How to Delete Data in Realm Database in Android?

In the previous series of articles on the realm database, we have seen adding, reading, and updating data using the realm database in android. In that articles, we were adding course details, reading them, and updating them. In this article, we will take a look at deleting these course details from our realm database in the Android app. 

What we are going to build in this article? 

We will be building a simple application in which we will be working on the existing applications in which we will be simply adding a new button to delete our course from the realm database. Below is the video in which we will get to see what we are going to build in this article. 

Step by Step Implementation

Step 1: Add google repository in the build.gradle file of the application project.

buildscript {

  repositories {

      google()

      mavenCentral()

 }

All Jetpack components are available in the Google Maven repository, include them in the build.gradle file

allprojects {

  repositories {

      google()

     mavenCentral()

  }

}

Step 2: Working with the activity_update_course.xml file

Navigate to the app > res > layout > activity_update_course.xml file and add a Button inside this layout for deleting a course. Below is the code for that file. 

XML

<!--Button for deleting your course to database-->
<Button
  android:id="@+id/idBtnDeleteCourse"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_margin="10dp"
  android:text="Delete Course"
  android:textAllCaps="false"
  android:visibility="visible" />

Below is the updated code for the activity_update_course.xml file after adding the above code snippet.

XML

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

    <!--Edit text to enter course name-->
    <EditText
        android:id="@+id/idEdtCourseName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="Enter Course Name" />

    <!--edit text to enter course duration-->
    <EditText
        android:id="@+id/idEdtCourseDuration"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="Enter Course Duration" />

    <!--edit text to display course tracks-->
    <EditText
        android:id="@+id/idEdtCourseTracks"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="Enter Course Tracks" />

    <!--edit text for course description-->
    <EditText
        android:id="@+id/idEdtCourseDescription"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:hint="Enter Course Description" />

    <!--button for updating course-->
    <Button
        android:id="@+id/idBtnUpdateCourse"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="Update Course"
        android:textAllCaps="false" />

    <!--Button for deleting your course to database-->
    <Button
        android:id="@+id/idBtnDeleteCourse"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="Delete Course"
        android:textAllCaps="false"
        android:visibility="visible" />

</LinearLayout>

Step 3: Working with the UpdateCourseActivity.java file

Initializing our button to delete our course. Navigate to the app > java > your app’s package name > UpdateCourseActivity.java file and add the below code to it. 

Java

// adding on click listener for delete course button.
deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // on below line we are calling a method to delete course.
                deleteCourse(id);
                // after deleting we are displaying a toast message as course deleted.
                Toast.makeText(UpdateCourseActivity.this, "Course Deleted.", Toast.LENGTH_SHORT).show();
                // after that we are opening a new activity via an intent.
                Intent i = new Intent(UpdateCourseActivity.this, ReadCoursesActivity.class);
                startActivity(i);
                finish();
            }
        });
    }

// deleteCourse() function
private void deleteCourse(long id) {
        // on below line we are finding data from our modal class by comparing it with the course id.
        DataModal modal = realm.where(DataModal.class).equalTo("id", id).findFirst();
        // on below line we are executing a realm transaction.
        realm.executeTransaction(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                // on below line we are calling a method for deleting this course
                modal.deleteFromRealm();
            }
     });
}

Below is the updated code for the UpdateCourseActivity.java file after adding the above code snippet.

Java

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

import androidx.appcompat.app.AppCompatActivity;

import io.realm.Realm;

public class UpdateCourseActivity extends AppCompatActivity {

    // creating variables for our edit text
    private EditText courseNameEdt, courseDurationEdt, courseDescriptionEdt, courseTracksEdt;

    // creating a strings for storing
    // our values from edittext fields.
    private String courseName, courseDuration, courseDescription, courseTracks;
    private long id;
    private Button updateCourseBtn, deleteCourseBtn;
    private Realm realm;

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

        // initializing our edittext and buttons
        realm = Realm.getDefaultInstance();
        courseNameEdt = findViewById(R.id.idEdtCourseName);
        courseDescriptionEdt = findViewById(R.id.idEdtCourseDescription);
        courseDurationEdt = findViewById(R.id.idEdtCourseDuration);
        courseTracksEdt = findViewById(R.id.idEdtCourseTracks);
        updateCourseBtn = findViewById(R.id.idBtnUpdateCourse);
        deleteCourseBtn = findViewById(R.id.idBtnDeleteCourse);

        // on below line we are getting data which is passed from intent.
        courseName = getIntent().getStringExtra("courseName");
        courseDuration = getIntent().getStringExtra("courseDuration");
        courseDescription = getIntent().getStringExtra("courseDescription");
        courseTracks = getIntent().getStringExtra("courseTracks");
        id = getIntent().getLongExtra("id", 0);

        // on below line we are setting data in our edit test fields.
        courseNameEdt.setText(courseName);
        courseDurationEdt.setText(courseDuration);
        courseDescriptionEdt.setText(courseDescription);
        courseTracksEdt.setText(courseTracks);

        // adding on click listener for update button.
        updateCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // getting data from edittext fields.
                String courseName = courseNameEdt.getText().toString();
                String courseDescription = courseDescriptionEdt.getText().toString();
                String courseDuration = courseDurationEdt.getText().toString();
                String courseTracks = courseTracksEdt.getText().toString();

                // validating the text fields if empty or not.
                if (TextUtils.isEmpty(courseName)) {
                    courseNameEdt.setError("Please enter Course Name");
                } else if (TextUtils.isEmpty(courseDescription)) {
                    courseDescriptionEdt.setError("Please enter Course Description");
                } else if (TextUtils.isEmpty(courseDuration)) {
                    courseDurationEdt.setError("Please enter Course Duration");
                } else if (TextUtils.isEmpty(courseTracks)) {
                    courseTracksEdt.setError("Please enter Course Tracks");
                } else {
                    // on below line we are getting data from our modal where
                    // the id of the course equals to which we passed previously.
                    final DataModal modal = realm.where(DataModal.class).equalTo("id", id).findFirst();
                    updateCourse(modal, courseName, courseDescription, courseDuration, courseTracks);
                }

                // on below line we are displaying a toast message when course is updated.
                Toast.makeText(UpdateCourseActivity.this, "Course Updated.", Toast.LENGTH_SHORT).show();

                // on below line we are opening our activity for read course activity to view updated course.
                Intent i = new Intent(UpdateCourseActivity.this, ReadCoursesActivity.class);
                startActivity(i);
                finish();
            }
        });

        // adding on click listener for delete course button.
        deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // on below line we are calling a method to delete course.
                deleteCourse(id);
                // after deleting we are displaying a toast message as course deleted.
                Toast.makeText(UpdateCourseActivity.this, "Course Deleted.", Toast.LENGTH_SHORT).show();
                // after that we are opening a new activity via an intent.
                Intent i = new Intent(UpdateCourseActivity.this, ReadCoursesActivity.class);
                startActivity(i);
                finish();
            }
        });
    }

    private void updateCourse(DataModal modal, String courseName, String courseDescription, String courseDuration, String courseTracks) {

        // on below line we are calling
        // a method to execute a transaction.
        realm.executeTransaction(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                // on below line we are setting data to our modal class
                // which we get from our edit text fields.
                modal.setCourseDescription(courseDescription);
                modal.setCourseName(courseName);
                modal.setCourseDuration(courseDuration);
                modal.setCourseTracks(courseTracks);

                // inside on execute method we are calling a method to copy
                // and update to real m database from our modal class.
                realm.copyToRealmOrUpdate(modal);
            }
        });
    }

    // deleteCourse() function
    private void deleteCourse(long id) {
        // on below line we are finding data from our modal class by comparing it with the course id.
        DataModal modal = realm.where(DataModal.class).equalTo("id", id).findFirst();
        // on below line we are executing a realm transaction.
        realm.executeTransaction(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                // on below line we are calling a method for deleting this course
                modal.deleteFromRealm();
            }
        });
    }
}

Now run your app and see the output of the code: 

Output:

Below is the complete project file structure after performing the CRUD operation:

Check out the project at the below link: https://github.com/ChaitanyaMunje/Realm-Db