How is it possible to display different fragment/view for each subclass? Android

I have a list of Message objects. Message is an abstract class, with two subclasses: TextMessage and ImageMessage

I would like to display the messages in a scrollable list, based on the type of the message. How is it possible to create a custom view/fragment with an abstract class as parameter, and create a TextView/ImageView inside it according to the actual subclass?

I've read the official android guide, but I still have no idea, how to do this.

Ok, So since a RecyclerView is the recommended view for a list of items with an undefined size, I'm gonna assume your want to use a RecyclerView for this.

If you then follow the sample code to make such a view, then you could make your Message class declare an abstract getViewType and bindToView method as follows:

Message.java

public abstract class Message {
    public abstract int getViewType();
    public abstract void bindToView(View messageView);

    public enum TYPE {
        TextMessage,
        ImageMessage
    }

    public static View createView(int typeOrdinal, Context context) {
        switch (TYPE.values()[typeOrdinal]) {
            case TextMessage:
                return TextMessage.createNewView(context);
            case ImageMessage:
                return ImageMessage.createNewView(context);
            default:
                throw new RuntimeException("Incorrect typeOrdinal: " + typeOrdinal);
        }
    }

}

TextMessage.java

public class TextMessage extends Message {

    public static TextView createNewView(Context context) {
        return new TextView(context);
    }

    @Override
    public void bindToView(View messageView) {
        ((TextView) messageView).setText("some specific text");
    }

    @Override
    int getViewType() {
        return TYPE.TextMessage.ordinal();
    }
}

ImageMessage.java

public class ImageMessage extends Message {
    public static ImageView createNewView(Context context) {
        return new ImageView(context);
    }

    @Override
    public void bindToView(View messageView) {
        Bitmap forExampleSomeBitmap = null; // TODO implement
        ((ImageView) messageView).setImageBitmap(forExampleSomeBitmap);
    }

    @Override
    int getViewType() {
        return TYPE.ImageMessage.ordinal();
    }
}

And then With all the rest of the code from that google sample code unchanged, your RecyclerView.Adapter could then look like this:

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
    private Message[] mDataset;

    public static class MyViewHolder extends RecyclerView.ViewHolder {
        public View messageView;

        public MyViewHolder(View v) {
            super(v);
            messageView = v;
        }
    }

    @NonNull
    @Override
    public MyAdapter.MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new MyViewHolder(Message.createView(viewType, parent.getContext()));
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        mDataset[position].bindToView(holder.messageView);
    }

    @Override
    public int getItemViewType(int position) {
        return mDataset[position].getViewType();
    }

    @Override
    public int getItemCount() {
        return mDataset.length;
    }
}

If you want to know more about how the recyclerview works, why it recycles views, then I would recommend this arcticle.

1 : Create new project -> select Basic Activity

there is to fragment inside your activity and a button to switch between fragment A to B

2 : create a RecyclerView resource in SecondFragment (fragment B )

add the following line into second fragment :

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerview"
    android:layout_width="409dp"
    android:layout_height="460dp"
    android:layout_marginStart="1dp"
    android:layout_marginTop="1dp"
    android:layout_marginEnd="1dp"
    android:layout_marginBottom="1dp"
    android:visibility="visible"
    app:layout_constraintBottom_toTopOf="@+id/button_second"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

3 : create a frame layout with textview
go to /res/layout/newlayout/layoutresource file and create an framlayout
inside the layout replace this codes

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_margin="8dp">

    <TextView
        android:id="@+id/randomText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        tools:text="This is some temp text" />

</FrameLayout>

4: Add RecyclerView in Fragment
go to second fragment and paste all of these

public class SecondFragment extends Fragment {

    // Add RecyclerView member
    private RecyclerView recyclerView;

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {

        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_second, container, false);

        // Add the following lines to create RecyclerView
        recyclerView = view.findViewById(R.id.recyclerview);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(view.getContext()));
        recyclerView.setAdapter(new RandomNumListAdapter(1234));

        return view;
    }

5 : create a class Viewholder for recyclerview
like this :

public class RecyclerViewHolder extends RecyclerView.ViewHolder {

    private TextView view;
    public RecyclerViewHolder(@NonNull View itemView) {
        super(itemView);
        view = itemView.findViewById(R.id.randomText);
    }

    public TextView getView(){
        return view;
    }
}

6 : Create ListAdapter

public class RandomNumListAdapter extends RecyclerView.Adapter<RecyclerViewHolder> {
    private Random random;

    public RandomNumListAdapter(int seed) {
        this.random = new Random(seed);
    }

    @Override
    public int getItemViewType(final int position) {
        return R.layout.frame_textview;
    }

    @NonNull
    @Override
    public RecyclerViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(viewType, parent, false);
        return new RecyclerViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerViewHolder holder, int position) {
        holder.getView().setText(String.valueOf(random.nextInt()));
    }

    @Override
    public int getItemCount() {
        return 100;
    }
}

you can replace your items with random numbers .