How to limit the amount of rows retreived from Android Room Database

In this android programming source code example, we are going to limit the amount of rows retreived from Android Room Database.

You can copy and adopt this source code example to your android project without reinventing the wheel.

Below is a step by step source code to limit the amount of rows retreived from Android Room Database.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

MainActivity.java

import android.content.Context;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.DividerItemDecoration;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;

import com.bluapp.androidview.R;

import java.util.List;

import io.reactivex.Completable;
import io.reactivex.CompletableObserver;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Action;
import io.reactivex.schedulers.Schedulers;

public class RoomDatabaseActivity20 extends AppCompatActivity {
    private RecyclerView list;
    private adapter recyclerAdapter;
    private UserDatabase userDatabase;
    private List<User> responseList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_room_database20);
        list = (RecyclerView)findViewById(R.id.list);
        userDatabase = UserDatabase.getInstance(this);
        readData();
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        list.setLayoutManager(layoutManager);
        recyclerAdapter = new adapter(RoomDatabaseActivity20.this, responseList);
        list.addItemDecoration(new DividerItemDecoration(RoomDatabaseActivity20.this, DividerItemDecoration.VERTICAL));
        list.setAdapter(recyclerAdapter);
    }

    private void readData(){
        Completable.fromAction(new Action() {
            @Override
            public void run() throws Exception {
                userDatabase.userDAO().getlimituserList();
            }
        }).observeOn(AndroidSchedulers.mainThread())
                .subscribeOn(Schedulers.io())
                .subscribe(new CompletableObserver() {
                    @Override
                    public void onSubscribe(Disposable d) {
                    }

                    @Override
                    public void onComplete() {
                        responseList = userDatabase.userDAO().getlimituserList();
                        recyclerAdapter.setList(responseList);
                    }

                    @Override
                    public void onError(Throwable e) {
                        Toast.makeText(RoomDatabaseActivity20.this, "Empty data",Toast.LENGTH_LONG).show();
                    }
                });
    }

    private class adapter extends RecyclerView.Adapter<adapter.myViewHolder> {
        Context context;
        List<User> responseList;

        public adapter(Context context, List<User> data) {
            this.context = context;
            this.responseList = data;
        }

        public void setList(List<User> resList) {
            this.responseList = resList;
            notifyDataSetChanged();
        }

        @Override
        public adapter.myViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(context).inflate(R.layout.room_adapter1, parent, false);
            return new myViewHolder(view);
        }

        @Override
        public void onBindViewHolder(adapter.myViewHolder holder, int position) {
            holder.name.setText(responseList.get(position).getName());
        }

        @Override
        public int getItemCount() {
            if(responseList != null){
                return responseList.size();
            }
            return 0;

        }

        public class myViewHolder extends RecyclerView.ViewHolder {
            TextView name;

            public myViewHolder(View itemView) {
                super(itemView);
                name = (TextView) itemView.findViewById(R.id.Name);
            }
        }
    }
}

room_adapter1.xml

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

    <TextView
        android:id="@+id/Name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_marginTop="10dp"
        android:lineSpacingMultiplier="0.6"
        android:layout_marginLeft="10dp"
        android:text="Emmanuel"
        android:textColor="#000000"
        android:textSize="18sp"/>
</RelativeLayout>

UserDatabase.java

import android.arch.persistence.db.SupportSQLiteDatabase;
import android.arch.persistence.room.Database;
import android.arch.persistence.room.Room;
import android.arch.persistence.room.RoomDatabase;
import android.content.Context;
import android.support.annotation.NonNull;

import java.util.concurrent.Executors;

@Database(entities = { User.class }, version = 1, exportSchema = false)
public abstract class UserDatabase extends RoomDatabase {
    private static final String DB_NAME ="UserDb";
    private static UserDatabase instance;
    public abstract UserDAO userDAO();
    
    public synchronized static UserDatabase getInstance(final Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context, UserDatabase.class, DB_NAME)
                    .allowMainThreadQueries().build();
        }
        return instance;
    }
}

User.java

import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.Ignore;
import android.arch.persistence.room.PrimaryKey;

@Entity(tableName = "users")
public class User {
    @PrimaryKey(autoGenerate = true)
    private int id;
    @ColumnInfo(name = "name")
    private String name;
    public User(int id, String name){
        this.id = id;
        this.name = name;
    }
    @Ignore
    public User(String name){
        this.name = name;
    }
    public int getId() {
        return id;
    }
    public String getName() {
        return name;
    }
}

UserDAO.java

import android.arch.persistence.room.Dao;
import android.arch.persistence.room.Delete;
import android.arch.persistence.room.Insert;
import android.arch.persistence.room.Query;
import android.arch.persistence.room.Update;

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

@Dao
public interface UserDAO {
    @Query("select * from users limit 1")
    List<User> getlimituserList();
    @Insert
    public void insert(User user);

    @Update
    public void update(User user);

    @Delete
    public void delete(User user);

    }

build.gradle

implementation 'io.reactivex.rxjava2:rxjava:2.1.9'
implementation 'io.reactivex.rxjava2:rxandroid:2.0.1'
implementation 'android.arch.persistence.room:runtime:1.0.0'
annotationProcessor 'android.arch.persistence.room:compiler:1.0.0'