Question

I am creating an object cache. I am having trouble writing a method return type (please see getCachedCMSObject) that is generic, so that I don’t have to cast the return as indicated in the comment. I guess I can live with it, but I’d rather use Generics.

cachedCMSObject is a separate object using a “Heterogeneous Collection” pattern, but I don’t think that matters in this case, and does not relate to my problem.

package util;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class CMSObjectCache {
    static Map<String, CMSObject> cachedCMSObject = new ConcurrentHashMap<String, CMSObject>();

    public static void putCachedCMSObject(String cmsKey, CMSObject cmsObject) {
        cachedCMSObject.put(cmsKey, cmsObject);
    }

    public static Object getCachedCMSObject(String objectKey, Class<?> clazz) {
        CMSObject c2 = cachedCMSObject.get(objectKey);
        return c2.getCMSObject(Integer.class);  // return generic type ?
    }

    public static void main(String[] args) {
        CMSObject cmsObject;

        // put object of type Integer with key "Int:3"
        putCachedCMSObject("Int:3", new CMSObject(Integer.class, 3));

        // Get that object from the cache
        Integer i3 = (Integer) getCachedCMSObject("Int:3", Integer.class);  // necessary? 
        System.out.println(i3);
    }
}

This is what CMSObject looks like (from Bloch).

package util;

import java.util.HashMap;
import java.util.Map;

public final class CMSObject {
    private Map<Class<?>, Object> cmsObject = new HashMap<Class<?>, Object>();

    public CMSObject() {
    }

    public <T> CMSObject(Class<T> type, T instance) {
        this.putCMSObject(type, instance);
    }

    public <T> void putCMSObject(Class<T> type, T instance) {
        if (type == null) {
            throw new NullPointerException("Type is null");
        }
        cmsObject.put(type, instance);
    }

    public <T> T getCMSObject(Class<T> type) {
        return type.cast(cmsObject.get(type));
    }
}

Answer 1

It’s not yet clear in your question, but I can only assume you are heading to something like this:

class CMSObject{
   public <T> T getCMSObject(Class<T> klass) {
       //...
   } 
}

And then your outer method should be somewhat like

public static <T> T getCachedCMSObject(String objectKey, Class<T> clazz) {
    CMSObject c2 = cachedCMSObject.get(objectKey);
    return c2.getCMSObject(clazz);  // return generic type ?
}