Why do I want to avoid non-default constructors in fragments?

Question

It seems like none of the answers actually answer "why use bundle for passing parameters rather than non default constructors"

The reason why you should be passing parameters through bundle is because when the system restores a fragment (e.g on config change), it will automatically restore your bundle.

The callbacks like onCreate or onCreateView should read the parameters from the bundle - this way you are guaranteed to restore the state of the fragment correctly to the same state the fragment was initialised with (note this state can be different from the onSaveInstanceState bundle that is passed to the onCreate/onCreateView)

The recommendation of using the static newInstance() method is just a recommendation. You can use a non default constructor but make sure you populate the initialisation parameters in the bundle inside the body of that constructor. And read those parameters in the onCreate() or onCreateView() methods.

Make a bundle object and insert your data (in this example your Category object). Be careful, you can't pass this object directly into the bundle, unless it's serializable. I think it's better to build your object in the fragment, and put only an id or something else into bundle. This is the code to create and attach a bundle:

Bundle args = new Bundle();
args.putLong("key", value);
yourFragment.setArguments(args);

After that, in your fragment access data:

Type value = getArguments().getType("key");

That's all.

Your Fragment shouldn't have constructors because of how the FragmentManager instantiates it. You should have a newInstance() static method defined with the parameters you need, then bundle them and set them as the arguments of the fragment, which you can later access with the Bundle parameter.

For example:

public static MyFragment newInstance(int title, String message) {
    MyFragment fragment = new MyFragment();
    Bundle bundle = new Bundle(2);
    bundle.putInt(EXTRA_TITLE, title);
    bundle.putString(EXTRA_MESSAGE, message);
    fragment.setArguments(bundle);
    return fragment ;
}

And read these arguments at onCreate:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    title = getArguments().getInt(EXTRA_TITLE);
    message = getArguments().getString(EXTRA_MESSAGE);

    //...
}

This way, if detached and re-attached, the object state can be stored through the arguments, much like bundles attached to Intents.

If you use parameter for some class. try this

SomeClass mSomeInstance;
public static final MyFragment newInstance(SomeClass someInstance){
    MyFragment f = new MyFragment();
    f.mSomeInstance = someInstance;
    return f;
}

I think, there is no difference between static constructor and two constructors (empty and parametrized one that stores arguments into a Fragment's arguments bundle), most probably, this rule of thumb is created to reduce probability of forgetting to implement no-arg constructor in Java, which is not implicitly generated when overload present.

In my projects I use Kotlin, and implement fragments with a primary no-arg constructor and secondary constructor for arguments which just stores them into a bundle and sets it as Fragment arguments, everything works fine.