ExpandableListView elv = (ExpandableListView)findViewById(R.id.expandableListView_categories);

        ArrayList<Category> categories3 = new ArrayList<Category>();
        categories3.add(new Category("__cat1"));
        categories3.add(new Category("__cat2"));
        categories3.add(new Category("__cat3"));
        categories3.add(new Category("__cat4"));

        ArrayList<Category> categories2 = new ArrayList<Category>();
        categories2.add(new Category("_cat1"));
        categories2.add(new Category("_cat2"));
        categories2.add(new Category("_cat3", categories3));
        categories2.add(new Category("_cat4"));

        ArrayList<Category> categories = new ArrayList<Category>();
        categories.add(new Category("cat1"));
        categories.add(new Category("cat2", categories2));
        categories.add(new Category("cat3"));
        categories.add(new Category("cat4"));

        CatAdapter adapter = new CatAdapter(this, categories);
        elv.setAdapter(adapter);