How to instantiate an ArrayList<?> and add an item through reflection with Java?

Question

I am writing a deserialization method that converts xml to a Java object. I would like to do this dynamically and avoid writing hard coded references to specific types.

For example this is a simplified version of one of my classes.

public class MyObject { 
   public ArrayList<SubObject> SubObjects = new ArrayList<SubObject>();
}

Here is a stripped down version of the method:

public class Serializer {    
    public static <T> T fromXml(String xml, Class<T> c) {
       T obj = c.newInstance();       
       Field field = obj.getClass().getField("SubObjects");    
       //help : create instance of ArrayList<SubObject> and add an item
       //help#2 : field.set(obj, newArrayList);

       return obj;
    }
}

Calling this method would look like this:

MyObject obj = Serializer.fromXml("myxmldata", MyObject.class);

Forgive me if this is a trivial problem as I am a C# developer learning Java.

Thanks!

Answer 1

In java generics are just syntax sugar and they do not exist in runtime. This is why you cannot programmatically create generic objects in runtime. Try to create non generic list and cast (however I am not sure as my experience is limited with GWT).

Answer 2

You'll have to create a raw typed List (i.e. with no type parameters).

List subs = deserialize( ... );

If you were to set the raw typed List on your class directly:

obj.setSubObjects(subs); //You'll get an "unchecked cast" warning at compile time

Although the compiler will emit a warning, it is legal Java code and will run just fine at runtime because the parameter-type information is lost (i.e. at runtime, there is no difference between a List and a List<SomeClass>). You can of course set it reflectively also.

Answer 3

Should be something pretty close to:

Object list = field.getType().newInstance();

Method add = List.class.getDeclaredMethod("add",Object.class);

add.invoke(list, addToAddToList);

Hope this helps.

Answer 4

It seems like the real issue here is going from XML to java objects. If that is the case (and since you are new to Java), a better solution than rolling your own may be to investigate existing technologies such as:

• castor
• xstream
• jaxb