How to add element to array with Drools (mvel) - drools

Question

I need insert a new value in a exist array with Drools. My example:
rule "insert new address"
dialect "java"
when
$data : Data( source.address != null)
then
Address address = (Address) $data.source.address
System.out.println("Element: "+address );
$data.target.addressList.add(address);
end
The error that happend is this:
Exception executing consequence for rule "insert new address" in rules: [Error: $data.target.addressList.add(address): null]
EDIT: Added the model
public class Data {
private Source source;
private Client target;
}
public class Source {
...
private Address address;
}
public class Client {
...
private List<Address> addressList;
}

Answer 1

In answer to the question in your title, which is how to add an element to array -- the answer is basically "the same way you would in Java."
To answer the question you actually asked, which has no arrays, your error is effectively a NullPointerException, or another indicator that the field cannot be modified (eg an immutable list.)
This:
Error: $data.target.addressList.add(address): null]
Means that either $data.target or $data.target.addressList is null, or possibly $data.target.addressList is an immutable list.
Make sure that whatever "target" is has been initialized, and that its "addressList" is also initialized as a mutable list type.