How to create a base class that works with two different objects?

Question

I have two classes, PersonAdapter and AnimalAdapter .

Both classes are almost identical so I'm trying to reduce as much boiler plate code as possible.

PersonAdapter extends from RecyclerView.Adapter .

AnimalAdapter extends from RecyclerView.Adapter . Both classes implements the Filterable interface.

PersonAdapter defines an inner class PersonFilter extending from Filter .

AnimalAdapter defines an inner class AnimalFilter extending from Filter .

I would like to replace PersonFilter and AnimalFilter with some kind of generic class, but I don't know how since these are the differences between both classes:

PersonFilter 
    - works with a List<Person>
    - uses Person#getName() to make a string comparison

AnimalFilter 
    - works with a List<Animal>
    - uses Animal#getType() to make a string comparison

Please take a quick look at both classes:

PersonAdapter

public class PersonAdapter extends RecyclerView.Adapter<PersonViewHolder> implements Filterable {
    private List<Person> persons;
    private PersonFilter personFilter;

    public PersonAdapter(List<Person> persons) {
        this.persons = persons;
    }

    ...

    @Override
    public Filter getFilter() {
        if(personFilter == null)
            personFilter = new PersonFilter(this, persons);
        return personFilter;
    }


    public static class PersonFilter extends Filter {
        private final PersonAdapter adapter;

        private final List<Person> originalList;

        private final List<Person> filteredList;

        private PersonFilter(PersonAdapter adapter, List<Person> originalList) {
            super();
            this.adapter = adapter;
            this.originalList = new LinkedList<>(originalList);
            this.filteredList = new ArrayList<>();
        }

        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            filteredList.clear();
            final FilterResults results = new FilterResults();

            if (constraint.length() == 0) {
                filteredList.addAll(originalList);
            } else {
                final String filterPattern = constraint.toString().toLowerCase().trim();

                for (final Person person : originalList) {
                    if (person.getName().toLowerCase().contains(filterPattern.toLowerCase())) {
                        filteredList.add(person);
                    }
                }
            }
            results.values = filteredList;
            results.count = filteredList.size();
            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            adapter.persons.clear();
            adapter.persons.addAll((ArrayList<Person>) results.values);
            adapter.notifyDataSetChanged();
        }
    }
}

AnimalAdapter

public class AnimalAdapter extends RecyclerView.Adapter<AnimalViewHolder> implements Filterable {
    private List<Animal> animals;
    private AnimalFilter animalFilter;

    public AnimalAdapter(List<Animal> animals) {
        this.animals = animals;
    }

    ...


    public static class AnimalFilter extends Filter {
        private final AnimalAdapter adapter;

        private final List<Animal> originalList;

        private final List<Animal> filteredList;

        private AnimalFilter(AnimalAdapter adapter, List<Animal> originalList) {
            super();
            this.adapter = adapter;
            this.originalList = new LinkedList<>(originalList);
            this.filteredList = new ArrayList<>();
        }

        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            filteredList.clear();
            final FilterResults results = new FilterResults();

            if (constraint.length() == 0) {
                filteredList.addAll(originalList);
            } else {
                final String filterPattern = constraint.toString().toLowerCase().trim();

                for (final Animal animal : originalList) {
                    if (animal.getType().toLowerCase().contains(filterPattern.toLowerCase())) {
                        filteredList.add(animal);
                    }
                }
            }
            results.values = filteredList;
            results.count = filteredList.size();
            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            adapter.animals.clear();
            adapter.animals.addAll((ArrayList<Animal>) results.values);
            adapter.notifyDataSetChanged();
        }
    }
}

How should I create a generic subclass of Filter that works both with Animal and Person objects and can be used by PersonAdapter and AnimalAdapter?

Answer 1

Here is a filter that uses generics and a predicate to pull out the two filters from the question. The code could be reduced further by using a method similar to Guava Iterable.filter() .

MyAdapterInterface will need to be an interface or shared parent class for the adapters that shares all of the adapter methods that this filter needs to call.

    public class MyFilter<A extends MyAdapterInterface,T> extends Filter {
        private final A adapter;
        private final Predicate<T> comparator; // Java 8 class - can easily be created by hand if you are using pre Java 8
        // see https://docs.oracle.com/javase/8/docs/api/java/util/function/Predicate.html

        private final List<T> originalList;
        private final List<T> filteredList;

        private MyFilter(A adapter, List<T> originalList, Predicate<T> predicate) {
            super();
            this.adapter = adapter;
            this.comparator = comparator;
            this.originalList = new LinkedList<>(originalList);
            this.filteredList = new ArrayList<>();
        }

        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            filteredList.clear();
            final FilterResults results = new FilterResults();

            if (constraint.length() == 0) {
                filteredList.addAll(originalList);
            } else {
                final String filterPattern = constraint.toString().toLowerCase().trim();

                for (final T animal : originalList) {
                    if (predicate.test(animal)) {
                        filteredList.add(animal);
                    }
                }
            }
            results.values = filteredList;
            results.count = filteredList.size();
            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            adapter.clearResults();   // method to be added to MyAdapterInterface
            adapter.addResults(results.values);  // method to be added to MyAdapterInterface
            adapter.notifyDataSetChanged();
        }
    }