如何在Android的Back4App数据库中删除数据？

我们已经看到将数据添加到Back4App数据库，以及读取和更新该数据。在本文中，我们将看一下删除此数据。

我们要建立什么?

我们将在要更新数据的更新屏幕内添加一个简单的按钮，并在该屏幕内添加用于删除课程的新按钮。删除后，该课程将从我们的数据库中删除。

分步实施

步骤1：创建一个新按钮以删除activity_update_course.xml文件中的数据

由于我们在上一篇文章中创建了一个新的更新课程活动。因此，我们只需向其添加一个新按钮。将以下代码片段添加到activity_update_course.xml文件中。

以下是activity_update_course.xml文件的更新代码。

步骤2：现在我们必须在UpdateCourseActivity中初始化此按钮。 Java文件并将onClickListner添加到它

转到UpdateCourseActivity。 Java文件，并添加以下代码段。

Java

// Adding on click listner 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 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() {
            @Override
            public void done(List 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();
                }
            }
     });
}

下面是UpdateCourseActivity的更新代码。 Java文件。

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 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() {
            @Override
            public void done(List 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 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() {
            @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() {
                        @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();
                }
            }
        });
    }
}

现在运行您的应用程序，并查看该应用程序的输出。确保在执行此操作之前添加课程。

输出：

下面是执行Delete操作后Android Studio中的文件结构：