Java generic classes and non-generic class

Question

I have a little concern about generics type in non-generic classes. I'm working on an API where i have a Data Model, which has a singleton DataModelManager. This DataModelManager creates DataFactory ? extends DMAbstractObject where DMAbstractObject is an abstract class telling "i'm a DataModel class".

So basically, when i want to create an object, i use this code :

DMLine line = DataModelManager.getInstance().getDataFactory(DMLine.class).newElement();

The problem is that i'm using the function

public DataFactory<? extends DMAbstractObject> getDataFactory(Class<? extends DMAbstractObject> clazz)

So Java can't know my returned DataFactory is from the same type that my parameter clazz. Because "? extends DMAbstractObject" tells "any object extending DMAbstractObject". In a generic class, i could use : "T extends DMAbstractObject" but not here.

My DataModelManager : public class DataModelManager {

     private static DataModelManager INSTANCE;

     public static DataModelManager getInstance(){
        if(INSTANCE == null)
            INSTANCE = new DataModelManager();
        return INSTANCE;
     }  


     private HashMap<Class<? extends DMAbstractObject>, DataFactory<? extends DMAbstractObject>> dataListMap;

      /**
      * Constructor
     */
     private DataModelManager(){
        dataListMap = new HashMap<Class<? extends DMAbstractObject>, DataFactory<? extends DMAbstractObject>>();

     }

     public DataFactory<? extends DMAbstractObject> getDataFactory(Class<? extends DMAbstractObject> clazz){

         if(!dataListMap.containsKey(clazz)){
            DataFactory<? extends DMAbstractObject> dataList = new DataFactory<>(clazz);
            dataListMap.put(clazz, dataList);
            return dataList;
         }

         return dataListMap.get(clazz);


     }
 }

It's not a major issue because it's only cause a Type safety Warning (Unchecked cast from DataFactory capture#2-of ? extends DMAbstractObject to DataFactory DMLine), but i would like to know if there was a proper way to do this.

Thanks for your answers !

John B · Accepted Answer · 2014-03-31 11:40:16Z

3

public <T extends DMAbstractObject> DataFactory<T> getDataFactory(Class<T> clazz)

use a generic to lock down the type. Generics can be defined at the method level as well as at the class level.

answered Mar 31, 2014 at 11:40

John B

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3 Comments

user3419255 Over a year ago

Oh right, i tried some stuffs with T extends... but didn't find the good syntax to make it works and it seems to be fine with your answer ! Now i don't have to cast each time where i'm using my getDataFactory(). Thanks !

Radiodef Over a year ago

This will still require an unchecked cast at return (DataFactory<T>)dataListMap.get(clazz); I believe but it is usually safe to do for this type of thing. There is not really a better way to do it.

John B Over a year ago

@Radiodef good point. Given the above code it is safe because of the conditions surrounding the dataListMap

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