How to initialize an interface defined in an Activity which is implemented by 3 fragments setup with ViewPager via TabLayout?

Question

I have an activity which contains TabLayout setup with ViewPager with 3 fragments These 3 fragments contain some methods unique to each and they need to be called on the click of a button which is a Menu item in a menu to be inflated for the parent activity of these 3 fragments.

Now i am unable to initialize my interface defined in the Activity named OnMenuSaveButonClickListener because im getting an error:

Caused by: java.lang.ClassCastException: com.blah.blah.EditProductInfoActivity cannot be cast to com.blah.blah.EditProductInfoActivity$OnMenuSaveButonClickListener
   at com.blah.blah.EditProductInfoActivity.onCreate(EditProductInfoActivity.java:53)

Code for Parent Activity with ViewPager(with fragments in TabLayout):

public class EditProductInfoActivity extends AppCompatActivity {

    TabLayout tabLayout;
    ViewPager mViewPager;
    OnMenuSaveButonClickListener mCallback;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_edit_product_info);

        tabLayout = (TabLayout) findViewById(R.id.tabs);
        mViewPager = (ViewPager) findViewById(R.id.v_pager_prod);

        mCallback = (OnMenuSaveButonClickListener) this;  //line:53 with error
        initViewPager();

    private void initViewPager() {
        ArrayList<Fragment> fragments = new ArrayList<>();
        fragments.add(new CatalogueItemInfoFragment());
        fragments.add(new CatalogueItemInventoryFragment());
        fragments.add(new CatalogueItemNotesFragment());

        ProductTabsViewPagerAdapter productTabs = new ProductTabsViewPagerAdapter(
                getSupportFragmentManager(),this, fragments);
        mViewPager.setAdapter(productTabs);

        tabLayout.setupWithViewPager(mViewPager);
        tabLayout.getTabAt(0).setText("Product Info");
        tabLayout.getTabAt(1).setText("Inventory");
        tabLayout.getTabAt(2).setText("Notes");

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.item_info_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        switch (item.getItemId()){
            case R.id.item_check:
                mCallback.onMenuButonClick();
                return true;

            default: return super.onOptionsItemSelected(item);
        }

    }

    public interface OnMenuSaveButonClickListener{
        void onMenuButonClick();
    }

}

Code for ViewPager Adpater Class:

public class ProductTabsViewPagerAdapter extends FragmentPagerAdapter {

    private Context context;
    private List<Fragment> fragments;

    public ProductTabsViewPagerAdapter(FragmentManager fm, Context context, List<Fragment> fragments) {

        super(fm);
        this.context = context;
        this.fragments = fragments;

    }

    @Override
    public Fragment getItem(int position) {
        return fragments.get(position);
    }

    @Override
    public int getCount() {
        return fragments.size();
    }
}

Code for Fragment 1:

public class CatalogueItemInfoFragment extends Fragment implements EditProductInfoActivity.OnMenuSaveButonClickListener {
    ...
    ...

    public void setItemFromItemForm(){

        productViewModel.setSize(productSizeText.getText().toString());
        productViewModel.setSizeSelection(productSizeSelectionText.getText().toString());
        productViewModel.setColor(productColorText.getText().toString());

    }

    @Override
    public void onMenuButonClick() {
        setItemFromItemForm();
    }

}

Code for Fragment 2:

public class CatalogueItemInventoryFragment extends Fragment implements EditProductInfoActivity.OnMenuSaveButonClickListener {

...
...

    private void setItemInventoryInfo() { 
         productViewModel.setAvailableQuantity(Integer.parseInt(availableQtyText.getText().toString()));
         productViewModel.setIsOutOfStock(outOfStockSwitch.isChecked());
         productViewModel.setIsShowOutOfStock(showOutOfStockSwitch.isChecked());
         productViewModel.setIsForceAllowOrder(forceAllowOrderSwitch.isChecked());
    }

    @Override
    public void onMenuButonClick() {

        setItemInventoryInfo();

    }

}

Code for Fragment 3:

public class CatalogueItemNotesFragment extends Fragment implements EditProductInfoActivity.OnMenuSaveButonClickListener {
    ...
    ...

    private void setItemMoreDetails() {

        mStaticProduct.setNotes(productNotesText.getText().toString());
        productViewModel.setNotes(productNotesText.getText().toString());

    }

    @Override
    public void onMenuButonClick() {
        setItemMoreDetails();
    }
}

Answer 1

1. get the selected tab position from TabLayout so you could target the current fragment visible

   int selectedTab = tabLayout.getSelectedTabPosition();

2. call the OnMenuSaveButonClickListener implementation method from that fragment

   ((OnMenuSaveButonClickListener) fragments.get(selectedTab)).onMenuButonClick();

this code goes in you activity in the onOptionsItemSelected you can put that logic in a function to avoid repeating the same lines for other menuitem option.

Make sure you remove mCallback and it's assignment from your code.

I suggest you also remove the OnMenuSaveButonClickListener interface definition from your activity and have it in it's own .java (OnMenuSaveButonClickListener.java) > loose coupling.

Answer 2

fragments are implementing the interface, so call method like this:

((OnMenuSaveButonClickListener)fragments.get(i)).onMenuButonClick();

and remove line 53.

Answer 3

So you have the list of fragments in the adapter, add one getter method to get the list of fragments

public List<Fragments> getFragments(){
    return fragments;
}

and on click do like this,

final List<Fragment> fragments = productTabs.getFragments();
    for (Fragment fragment : fragments) {
        if (fragment instanceof OnMenuSaveButonClickListener) {
            ((OnMenuSaveButonClickListener) fragment).onMenuButonClick();
        }
    }