原文:https://www . geesforgeks . org/如何从安卓领域数据库读取数据/
在前一篇文章中,我们已经看到在安卓中向领域数据库添加数据。在本文中,我们将看一下在安卓应用程序中从我们的领域数据库中读取这些数据。
在本文中,我们将简单地添加一个按钮来打开我们在上一篇文章中创建的项目中的一个新活动。在新活动中,我们将以回收视图的形式显示数据。下面是视频,我们将在其中看到我们将在本文中构建的内容。
[https://media.geeksforgeeks.org/wp-content/uploads/20210319223942/20210319_222916.mp4](https://media.geeksforgeeks.org/wp-content/uploads/20210319223942/20210319_222916.mp4)第一步:在应用项目的 build.gradle 文件中添加 google repository。
buildscript {
存储库{
谷歌()
mavenuentral _)
}
所有的 Jetpack 组件都可以在 Google Maven 存储库中找到,包括在 build.gradle 文件中。
所有项目{
存储库{
谷歌()
mavenuentral _)
}
}
步骤 2:使用 activity_main.xml 文件
转到 activity_main.xml 文件,添加一个新的 按钮 打开一个新的活动来显示我们的课程列表。
<Button
android:id="@+id/idBtnReadData"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:visibility="visible"
android:layout_margin="10dp"
android:text="Read Course Details"
android:textAllCaps="false" />
下面是添加上述代码片段后 activity_main.xml 文件的更新代码。
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=".MainActivity">
<EditText
android:id="@+id/idEdtCourseName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter course Name" />
<EditText
android:id="@+id/idEdtCourseDuration"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter Course Duration" />
<EditText
android:id="@+id/idEdtCourseTracks"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter Course Tracks" />
<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
android:id="@+id/idBtnAddCourse"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:text="Add Course"
android:textAllCaps="false" />
<Button
android:id="@+id/idBtnReadData"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:text="Read Course Details"
android:textAllCaps="false"
android:visibility="visible" />
LinearLayout>
步骤 3:使用 MainActivity.java 文件
由于我们在 activity_main.xml 文件中添加了一个新按钮,因此我们必须在MainActivity.java文件中的那个按钮上添加 setOnClickListener() 。
readCourseBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(MainActivity.this, ReadCoursesActivity.class);
startActivity(i);
}
});
以下是添加上述代码片段后MainActivity.java文件的更新代码。
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 MainActivity extends AppCompatActivity {
// creating variables for our edit text
private EditText courseNameEdt, courseDurationEdt, courseDescriptionEdt, courseTracksEdt;
private Realm realm;
private Button readCourseBtn;
// creating a strings for storing
// our values from edittext fields.
private String courseName, courseDuration, courseDescription, courseTracks;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// initializing our edittext and buttons
realm = Realm.getDefaultInstance();
courseNameEdt = findViewById(R.id.idEdtCourseName);
courseDescriptionEdt = findViewById(R.id.idEdtCourseDescription);
courseDurationEdt = findViewById(R.id.idEdtCourseDuration);
readCourseBtn = findViewById(R.id.idBtnReadData);
// creating variable for button
Button submitCourseBtn = findViewById(R.id.idBtnAddCourse);
courseTracksEdt = findViewById(R.id.idEdtCourseTracks);
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();
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 {
// calling method to add data to Realm database..
addDataToDatabase(courseName, courseDescription, courseDuration, courseTracks);
Toast.makeText(MainActivity.this, "Course added to database..", Toast.LENGTH_SHORT).show();
courseNameEdt.setText("");
courseDescriptionEdt.setText("");
courseDurationEdt.setText("");
courseTracksEdt.setText("");
}
}
});
readCourseBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(MainActivity.this, ReadCoursesActivity.class);
startActivity(i);
}
});
}
private void addDataToDatabase(String courseName, String courseDescription, String courseDuration, String courseTracks) {
// on below line we are creating
// a variable for our modal class.
DataModal modal = new DataModal();
// on below line we are getting id for the course which we are storing.
Number id = realm.where(DataModal.class).max("id");
// on below line we are
// creating a variable for our id.
long nextId;
// validating if id is null or not.
if (id == null) {
// if id is null
// we are passing it as 1.
nextId = 1;
} else {
// if id is not null then
// we are incrementing it by 1
nextId = id.intValue() + 1;
}
// on below line we are setting the
// data entered by user in our modal class.
modal.setId(nextId);
modal.setCourseDescription(courseDescription);
modal.setCourseName(courseName);
modal.setCourseDuration(courseDuration);
modal.setCourseTracks(courseTracks);
// on below line we are calling a method to execute a transaction.
realm.executeTransaction(new Realm.Transaction() {
@Override
public void execute(Realm realm) {
// inside on execute method we are calling a method
// to copy to real m database from our modal class.
realm.copyToRealm(modal);
}
});
}
}
第 4 步:创建一个新的活动来显示我们的课程列表
要创建新的活动,我们必须导航到应用程序> java >您的应用程序的包名>右键单击包名>新建>空活动并将您的活动命名为readcourses 活动并创建新的活动。确保选择空活动。
第五步:为我们的回收视图创建一个新的布局文件
导航至 app > res >布局>右键点击>新建>布局资源文件并将其命名为 course_rv_item 并添加以下代码。
xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:elevation="8dp"
app:cardCornerRadius="4dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="3dp"
android:orientation="vertical">
<TextView
android:id="@+id/idTVCourseName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="3dp"
android:text="Course Name"
android:textColor="@color/black" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:weightSum="2">
<TextView
android:id="@+id/idTVCourseTracks"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:padding="3dp"
android:text="Course Tracks"
android:textColor="@color/black" />
<TextView
android:id="@+id/idTVCourseDuration"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:padding="3dp"
android:text="Duration"
android:textColor="@color/black" />
LinearLayout>
<TextView
android:id="@+id/idTVCourseDescription"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:padding="3dp"
android:text="Description"
android:textColor="@color/black" />
LinearLayout>
androidx.cardview.widget.CardView>
第 6 步:创建一个适配器类,用于将数据设置到我们的 RecyclerView 项目中
导航到应用> java >你的应用的包名>右键点击它>新建> Java 类并将其命名为 CourseRVAdapter 并添加下面的代码。代码中添加了注释,以更详细地理解代码。
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.List;
public class CourseRVAdapter extends RecyclerView.Adapter<CourseRVAdapter.ViewHolder> {
// variable for our array list and context
private List<DataModal> dataModalArrayList;
private Context context;
public CourseRVAdapter(List<DataModal> dataModalArrayList, Context context) {
this.dataModalArrayList = dataModalArrayList;
this.context = context;
}
@NonNull
@Override
public CourseRVAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
// on below line we are inflating our layout
// file for our recycler view items.
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.course_rv_item, parent, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(@NonNull CourseRVAdapter.ViewHolder holder, int position) {
DataModal modal = dataModalArrayList.get(position);
holder.courseNameTV.setText(modal.getCourseName());
holder.courseDescTV.setText(modal.getCourseDescription());
holder.courseDurationTV.setText(modal.getCourseDuration());
holder.courseTracksTV.setText(modal.getCourseTracks());
}
@Override
public int getItemCount() {
return dataModalArrayList.size();
}
public class ViewHolder extends RecyclerView.ViewHolder {
// creating variables for our text views.
private TextView courseNameTV, courseDescTV, courseDurationTV, courseTracksTV;
public ViewHolder(@NonNull View itemView) {
super(itemView);
// initializing our text views
courseNameTV = itemView.findViewById(R.id.idTVCourseName);
courseDescTV = itemView.findViewById(R.id.idTVCourseDescription);
courseDurationTV = itemView.findViewById(R.id.idTVCourseDuration);
courseTracksTV = itemView.findViewById(R.id.idTVCourseTracks);
}
}
}
第 7 步:使用 activity_read_courses.xml 文件
导航到应用程序> res >布局>activity _ read _ courses . XML并将下面的代码添加到该文件中。下面是活动文件的代码。
xml version="1.0" encoding="utf-8"?>
<RelativeLayout
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"
tools:context=".ReadCoursesActivity">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/idRVCourses"
android:layout_width="match_parent"
android:layout_height="match_parent" />
RelativeLayout>
步骤 8:使用 ReadCoursesActivity.java 文件
导航到应用程序> java >你的应用程序的包名>ReadCoursesActivity.java文件,并添加下面的代码。代码中添加了注释,以更详细地理解代码。
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import java.util.ArrayList;
import java.util.List;
import io.realm.Realm;
public class ReadCoursesActivity extends AppCompatActivity {
List<DataModal> dataModals;
// creating variables for realm,
// recycler view, adapter and our list.
private Realm realm;
private RecyclerView coursesRV;
private CourseRVAdapter courseRVAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_read_courses);
// on below lines we are initializing our variables.
coursesRV = findViewById(R.id.idRVCourses);
realm = Realm.getDefaultInstance();
dataModals = new ArrayList<>();
// calling a method to load
// our recycler view with data.
prepareRecyclerView();
}
private void prepareRecyclerView() {
// on below line we are getting data from realm database in our list.
dataModals = realm.where(DataModal.class).findAll();
// on below line we are adding our list to our adapter class.
courseRVAdapter = new CourseRVAdapter(dataModals, this);
// on below line we are setting layout manager to our recycler view.
coursesRV.setLayoutManager(new LinearLayoutManager(this));
// at last we are setting adapter to our recycler view.
coursesRV.setAdapter(courseRVAdapter);
}
}
现在运行您的应用程序,并查看应用程序的输出。
输出:
[https://media.geeksforgeeks.org/wp-content/uploads/20210319223942/20210319_222916.mp4](https://media.geeksforgeeks.org/wp-content/uploads/20210319223942/20210319_222916.mp4)以下是执行读取操作后的完整项目文件结构: