Object Oriented Question

Lets say I have some interfaces which make up a framework:

public interface Gum {
public String getFlavor();
}

public interface Package {
public Gum getGum();
}

public interface

There are no setter methods because the values may be set/retrieved different ways.

And I have a particular implementation

public class GumImpl implements Gum{
private String flavor;
public String getFlavor() { return this.flavor; }
public void setFlavor(String flavor) { this.flavor = flavor; }
}


public class PackageImpl implements Package{
private GumImpl gum;
public GumImpl getGum(){return this.gum;}
public void setGum(GumImpl gum){ this.gum = gum; }
}


First of all, notice that I am returning GumImpl from my PackageImpl.getGum and the interface defines that Package.getGum() should return a Gum type. To me, this seems appropriate because that classes that interact with the implementation shouldn't have to deal/cast to the from the interface to the implementation.

Does anyone have an opinion if changing the method signature to return a subclass like this is appropriate?

I am having problems using various tools with this approach such as Spring and Netbeans. Spring will complain that the PackageImpl is read-only.