MainActivity.java

public class MainActivity extends AppCompatActivity implements Callbacks{

    private CoordinatorLayout coordinatorLayout;
    private BottomSheetBehavior behavior;
    private View persistentbottomSheet;
    private TextView cart;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        coordinatorLayout = (CoordinatorLayout)findViewById(R.id.coordinator);
        persistentbottomSheet = coordinatorLayout.findViewById(R.id.bottomsheet);
        bottomSheetRecyclerview = coordinatorLayout.findViewById(R.id.recyclerview_bottom_sheet);
        BottomSheetBehavior behavior = BottomSheetBehavior.from(persistentbottomSheet);
    }
        public void ExpandBottomSheet(View view){
        if (behavior.getState() == BottomSheetBehavior.STATE_COLLAPSED) {
            behavior.setState(BottomSheetBehavior.STATE_EXPANDED);
        } else {
            behavior.setState(BottomSheetBehavior.STATE_COLLAPSED);
        }
    }
}