Avoid inheritance for implementation
The core idea here is not to create a subclass merely to access the functionality of the superclass. An example:
class Grapher extends DataList
Now the Graphing class can use all the functionality of the DataList class. But intuitively, the declaration looks wrong, mainly because a Grapher isn't a type of DataList. A graph should have have a DataList, but it isn't strictly a list of data itself.
This will cause problems since Grapher exposes its internal data list to outside classes (bad for encapsulation), we can't extend the class easily if we want a graph that draws two lists of data, rather than a single one, and also, we are creating unnecessary class hierarchy.
Instead, the Grapher should contain a DataList object internally. This avoids the above issues, as well as being more semantically correct (a Graph has a DataList)