I'm trying to follow the advice whereby "using getters/setters in your classes is evil". So, say I have the aggregate class `Invoice` with the following signature:

```
public class Invoice{
    ISet<Line> _lines;
    public void ChangeLineAmount(int LineId, double newAmount){
               //Your answer here
           }
    }

```

How I can find the specific line for the change, if the `Line` class has no getters?