For instance  if your code is   HashMap dictMap   new HashMap lt String  ArrayList gt         ArrayList current   dictMap get dictCode     that will not work  Instead you want   HashMap lt String  ArrayList gt  dictMap   new HashMap lt String  Arraylist gt         ArrayList current   dictMap get dictCode     The way generics work is that the type information is available to the compiler  but is not available at runtime  This is called type erasure  The implementation of HashMap  or any other generics implementation  is dealing with Object  The type information is there for type safety checks during compile time  See the Generics documentation   Also note that ArrayList is also implemented as a generic class  and thus you might want to specify a type there as well  Assuming your ArrayList contains your class MyClass  the line above might be   HashMap lt String  ArrayList lt MyClass gt  gt  dictMap

User · Answer

public static void main String arg          HashMap lt String  ArrayList lt String gt  gt  hashmap            new HashMap lt String  ArrayList lt String gt  gt         ArrayList lt String gt  arraylist   new ArrayList lt String gt         arraylist add  Hello        arraylist add  World         hashmap put  my key   arraylist       arraylist   hashmap get  not inserted        System out println arraylist       arraylist   hashmap get  my key        System out println arraylist      null  Hello  World     Works fine    maybe you find your mistake in my code

[java] HashMap(key: String, value: ArrayList) returns an Object instead of ArrayList?

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