How to Delete Data in Back4App Database in Android?

We have seen adding data to our Back4App database along with reading and updating this data. In this article, we will take a look at Deleting this data. 

What we are going to build? 

We will be adding a simple button inside our update screen where we were updating our data and inside that screen, we will be adding a new button for deleting the course. After deleting, that course will be deleted from our database. 

Step by Step Implementation

Step 1: Creating a new button for deleting the data inside the activity_update_course.xml file

As we have created a new Update Course Activity in the previous article. So we will simply add a new button to it. Add the following code snippet to the activity_update_course.xml file. 

XML

<!--button for deleting our course-->
<Button
  android:id="@+id/idBtnDelete"
  android:layout_width="0dp"
  android:layout_height="wrap_content"
  android:layout_margin="3dp"
  android:layout_weight="1"
  android:text="Delete Course"
  android:textAllCaps="false" />

Below is the updated code for the activity_update_course.xml file. 

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 for getting course Name-->
    <EditText
        android:id="@+id/idEdtCourseName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="10dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="10dp"
        android:hint="Course Name"
        android:importantForAutofill="no"
        android:inputType="text" />

    <!--Edittext for getting course Duration-->
    <EditText
        android:id="@+id/idEdtCourseDuration"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="10dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="10dp"
        android:hint="Course Duration in min"
        android:importantForAutofill="no"
        android:inputType="time" />

    <!--Edittext for getting course Description-->
    <EditText
        android:id="@+id/idEdtCourseDescription"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="10dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="10dp"
        android:hint="Course Description"
        android:importantForAutofill="no"
        android:inputType="text" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="10dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="10dp"
        android:orientation="horizontal"
        android:weightSum="2">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginStart="10dp"
            android:layout_marginTop="20dp"
            android:layout_marginEnd="10dp"
            android:orientation="horizontal"
            android:weightSum="2">

            <!--button for updating our course-->
            <Button
                android:id="@+id/idBtnUpdate"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_margin="3dp"
                android:layout_weight="1"
                android:text="Update Course"
                android:textAllCaps="false" />

            <!--button for deleting our course-->
            <Button
                android:id="@+id/idBtnDelete"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_margin="3dp"
                android:layout_weight="1"
                android:text="Delete Course"
                android:textAllCaps="false" />

        </LinearLayout>

    </LinearLayout>

</LinearLayout>

Step 2: Now we have to initialize this button in the UpdateCourseActivity.java file and add onClickListener to it 

Go to the UpdateCourseActivity.java file and add the following code snippet.

Java

// Adding on click listener for delete button
deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // calling a method to delete a course.
                deleteCourse(originalCourseName);
        }
});

private void deleteCourse(String courseName) {

        // Configure Query with our query.
        ParseQuery<ParseObject> query = ParseQuery.getQuery("courses");

        // adding a condition where our course name
        // must be equal to the original course name
        query.whereEqualTo("courseName", courseName);

        // on below line we are finding the course with the course name.
        query.findInBackground(new FindCallback<ParseObject>() {
            @Override
            public void done(List<ParseObject> objects, ParseException e) {
                // if the error is null.
                if (e == null) {
                    // on below line we are getting the first course and
                    // calling a delete method to delete this course.
                    objects.get(0).deleteInBackground(new DeleteCallback() {
                        @Override
                        public void done(ParseException e) {
                            // inside done method checking if the error is null or not.
                            if (e == null) {
                                // if the error is not null then we are displaying a toast message and opening our home activity.
                                Toast.makeText(UpdateCourseActivity.this, "Course Deleted..", Toast.LENGTH_SHORT).show();
                                Intent i = new Intent(UpdateCourseActivity.this, HomeActivity.class);
                                startActivity(i);
                            } else {
                                // if we get error we are displaying it in below line.
                                Toast.makeText(UpdateCourseActivity.this, "Fail to delete course..", Toast.LENGTH_SHORT).show();
                            }
                        }
                    });
                } else {
                    // if we don't get the data in our database then we are displaying below message.
                    Toast.makeText(UpdateCourseActivity.this, "Fail to get the object..", Toast.LENGTH_SHORT).show();
                }
            }
     });
}

Below is the updated code for the UpdateCourseActivity.java file. 

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 com.parse.DeleteCallback;
import com.parse.FindCallback;
import com.parse.GetCallback;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
import com.parse.SaveCallback;

import java.util.List;

public class UpdateCourseActivity extends AppCompatActivity {

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

    // creating variable for button
    private Button updateCourseBtn, deleteCourseBtn;

    // creating a strings for storing our values from edittext fields.
    private String courseName, courseDuration, courseDescription, originalCourseName, objectID;

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

        // initializing our edittext and buttons
        deleteCourseBtn = findViewById(R.id.idBtnDelete);
        updateCourseBtn = findViewById(R.id.idBtnUpdate);
        courseNameEdt = findViewById(R.id.idEdtCourseName);
        courseDescriptionEdt = findViewById(R.id.idEdtCourseDescription);
        courseDurationEdt = findViewById(R.id.idEdtCourseDuration);

        // on below line we are setting data to our edit text field.
        courseNameEdt.setText(getIntent().getStringExtra("courseName"));
        courseDescriptionEdt.setText(getIntent().getStringExtra("courseDescription"));
        courseDurationEdt.setText(getIntent().getStringExtra("courseDuration"));
        originalCourseName = getIntent().getStringExtra("courseName");

        // Adding on click listener for delete button
        deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // calling a method to delete a course.
                deleteCourse(originalCourseName);
            }
        });

        updateCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                courseName = courseNameEdt.getText().toString();
                courseDescription = courseDescriptionEdt.getText().toString();
                courseDuration = courseDurationEdt.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 {
                    // calling method to update data.
                    updateData(originalCourseName, courseName, courseDescription, courseDuration);
                }
            }
        });
    }

    private void deleteCourse(String courseName) {

        // Configure Query with our query.
        ParseQuery<ParseObject> query = ParseQuery.getQuery("courses");

        // adding a condition where our course name
        // must be equal to the original course name
        query.whereEqualTo("courseName", courseName);

        // on below line we are finding the course with the course name.
        query.findInBackground(new FindCallback<ParseObject>() {
            @Override
            public void done(List<ParseObject> objects, ParseException e) {
                // if the error is null.
                if (e == null) {
                    // on below line we are getting the first course and
                    // calling a delete method to delete this course.
                    objects.get(0).deleteInBackground(new DeleteCallback() {
                        @Override
                        public void done(ParseException e) {
                            // inside done method checking if the error is null or not.
                            if (e == null) {
                                // if the error is not null then we are displaying a toast message and opening our home activity.
                                Toast.makeText(UpdateCourseActivity.this, "Course Deleted..", Toast.LENGTH_SHORT).show();
                                Intent i = new Intent(UpdateCourseActivity.this, HomeActivity.class);
                                startActivity(i);
                            } else {
                                // if we get error we are displaying it in below line.
                                Toast.makeText(UpdateCourseActivity.this, "Fail to delete course..", Toast.LENGTH_SHORT).show();
                            }
                        }
                    });
                } else {
                    // if we don't get the data in our database then we are displaying below message.
                    Toast.makeText(UpdateCourseActivity.this, "Fail to get the object..", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

    private void updateData(String originalCourseName, String courseName, String courseDescription, String courseDuration) {

        // Configure Query with our query.
        ParseQuery<ParseObject> query = ParseQuery.getQuery("courses");

        // adding a condition where our course name must be equal to the original course name
        query.whereEqualTo("courseName", originalCourseName);

        // in below method we are getting the unique id
        // of the course which we have to make update.
        query.getFirstInBackground(new GetCallback<ParseObject>() {
            @Override
            public void done(ParseObject object, ParseException e) {
                // inside done method we check
                // if the error is null or not.
                if (e == null) {

                    // if the error is null then we are getting
                    // our object id in below line.
                    objectID = object.getObjectId().toString();

                    // after getting our object id we will
                    // move towards updating our course.
                    // calling below method to update our course.
                    query.getInBackground(objectID, new GetCallback<ParseObject>() {
                        @Override
                        public void done(ParseObject object, ParseException e) {
                            // in this method we are getting the
                            // object which we have to update.
                            if (e == null) {

                                // in below line we are adding new data
                                // to the object which we get from its id.
                                // on below line we are adding our data
                                // with their key value in our object.
                                object.put("courseName", courseName);
                                object.put("courseDescription", courseDescription);
                                object.put("courseDuration", courseDuration);

                                // after adding new data then we are
                                // calling a method to save this data
                                object.saveInBackground(new SaveCallback() {
                                    @Override
                                    public void done(ParseException e) {
                                        // inside on done method we are checking
                                        // if the error is null or not.
                                        if (e == null) {
                                            // if the error is null our data has been updated.
                                            // we are displaying a toast message and redirecting
                                            // our user to home activity where we are displaying course list.
                                            Toast.makeText(UpdateCourseActivity.this, "Course Updated..", Toast.LENGTH_SHORT).show();
                                            Intent i = new Intent(UpdateCourseActivity.this, HomeActivity.class);
                                            startActivity(i);
                                        } else {
                                            // below line is for error handling.
                                            Toast.makeText(UpdateCourseActivity.this, "Fail to update data " + e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
                                        }
                                    }
                                });
                            } else {
                                // on below line we are displaying a message
                                // if we don't get the object from its id.
                                Toast.makeText(UpdateCourseActivity.this, "Fail to update course " + e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
                            }
                        }
                    });
                } else {
                    // this is error handling if we don't get the id for our object
                    Toast.makeText(UpdateCourseActivity.this, "Fail to get object ID..", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

Now run your app and see the output of the app. Make sure to add the course before performing this operation. 

Output:

Below is the file structure in Android Studio after performing the Delete operation: