如何从Android中的Back4App数据库读取数据？

我们已经看到了将数据添加到Android应用程序中的Back4App数据库中。在本文中，我们将看一下从Android应用程序中的数据库中读取此数据的方法。

我们将在本文中构建什么?

我们将在先前的应用程序中创建一个新屏幕，在该屏幕中，我们将显示在RecyclerView中添加到Back4App数据库中的数据。

分步实施

步骤1：使用activity_main.xml文件

转到activity_main.xml文件，再添加一个按钮，以显示所有已添加课程的列表。下面是代码片段，最后添加了代码。

现在下面是activity_main.xml文件的更新代码 添加以上代码段后。

第2步：现在，我们将创建新的Activity，以在RecyclerView中显示Firebase Firestore中的数据

要创建一个新的活动，我们必须导航到应用程序> Java >您的应用程序的包名称>右键单击包名称>新建>空活动，并将您的活动命名为HomeActivity并创建新的活动。确保选择空白活动。

第3步：创建用于存储数据的模式类。

导航到应用程序> Java >应用程序的程序包名称>右键单击它>新建> Java类，并将其命名为CourseModal，然后将以下代码添加到其中。在代码中添加了注释，以便更详细地了解。

Java

public class CourseModal {
 
    // variables for our course name,
    // description and duration.
    private String courseName;
    private String courseDescription;
    private String courseDuration;
 
    // constructor class.
    public CourseModal(String courseName, String courseDescription, String courseDuration) {
        this.courseName = courseName;
        this.courseDescription = courseDescription;
        this.courseDuration = courseDuration;
    }
 
    // 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;
    }
}

步骤4：现在，我们将为RecyclerView项目创建一个卡片布局

要为RecyclerView创建新项目，请导航至应用程序> res>布局>右键单击layout>新建>布局资源文件，并将其命名为course_rv_item.xml，然后在其中添加以下代码。

步骤5：现在我们将创建Adapter类，该类将处理RecyclerView项的数据

要为我们的Recycler View创建Adapter类，请导航至应用程序> Java >您的应用程序包名称>右键单击它，然后单击>新建> Java类，并将您的类命名为CourseRVAdapter 。创建这些类后，将以下代码添加到其中。在代码中添加了注释，以更详细地了解。

Java

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
 
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
 
import java.util.ArrayList;
 
public class CourseRVAdapter extends RecyclerView.Adapter {
    private Context context;
    private ArrayList courseModalArrayList;
 
    // creating a constructor class.
    public CourseRVAdapter(Context context, ArrayList courseModalArrayList) {
        this.context = context;
        this.courseModalArrayList = courseModalArrayList;
    }
 
    @NonNull
    @Override
    public CourseRVAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        // passing our layout file for displaying our card item
        return new ViewHolder(LayoutInflater.from(context).inflate(R.layout.course_rv_item, parent, false));
    }
 
    @Override
    public void onBindViewHolder(@NonNull CourseRVAdapter.ViewHolder holder, int position) {
        // setting data to our text views from our modal class.
        CourseModal courses = courseModalArrayList.get(position);
        holder.courseNameTV.setText(courses.getCourseName());
        holder.courseDurationTV.setText(courses.getCourseDuration());
        holder.courseDescTV.setText(courses.getCourseDescription());
    }
 
    @Override
    public int getItemCount() {
        return courseModalArrayList.size();
    }
 
    public class ViewHolder extends RecyclerView.ViewHolder {
        // creating variables for our text views.
        private final TextView courseNameTV;
        private final TextView courseDurationTV;
        private final TextView courseDescTV;
 
        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            // initializing our text views.
            courseNameTV = itemView.findViewById(R.id.idTVCourseName);
            courseDurationTV = itemView.findViewById(R.id.idTVCourseDuration);
            courseDescTV = itemView.findViewById(R.id.idTVCourseDescription);
        }
    }
}

步骤6：使用activity_home.xml文件

导航至应用程序> res>布局> activity_home.xml文件，然后将以下代码添加到其中。

步骤7：使用HomeActivity。 Java文件

导航到应用程序> Java >应用程序的程序包名称> HomeActivity。 Java文件，并向其中添加以下代码。

Java

import android.os.Bundle;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;
 
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
 
import com.parse.FindCallback;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
 
import java.util.ArrayList;
import java.util.List;
 
public class HomeActivity extends AppCompatActivity {
     
    // creating variables for our recycler view,
    // progressbar, array list and adapter class.
    private RecyclerView coursesRV;
    private ProgressBar loadingPB;
    private ArrayList courseModalArrayList;
    private CourseRVAdapter courseRVAdapter;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
         
        // initializing our views.
        loadingPB = findViewById(R.id.idProgressBar);
        coursesRV = findViewById(R.id.idRVCourses);
        courseModalArrayList = new ArrayList<>();
         
        // calling a method to
        // load recycler view.
        prepareCourseRV();
        
        // calling a method to get
        // the data from database.
        getDataFromServer();
    }
 
    private void getDataFromServer() {
        // Configure Query with our query.
        ParseQuery query = ParseQuery.getQuery("courses");
        query.findInBackground(new FindCallback() {
            @Override
            public void done(List objects, ParseException e) {
                // in done method checking if the error is null or not.
                if (e == null) {
                    // Adding objects into the Array
                    // if error is not null we are getting data from
                    // our object and passing it to our array list.
                    for (int i = 0; i < objects.size(); i++) {
                         
                        // on below line we are extracting our
                        // data and adding it ot our array list
                        String courseName = objects.get(i).getString("courseName");
                        String courseDescription = objects.get(i).getString("courseDescription");
                        String courseDuration = objects.get(i).getString("courseDuration");
                         
                        // on below line we are adding data to our array list.
                        courseModalArrayList.add(new CourseModal(courseName, courseDescription, courseDuration));
                    }
                    // notifying adapter class on adding new data.
                    courseRVAdapter.notifyDataSetChanged();
                    loadingPB.setVisibility(View.GONE);
                } else {
                    // handling error if we get any error.
                    Toast.makeText(HomeActivity.this, "Fail to get data..", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
 
    private void prepareCourseRV() {
         
        coursesRV.setHasFixedSize(true);
        coursesRV.setLayoutManager(new LinearLayoutManager(this));
         
        // adding our array list to our recycler view adapter class.
        courseRVAdapter = new CourseRVAdapter(this, courseModalArrayList);
         
        // setting adapter to our recycler view.
        coursesRV.setAdapter(courseRVAdapter);
    }
}

步骤8：使用MainActivity。 Java文件

在MainActivity中。 Java文件，我们必须向HomeActivity文件添加一个Intent 。下面是执行此操作的代码段。

Java

readCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // opening new intent to read our data.
                Intent i = new Intent(MainActivity.this, HomeActivity.class);
                startActivity(i);
       }
});

以下是MainActivity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.ParseException;
import com.parse.ParseObject;
import com.parse.SaveCallback;
 
public class MainActivity extends AppCompatActivity {
 
    // creating variables for our edit text
    private EditText courseNameEdt, courseDurationEdt, courseDescriptionEdt;
 
    // creating variable for button
    private Button submitCourseBtn, readCourseBtn;
 
    // creating a strings for storing our values from edittext fields.
    private String courseName, courseDuration, courseDescription;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        // initializing our edittext and buttons
        readCourseBtn = findViewById(R.id.idBtnReadCourse);
        courseNameEdt = findViewById(R.id.idEdtCourseName);
        courseDescriptionEdt = findViewById(R.id.idEdtCourseDescription);
        courseDurationEdt = findViewById(R.id.idEdtCourseDuration);
        submitCourseBtn = findViewById(R.id.idBtnSubmitCourse);
 
        readCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // opening new intent to read our data.
                Intent i = new Intent(MainActivity.this, HomeActivity.class);
                startActivity(i);
            }
        });
 
        submitCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
 
                // getting data from edittext fields.
                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 add data to Firebase Firestore.
                    addDataToDatabase(courseName, courseDescription, courseDuration);
                }
            }
        });
    }
 
    private void addDataToDatabase(String courseName, String courseDescription, String courseDuration) {
 
        // Configure Query
        ParseObject courseList = new ParseObject("courses");
 
        // on below line we are adding our data with their key value in our object.
        courseList.put("courseName", courseName);
        courseList.put("courseDescription", courseDescription);
        courseList.put("courseDuration", courseDuration);
 
        // after adding all data we are calling a
        // method to save our data in background.
        courseList.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 we are displaying a simple toast message.
                    Toast.makeText(MainActivity.this, "Data has been successfully added to Database", Toast.LENGTH_SHORT).show();
 
                    // on below line we are setting our edit text fields to empty value.
                    courseNameEdt.setText("");
                    courseDescriptionEdt.setText("");
                    courseDurationEdt.setText("");
                } else {
                    // if the error is not null we will be
                    // displaying an error message to our user.
                    Toast.makeText(getApplicationContext(), e.getMessage().toString(), Toast.LENGTH_LONG).show();
                }
            }
        });
    }
}

现在运行您的应用程序，并查看该应用程序的输出。

输出：

以下是执行读取操作后Android Studio中的文件结构：