Expandable RecyclerView with parent and child item selection.

Category.java

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by rajnit on 22/08/17.
 */

public class Category implements Serializable {

    private int id;
    private String name;

    public List<SubCategory> getSubCategoryList() {
        return subCategoryList;
    }

    public void setSubCategoryList(List<SubCategory> subCategoryList) {
        this.subCategoryList = subCategoryList;
    }

    private List<SubCategory> subCategoryList = new ArrayList<>();

    public Category(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

category_row.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:layout_margin="4dp">

    <LinearLayout
        android:id="@+id/mRow"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="8dp">

        <TextView
            android:id="@+id/mPosition"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="0" />

        <TextView
            android:id="@+id/mName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="32dp"
            android:text="one" />

    </LinearLayout>

    <android.support.v7.widget.RecyclerView
        android:id="@+id/mSubRecyclerView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="#f1f1f1"/>

</LinearLayout>

CategoryAdatper.java

import android.content.Context;
import android.graphics.Typeface;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.example.rajnit.keylivemoney.R;

import java.util.List;

/**
 * Created by rajnit on 22/08/17.
 */

public class CategoryAdatper extends RecyclerView.Adapter<CategoryAdatper.MyViewHolder> {

    private List<Category> categoryList;
    private Context context;
    private int clickedPosition = -1;
    private int subClickedPosition = -1;

    public CategoryAdatper(Context context, List<Category> categoryList, int clickedPosition) {
        this.categoryList = categoryList;
        this.context = context;
        this.clickedPosition = clickedPosition;
    }

    @Override
    public CategoryAdatper.MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.movie_list_row, parent, false);

        return new MyViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(final CategoryAdatper.MyViewHolder holder, final int position) {

        holder.mName.setText(categoryList.get(position).getName());
        holder.mPosition.setText(String.valueOf(categoryList.get(position).getId()));

        LinearLayoutManager manager = new LinearLayoutManager(context);
        SubRecyclerViewAdapter adapter = new SubRecyclerViewAdapter(context, categoryList.get(position).getSubCategoryList(), -1, new SubRecyclerViewAdapter.SubSelectionInterface() {
            @Override
            public void onsubselection(int position) {
                subClickedPosition = position;
            }
        });
        holder.mSubRecyclerView.setLayoutManager(manager);
        holder.mSubRecyclerView.setAdapter(adapter);

        if (position == clickedPosition) {
            holder.mSubRecyclerView.setVisibility(View.VISIBLE);

            holder.mName.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);
            holder.mName.setTypeface(null, Typeface.BOLD);
            holder.mPosition.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);
            holder.mPosition.setTypeface(null, Typeface.BOLD);
        } else {
            holder.mSubRecyclerView.setVisibility(View.GONE);

            holder.mName.setTextSize(TypedValue.COMPLEX_UNIT_SP, 14);
            holder.mName.setTypeface(null, Typeface.NORMAL);
            holder.mPosition.setTextSize(TypedValue.COMPLEX_UNIT_SP, 14);
            holder.mPosition.setTypeface(null, Typeface.NORMAL);
        }

        holder.row.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (clickedPosition != position) {
                    clickedPosition = position;
                    subClickedPosition = -1;
                    notifyDataSetChanged();
                } else {
                    holder.mSubRecyclerView.setVisibility(View.GONE);
                    clickedPosition = -1;
                    subClickedPosition = -1;
                    notifyDataSetChanged();
                }
            }
        });

    }

    @Override
    public int getItemCount() {
        return categoryList.size();
    }

    public int getCategoryPosition() {
        return clickedPosition;
    }

    public int getSubCategoryPosition() {
        return subClickedPosition;
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        public TextView mPosition, mName;
        public RecyclerView mSubRecyclerView;
        public LinearLayout row;

        public MyViewHolder(View view) {
            super(view);
            mPosition = (TextView) view.findViewById(R.id.mPosition);
            mName = (TextView) view.findViewById(R.id.mName);
            mSubRecyclerView = (RecyclerView) view.findViewById(R.id.mSubRecyclerView);
            row = (LinearLayout) view.findViewById(R.id.mRow);
        }
    }

}

ExpandableRecyclerViewActivity.java

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import com.example.rajnit.keylivemoney.R;

import java.util.ArrayList;
import java.util.List;

public class ExpandableRecyclerViewActivity extends AppCompatActivity {
    private static final String TAG = "ExpandableRecyclerViewA";
    RecyclerView recyclerView;
    Button mShowSelection;

    List<Category> categoryList = new ArrayList<>();

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

        recyclerView = (RecyclerView) findViewById(R.id.mRecyclerView);
        mShowSelection = (Button) findViewById(R.id.mShowSelection);

        createDummyData();

        LinearLayoutManager manager = new LinearLayoutManager(this);
        final RecyclerViewAdapter adapter = new RecyclerViewAdapter(this, categoryList, -1);
        recyclerView.setLayoutManager(manager);
        recyclerView.setAdapter(adapter);

        mShowSelection.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: " + adapter.getCategoryPosition() + " -> " + adapter.getSubCategoryPosition());
            }
        });

    }

    private void createDummyData() {

        categoryList.add(new Category(1, "one"));
        categoryList.get(0).getSubCategoryList().add(new SubCategory(1, "sub one"));
        categoryList.get(0).getSubCategoryList().add(new SubCategory(2, "sub two"));
        categoryList.get(0).getSubCategoryList().add(new SubCategory(3, "sub three"));

        categoryList.add(new Category(2, "two"));
        categoryList.get(1).getSubCategoryList().add(new SubCategory(1, "sub one"));
        categoryList.get(1).getSubCategoryList().add(new SubCategory(2, "sub two"));
        categoryList.get(1).getSubCategoryList().add(new SubCategory(3, "sub three"));

        categoryList.add(new Category(3, "three"));
        categoryList.get(2).getSubCategoryList().add(new SubCategory(1, "sub one"));
        categoryList.get(2).getSubCategoryList().add(new SubCategory(2, "sub two"));
        categoryList.get(2).getSubCategoryList().add(new SubCategory(3, "sub three"));

        categoryList.add(new Category(4, "four"));
        categoryList.get(3).getSubCategoryList().add(new SubCategory(1, "sub one"));
        categoryList.get(3).getSubCategoryList().add(new SubCategory(2, "sub two"));
        categoryList.get(3).getSubCategoryList().add(new SubCategory(3, "sub three"));

        categoryList.add(new Category(5, "five"));
        categoryList.get(4).getSubCategoryList().add(new SubCategory(1, "sub one"));
        categoryList.get(4).getSubCategoryList().add(new SubCategory(2, "sub two"));
        categoryList.get(4).getSubCategoryList().add(new SubCategory(3, "sub three"));

    }
}

sub_category_row.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:layout_margin="12dp">

    <LinearLayout
        android:id="@+id/mRow"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/mPosition"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="0" />

        <TextView
            android:id="@+id/mName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="32dp"
            android:text="one" />

    </LinearLayout>

</LinearLayout> 

SubCategory.java

import java.io.Serializable;

/**
 * Created by rajnit on 22/08/17.
 */

public class SubCategory implements Serializable {

    private int subId;
    private String subName;

    public SubCategory(int subId, String subName) {
        this.subId = subId;
        this.subName = subName;
    }

    public int getSubId() {
        return subId;
    }

    public void setSubId(int subId) {
        this.subId = subId;
    }

    public String getSubName() {
        return subName;
    }

    public void setSubName(String subName) {
        this.subName = subName;
    }
}

SubCategoryAdatper.java

import android.content.Context;
import android.graphics.Typeface;
import android.support.v7.widget.RecyclerView;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.example.rajnit.keylivemoney.R;

import java.util.List;

/**
 * Created by rajnit on 22/08/17.
 */

public class SubCategoryAdatper extends RecyclerView.Adapter<SubCategoryAdatper.MyViewHolder> {

    SubSelectionInterface subSelectionInterface;
    private List<SubCategory> categoryList;
    private Context context;
    private int clickedPosition = -1;

    public SubCategoryAdatper(Context context, List<SubCategory> categoryList, int clickedPosition, SubSelectionInterface subSelectionInterface) {
        this.categoryList = categoryList;
        this.context = context;
        this.clickedPosition = clickedPosition;
        this.subSelectionInterface = subSelectionInterface;
    }

    @Override
    public SubCategoryAdatper.MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.sub_movie_list_row, parent, false);

        return new MyViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(SubCategoryAdatper.MyViewHolder holder, final int position) {

        holder.mName.setText(categoryList.get(position).getSubName());
        holder.mPosition.setText(String.valueOf(categoryList.get(position).getSubId()));

        if (position == clickedPosition) {
            holder.mName.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);
            holder.mName.setTypeface(null, Typeface.BOLD);
            holder.mPosition.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);
            holder.mPosition.setTypeface(null, Typeface.BOLD);
        } else {
            holder.mName.setTextSize(TypedValue.COMPLEX_UNIT_SP, 14);
            holder.mName.setTypeface(null, Typeface.NORMAL);
            holder.mPosition.setTextSize(TypedValue.COMPLEX_UNIT_SP, 14);
            holder.mPosition.setTypeface(null, Typeface.NORMAL);
        }

        holder.row.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (clickedPosition != position) {
                    clickedPosition = position;
                    subSelectionInterface.onsubselection(position);
                    notifyDataSetChanged();
                }
            }
        });

    }

    @Override
    public int getItemCount() {
        return categoryList.size();
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        public TextView mPosition, mName;
        public LinearLayout row;

        public MyViewHolder(View view) {
            super(view);
            mPosition = (TextView) view.findViewById(R.id.mPosition);
            mName = (TextView) view.findViewById(R.id.mName);
            row = (LinearLayout) view.findViewById(R.id.mRow);
        }
    }

    public interface SubSelectionInterface {
        void onsubselection(int position);
    }
}

where is the activity_expandable_recycler_view.xml