package com.ggj.game;

import java.util.ArrayList;
import java.util.List;

import com.badlogic.gdx.scenes.scene2d.Actor;
import com.badlogic.gdx.utils.Array;
import com.ggj.model.Player;
import com.ggj.model.Rock;

public class ObjectController {
  public static synchronized int register(Actor actor) {
    objects.add(actor);
    
    return ++objectId;
  }
  
  public static <T> T getObject(Class<T> callee)
  {
    for(Actor actor : objects)
    {
      if(callee.equals(actor.getClass()))
      {
        return (T)actor;
      }
    }
    
    return null;
  }
  
  public static <T> List<T> getList(Class<T> callee)
  {
    List<T> result = new ArrayList<T>();
    for(Actor actor : objects)
    {
      if(callee.equals(actor.getClass()))
      {
        result.add((T)actor);
      }
    }
    
    return result;
  }
  
  private static int objectId = 0;
  private static Array<Actor> objects = new Array<Actor>();
}

package com.ggj.game;

import com.badlogic.gdx.scenes.scene2d.Actor;
import com.badlogic.gdx.utils.Array;
import com.ggj.model.Player;
import com.ggj.model.Rock;

public class ObjectController {
  public static synchronized int register(Actor actor) {
    objects.add(actor);
    
    return ++objectId;
  }
  
  public static <T> T getObject(Class<T> callee)
  {
    for(Actor actor : objects)
    {
      if(callee.equals(actor.getClass()))
      {
        return (T)actor;
      }
    }
    
    return null;
  }
  
  private static int objectId = 0;
  private static Array<Actor> objects = new Array<Actor>();
}

--- 
+++ 
@@ -1,4 +1,7 @@
 package com.ggj.game;
+
+import java.util.ArrayList;
+import java.util.List;
 
 import com.badlogic.gdx.scenes.scene2d.Actor;
 import com.badlogic.gdx.utils.Array;
@@ -25,6 +28,20 @@
     return null;
   }
   
+  public static <T> List<T> getList(Class<T> callee)
+  {
+    List<T> result = new ArrayList<T>();
+    for(Actor actor : objects)
+    {
+      if(callee.equals(actor.getClass()))
+      {
+        result.add((T)actor);
+      }
+    }
+    
+    return result;
+  }
+  
   private static int objectId = 0;
   private static Array<Actor> objects = new Array<Actor>();
 }
Add generic way to get a list of objects by class
