How to Perform CRUD Operations in Room Database in Android?

import androidx.room.Entity;
import androidx.room.PrimaryKey;

// below line is for setting table name.
@Entity(tableName = "course_table")
public class CourseModal {

    // below line is to auto increment
    // id for each course.
    @PrimaryKey(autoGenerate = true)
    
    // variable for our id.
    private int id;
    
    // below line is a variable 
    // for course name.
    private String courseName;
   
    // below line is use for 
    // course description.
    private String courseDescription;
    
    // below line is use 
    // for course duration.
    private String courseDuration;

    // below line we are creating constructor class.
    // inside constructor class we are not passing 
    // our id because it is incrementing automatically
    public CourseModal(String courseName, String courseDescription, String courseDuration) {
        this.courseName = courseName;
        this.courseDescription = courseDescription;
        this.courseDuration = courseDuration;
    }

    // on below line we are creating
    // getter and setter methods.
    public String getCourseName() {
        return courseName;
    }

    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }

    public String getCourseDescription() {
        return courseDescription;
    }

    public void setCourseDescription(String courseDescription) {
        this.courseDescription = courseDescription;
    }

    public String getCourseDuration() {
        return courseDuration;
    }

    public void setCourseDuration(String courseDuration) {
        this.courseDuration = courseDuration;
    }

    public int getId() {
        return id;
    }

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

import androidx.lifecycle.LiveData;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;

// Adding annotation 
// to our Dao class
@androidx.room.Dao
public interface Dao {

    // below method is use to 
    // add data to database.
    @Insert
    void insert(CourseModal model);

    // below method is use to update 
    // the data in our database.
    @Update
    void update(CourseModal model);

    // below line is use to delete a 
    // specific course in our database.
    @Delete
    void delete(CourseModal model);

    // on below line we are making query to
    // delete all courses from our database.
    @Query("DELETE FROM course_table")
    void deleteAllCourses();

    // below line is to read all the courses from our database.
    // in this we are ordering our courses in ascending order 
    // with our course name.
    @Query("SELECT * FROM course_table ORDER BY courseName ASC")
    LiveData<List<CourseModal>> getAllCourses();
}

import android.content.Context;
import android.os.AsyncTask;

import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.sqlite.db.SupportSQLiteDatabase;

// adding annotation for our database entities and db version.
@Database(entities = {CourseModal.class}, version = 1)
public abstract class CourseDatabase extends RoomDatabase {

    // below line is to create instance
    // for our database class.
    private static CourseDatabase instance;

    // below line is to create
    // abstract variable for dao.
    public abstract Dao Dao();

    // on below line we are getting instance for our database.
    public static synchronized CourseDatabase getInstance(Context context) {
        // below line is to check if 
        // the instance is null or not.
        if (instance == null) {
            // if the instance is null we
            // are creating a new instance
            instance =
                    // for creating a instance for our database 
                    // we are creating a database builder and passing 
                    // our database class with our database name.
                    Room.databaseBuilder(context.getApplicationContext(),
                            CourseDatabase.class, "course_database")
                            // below line is use to add fall back to 
                            // destructive migration to our database.
                            .fallbackToDestructiveMigration()
                            // below line is to add callback 
                            // to our database.
                            .addCallback(roomCallback)
                            // below line is to 
                            // build our database.
                            .build();
        }
        // after creating an instance 
        // we are returning our instance
        return instance;
    }

    // below line is to create a callback for our room database.
    private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback() {
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            // this method is called when database is created
            // and below line is to populate our data.
            new PopulateDbAsyncTask(instance).execute();
        }
    };

    // we are creating an async task class to perform task in background.
    private static class PopulateDbAsyncTask extends AsyncTask<Void, Void, Void> {
        PopulateDbAsyncTask(CourseDatabase instance) {
            Dao dao = instance.Dao();
        }
        @Override
        protected Void doInBackground(Void... voids) {
            return null;
        }
    }
}

import android.app.Application;
import android.os.AsyncTask;

import androidx.lifecycle.LiveData;

import java.util.List;

public class CourseRepository {
    
    // below line is the create a variable 
    // for dao and list for all courses.
    private Dao dao;
    private LiveData<List<CourseModal>> allCourses;

    // creating a constructor for our variables
    // and passing the variables to it.
    public CourseRepository(Application application) {
        CourseDatabase database = CourseDatabase.getInstance(application);
        dao = database.Dao();
        allCourses = dao.getAllCourses();
    }

    // creating a method to insert the data to our database.
    public void insert(CourseModal model) {
        new InsertCourseAsyncTask(dao).execute(model);
    }

    // creating a method to update data in database.
    public void update(CourseModal model) {
        new UpdateCourseAsyncTask(dao).execute(model);
    }

    // creating a method to delete the data in our database.
    public void delete(CourseModal model) {
        new DeleteCourseAsyncTask(dao).execute(model);
    }

    // below is the method to delete all the courses.
    public void deleteAllCourses() {
        new DeleteAllCoursesAsyncTask(dao).execute();
    }

    // below method is to read all the courses.
    public LiveData<List<CourseModal>> getAllCourses() {
        return allCourses;
    }

    // we are creating a async task method to insert new course.
    private static class InsertCourseAsyncTask extends AsyncTask<CourseModal, Void, Void> {
        private Dao dao;

        private InsertCourseAsyncTask(Dao dao) {
            this.dao = dao;
        }

        @Override
        protected Void doInBackground(CourseModal... model) {
            // below line is use to insert our modal in dao.
            dao.insert(model[0]);
            return null;
        }
    }

    // we are creating a async task method to update our course.
    private static class UpdateCourseAsyncTask extends AsyncTask<CourseModal, Void, Void> {
        private Dao dao;

        private UpdateCourseAsyncTask(Dao dao) {
            this.dao = dao;
        }

        @Override
        protected Void doInBackground(CourseModal... models) {
            // below line is use to update
            // our modal in dao.
            dao.update(models[0]);
            return null;
        }
    }

    // we are creating a async task method to delete course.
    private static class DeleteCourseAsyncTask extends AsyncTask<CourseModal, Void, Void> {
        private Dao dao;

        private DeleteCourseAsyncTask(Dao dao) {
            this.dao = dao;
        }

        @Override
        protected Void doInBackground(CourseModal... models) {
            // below line is use to delete 
            // our course modal in dao.
            dao.delete(models[0]);
            return null;
        }
    }

    // we are creating a async task method to delete all courses.
    private static class DeleteAllCoursesAsyncTask extends AsyncTask<Void, Void, Void> {
        private Dao dao;
        private DeleteAllCoursesAsyncTask(Dao dao) {
            this.dao = dao;
        }
        @Override
        protected Void doInBackground(Void... voids) {
            // on below line calling method
            // to delete all courses.
            dao.deleteAllCourses();
            return null;
        }
    }
}

import android.app.Application;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;

import java.util.List;

public class ViewModal extends AndroidViewModel {
    
    // creating a new variable for course repository.
    private CourseRepository repository;
   
    // below line is to create a variable for live 
    // data where all the courses are present.
    private LiveData<List<CourseModal>> allCourses;

    // constructor for our view modal.
    public ViewModal(@NonNull Application application) {
        super(application);
        repository = new CourseRepository(application);
        allCourses = repository.getAllCourses();
    }
    
    // below method is use to insert the data to our repository.
    public void insert(CourseModal model) {
        repository.insert(model);
    }

    // below line is to update data in our repository.
    public void update(CourseModal model) {
        repository.update(model);
    }

    // below line is to delete the data in our repository.
    public void delete(CourseModal model) {
        repository.delete(model);
    }

    // below method is to delete all the courses in our list.
    public void deleteAllCourses() {
        repository.deleteAllCourses();
    }

    // below method is to get all the courses in our list.
    public LiveData<List<CourseModal>> getAllCourses() {
        return allCourses;
    }
}

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.DiffUtil;
import androidx.recyclerview.widget.ListAdapter;
import androidx.recyclerview.widget.RecyclerView;

public class CourseRVAdapter extends ListAdapter<CourseModal, CourseRVAdapter.ViewHolder> {
    
    // creating a variable for on item click listener.
    private OnItemClickListener listener;

    // creating a constructor class for our adapter class.
    CourseRVAdapter() {
        super(DIFF_CALLBACK);
    }

    // creating a call back for item of recycler view.
    private static final DiffUtil.ItemCallback<CourseModal> DIFF_CALLBACK = new DiffUtil.ItemCallback<CourseModal>() {
        @Override
        public boolean areItemsTheSame(CourseModal oldItem, CourseModal newItem) {
            return oldItem.getId() == newItem.getId();
        }

        @Override
        public boolean areContentsTheSame(CourseModal oldItem, CourseModal newItem) {
            // below line is to check the course name, description and course duration.
            return oldItem.getCourseName().equals(newItem.getCourseName()) &&
                    oldItem.getCourseDescription().equals(newItem.getCourseDescription()) &&
                    oldItem.getCourseDuration().equals(newItem.getCourseDuration());
        }
    };

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        // below line is use to inflate our layout
        // file for each item of our recycler view.
        View item = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.course_rv_item, parent, false);
        return new ViewHolder(item);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        // below line of code is use to set data to
        // each item of our recycler view.
        CourseModal model = getCourseAt(position);
        holder.courseNameTV.setText(model.getCourseName());
        holder.courseDescTV.setText(model.getCourseDescription());
        holder.courseDurationTV.setText(model.getCourseDuration());
    }

    // creating a method to get course modal for a specific position.
    public CourseModal getCourseAt(int position) {
        return getItem(position);
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        // view holder class to create a variable for each view.
        TextView courseNameTV, courseDescTV, courseDurationTV;

        ViewHolder(@NonNull View itemView) {
            super(itemView);
            // initializing each view of our recycler view.
            courseNameTV = itemView.findViewById(R.id.idTVCourseName);
            courseDescTV = itemView.findViewById(R.id.idTVCourseDescription);
            courseDurationTV = itemView.findViewById(R.id.idTVCourseDuration);
            
            // adding on click listener for each item of recycler view.
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    // inside on click listener we are passing 
                    // position to our item of recycler view.
                    int position = getAdapterPosition();
                    if (listener != null && position != RecyclerView.NO_POSITION) {
                        listener.onItemClick(getItem(position));
                    }
                }
            });
        }
    }

    public interface OnItemClickListener {
        void onItemClick(CourseModal model);
    }
    public void setOnItemClickListener(OnItemClickListener listener) {
        this.listener = listener;
    }
}

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class NewCourseActivity extends AppCompatActivity {

    // creating a variables for our button and edittext.
    private EditText courseNameEdt, courseDescEdt, courseDurationEdt;
    private Button courseBtn;

    // creating a constant string variable for our 
    // course name, description and duration.
    public static final String EXTRA_ID = "com.gtappdevelopers.gfgroomdatabase.EXTRA_ID";
    public static final String EXTRA_COURSE_NAME = "com.gtappdevelopers.gfgroomdatabase.EXTRA_COURSE_NAME";
    public static final String EXTRA_DESCRIPTION = "com.gtappdevelopers.gfgroomdatabase.EXTRA_COURSE_DESCRIPTION";
    public static final String EXTRA_DURATION = "com.gtappdevelopers.gfgroomdatabase.EXTRA_COURSE_DURATION";
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_new_course);
        
        // initializing our variables for each view.
        courseNameEdt = findViewById(R.id.idEdtCourseName);
        courseDescEdt = findViewById(R.id.idEdtCourseDescription);
        courseDurationEdt = findViewById(R.id.idEdtCourseDuration);
        courseBtn = findViewById(R.id.idBtnSaveCourse);
        
        // below line is to get intent as we
        // are getting data via an intent.
        Intent intent = getIntent();
        if (intent.hasExtra(EXTRA_ID)) {
            // if we get id for our data then we are
            // setting values to our edit text fields.
            courseNameEdt.setText(intent.getStringExtra(EXTRA_COURSE_NAME));
            courseDescEdt.setText(intent.getStringExtra(EXTRA_DESCRIPTION));
            courseDurationEdt.setText(intent.getStringExtra(EXTRA_DURATION));
        }
        // adding on click listener for our save button.
        courseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // getting text value from edittext and validating if
                // the text fields are empty or not.
                String courseName = courseNameEdt.getText().toString();
                String courseDesc = courseDescEdt.getText().toString();
                String courseDuration = courseDurationEdt.getText().toString();
                if (courseName.isEmpty() || courseDesc.isEmpty() || courseDuration.isEmpty()) {
                    Toast.makeText(NewCourseActivity.this, "Please enter the valid course details.", Toast.LENGTH_SHORT).show();
                    return;
                }
                // calling a method to save our course.
                saveCourse(courseName, courseDesc, courseDuration);
            }
        });
    }

    private void saveCourse(String courseName, String courseDescription, String courseDuration) {
        // inside this method we are passing 
        // all the data via an intent.
        Intent data = new Intent();
        
        // in below line we are passing all our course detail.
        data.putExtra(EXTRA_COURSE_NAME, courseName);
        data.putExtra(EXTRA_DESCRIPTION, courseDescription);
        data.putExtra(EXTRA_DURATION, courseDuration);
        int id = getIntent().getIntExtra(EXTRA_ID, -1);
        if (id != -1) {
            // in below line we are passing our id.
            data.putExtra(EXTRA_ID, id);
        }
        
        // at last we are setting result as data.
        setResult(RESULT_OK, data);
        
        // displaying a toast message after adding the data
        Toast.makeText(this, "Course has been saved to Room Database. ", Toast.LENGTH_SHORT).show();
    }
}

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import androidx.recyclerview.widget.ItemTouchHelper;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.android.material.floatingactionbutton.FloatingActionButton;

import java.util.List;

public class MainActivity extends AppCompatActivity {

    // creating a variables for our recycler view.
    private RecyclerView coursesRV;
    private static final int ADD_COURSE_REQUEST = 1;
    private static final int EDIT_COURSE_REQUEST = 2;
    private ViewModal viewmodal;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       
        // initializing our variable for our recycler view and fab.
        coursesRV = findViewById(R.id.idRVCourses);
        FloatingActionButton fab = findViewById(R.id.idFABAdd);

        // adding on click listener for floating action button.
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // starting a new activity for adding a new course 
                // and passing a constant value in it.
                Intent intent = new Intent(MainActivity.this, NewCourseActivity.class);
                startActivityForResult(intent, ADD_COURSE_REQUEST);
            }
        });

        // setting layout manager to our adapter class.
        coursesRV.setLayoutManager(new LinearLayoutManager(this));
        coursesRV.setHasFixedSize(true);
       
        // initializing adapter for recycler view.
        final CourseRVAdapter adapter = new CourseRVAdapter();
        
        // setting adapter class for recycler view.
        coursesRV.setAdapter(adapter);
        
        // passing a data from view modal.
        viewmodal = ViewModelProviders.of(this).get(ViewModal.class);
       
        // below line is use to get all the courses from view modal.
        viewmodal.getAllCourses().observe(this, new Observer<List<CourseModal>>() {
            @Override
            public void onChanged(List<CourseModal> models) {
                // when the data is changed in our models we are
                // adding that list to our adapter class.
                adapter.submitList(models);
            }
        });
        // below method is use to add swipe to delete method for item of recycler view.
        new ItemTouchHelper(new ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT) {
            @Override
            public boolean onMove(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, @NonNull RecyclerView.ViewHolder target) {
                return false;
            }

            @Override
            public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
                // on recycler view item swiped then we are deleting the item of our recycler view.
                viewmodal.delete(adapter.getCourseAt(viewHolder.getAdapterPosition()));
                Toast.makeText(MainActivity.this, "Course deleted", Toast.LENGTH_SHORT).show();
            }
        }).
                // below line is use to attach this to recycler view.
                        attachToRecyclerView(coursesRV);
        // below line is use to set item click listener for our item of recycler view.
        adapter.setOnItemClickListener(new CourseRVAdapter.OnItemClickListener() {
            @Override
            public void onItemClick(CourseModal model) {
                // after clicking on item of recycler view
                // we are opening a new activity and passing 
                // a data to our activity.
                Intent intent = new Intent(MainActivity.this, NewCourseActivity.class);
                intent.putExtra(NewCourseActivity.EXTRA_ID, model.getId());
                intent.putExtra(NewCourseActivity.EXTRA_COURSE_NAME, model.getCourseName());
                intent.putExtra(NewCourseActivity.EXTRA_DESCRIPTION, model.getCourseDescription());
                intent.putExtra(NewCourseActivity.EXTRA_DURATION, model.getCourseDuration());
                
                // below line is to start a new activity and 
                // adding a edit course constant.
                startActivityForResult(intent, EDIT_COURSE_REQUEST);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == ADD_COURSE_REQUEST && resultCode == RESULT_OK) {
            String courseName = data.getStringExtra(NewCourseActivity.EXTRA_COURSE_NAME);
            String courseDescription = data.getStringExtra(NewCourseActivity.EXTRA_DESCRIPTION);
            String courseDuration = data.getStringExtra(NewCourseActivity.EXTRA_DURATION);
            CourseModal model = new CourseModal(courseName, courseDescription, courseDuration);
            viewmodal.insert(model);
            Toast.makeText(this, "Course saved", Toast.LENGTH_SHORT).show();
        } else if (requestCode == EDIT_COURSE_REQUEST && resultCode == RESULT_OK) {
            int id = data.getIntExtra(NewCourseActivity.EXTRA_ID, -1);
            if (id == -1) {
                Toast.makeText(this, "Course can't be updated", Toast.LENGTH_SHORT).show();
                return;
            }
            String courseName = data.getStringExtra(NewCourseActivity.EXTRA_COURSE_NAME);
            String courseDesc = data.getStringExtra(NewCourseActivity.EXTRA_DESCRIPTION);
            String courseDuration = data.getStringExtra(NewCourseActivity.EXTRA_DURATION);
            CourseModal model = new CourseModal(courseName, courseDesc, courseDuration);
            model.setId(id);
            viewmodal.update(model);
            Toast.makeText(this, "Course updated", Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(this, "Course not saved", Toast.LENGTH_SHORT).show();
        }
    }
}