I have an object that stores some data in a list. The implementation could change later, and I don't want to expose the internal implementation to the end user. However, the user must have the ability to modify and access this collection of data. Currently I have something like this:

public List<SomeDataType> getData() {
   return this.data;
}

public void setData(List<SomeDataType> data) {
   this.data = data;
}

Does this mean that I have allowed the internal implementation details to leak out? Should I be doing this instead?

public Collection<SomeDataType> getData() {
   return this.data;
}

public void setData(Collection<SomeDataType> data) {
   this.data = new ArrayList<SomeDataType>(data);
}