为什么我的循环只输出最后一个结果？
Why is my loop only outputting the last result?
我正在构建一个 Android Studio 应用程序，可以在其中将数据添加到 SQLite 数据库。当我想将数据输出到页面时，它只显示数据库中的最后一个结果（它会覆盖）。我怎样才能防止这种情况发生并输出所有数据？
这是我的代码：
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
TextView textViewID = (TextView) findViewById(R.id.textViewID);
textViewID.setText("Id: " + res.getString(0) + "\n");
TextView textViewName = (TextView) findViewById(R.id.textViewName);
textViewName.setText("Name: " + res.getString(1) + "\n");
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
textViewDescription.setText("Description: " + res.getString(2) + "\n");
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
textViewLocation.setText("Location: " + res.getString(4) + "\n");
}
// show all the data
}
预期的结果是数据库中的所有数据，但它只输出最后一个条目。
编辑
MyActivity.java
package com.example.triptracker;
import android.content.Intent;
import android.database.Cursor;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
public class MomentsActivity extends AppCompatActivity {
private TextView mTextMessage;
private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
= new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.navigation_moments:
Intent intent = new Intent(MomentsActivity.this, MomentsActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivity(intent);
break;
case R.id.navigation_addmoment:
Intent intent2 = new Intent(MomentsActivity.this, AddMomentActivity.class);
intent2.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivity(intent2);
break;
}
return false;
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
myDb = new DatabaseHelper(this);
setContentView(R.layout.activity_moments);
BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
mList = this.findViewById(R.id.my_listview);
manageListView();
}
DatabaseHelper myDb;
EditText editTitle, editDescription, editLocation;
Button btnAddData;
Button btnViewAll;
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
String[] columns_to_list = new String[]{
DatabaseHelper.COL_1,
DatabaseHelper.COL_2,
DatabaseHelper.COL_3,
DatabaseHelper.COL_4
};
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
}
DatabaseHelper.java
package com.example.triptracker;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "triptracker.db";
public static final String TABLE_NAME = "trip_table";
public static final String COL_1 = "trip_id";
public static final String COL_2 = "trip_title";
public static final String COL_3 = "trip_description";
public static final String COL_4 = "trip_image";
public static final String COL_5 = "trip_location";
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table " + TABLE_NAME + " (" + COL_1 + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COL_2 + " TEXT, " + COL_3 + " TEXT, " + COL_4 + " TEXT, " + COL_5 + " TEXT)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("drop table if exists " + TABLE_NAME);
onCreate(db);
}
// TODO: Add image functionality.
public boolean insertData(String title, String description, String location) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(COL_2, title);
contentValues.put(COL_3, description);
// contentValues.put(COL_4, image);
contentValues.put(COL_5, location);
long result = db.insert(TABLE_NAME, null, contentValues);
if (result == -1)
return false;
else
return true;
}
public Cursor getAllData() {
SQLiteDatabase db = this.getWritableDatabase();
Cursor res = db.rawQuery("select * from " + TABLE_NAME, null);
return res;
}
}
输出这些错误：
2019-05-09 14:23:47.959 21253-21253/com.example.triptracker E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.triptracker, PID: 21253
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.triptracker/com.example.triptracker.MomentsActivity}: java.lang.IllegalArgumentException: column '_id' does not exist. Available columns: [trip_id, trip_title, trip_description, trip_image, trip_location]
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.IllegalArgumentException: column '_id' does not exist. Available columns: [trip_id, trip_title, trip_description, trip_image, trip_location]
at android.database.AbstractCursor.getColumnIndexOrThrow(AbstractCursor.java:340)
at android.widget.CursorAdapter.init(CursorAdapter.java:180)
at android.widget.CursorAdapter.<init>(CursorAdapter.java:157)
at android.widget.ResourceCursorAdapter.<init>(ResourceCursorAdapter.java:96)
at android.widget.SimpleCursorAdapter.<init>(SimpleCursorAdapter.java:104)
at com.example.triptracker.MomentsActivity.manageListView(MomentsActivity.java:81)
at com.example.triptracker.MomentsActivity.onCreate(MomentsActivity.java:52)
at android.app.Activity.performCreate(Activity.java:7136)
at android.app.Activity.performCreate(Activity.java:7127)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-05-09 14:23:47.983 21253-21253/com.example.triptracker I/Process: Sending signal. PID: 21253 SIG: 9
原因是对于循环的每次迭代，您都将 TextView 的值设置为该迭代的值，因此您可能只会看到最后一行。
你可以使用：-
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
TextView textViewID = (TextView) findViewById(R.id.textViewID);
textViewID.setText("");
TextView textViewName = (TextView) findViewById(R.id.textViewName);
textViewName.setText("");
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
textViewDescription.setText("");
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
textViewLocation.setText("");
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
textViewId.setText(textViewID.getText().toString()+res.getString(0) + "\n");
textViewName.setText(textViewName.getText().toString()+"Name: " + res.getString(1) + "\n");
textViewDescription.setText(textViewDescription.getText().toString() + "Description: " + res.getString(2) + "\n");
textViewLocation.setText(textViewLocation.getText().toString() + "Location: " + res.getString(4) + "\n");
}
}
或者更好的是
- 循环中的字符串连接效率低下
- 这也只在第一个之后附加一个换行符。
:-
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
StringBuilder idsb = new StringBuilder();
StringBuilder namesb = new StringBuilder();
StringBuilder descsb = new StringBuilder();
StringBuilder locsb = new StringBuilder();
String endofline = "";
TextView textViewID = (TextView) findViewById(R.id.textViewID);
TextView textViewName = (TextView) findViewById(R.id.textViewName);
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
idsb.append("Id: ").append(res.getString(0)).append(endofline);
namesb.append("Name: ").append(res.getString(1)).append(endofline);
descsb.append("Description: ").append(res.getString(2)).append(endofline);
locsb.append("Location: ").append(res.getString(4)).append(endofline);
if (endofline.length() == 0) {
endofline = "\n";
}
}
textViewId.setText(idsb.toString());
textViewName.setText(namesb.toString());
textViewDescription.setText(descsb.toString());
textViewLocation.setText(locsb.toString());
}
但是，您可能想要使用 ListView 或 RecyclerView。
附加 - 使用 ListView
重要提示
这个例子使用了一个 CursorAdapter，这让生活变得非常轻松，但他们要求存在一个专门命名为 _id 的列（id 不会）。您可能需要更改 table 以便将列命名为 _id（您可以使用常量 BaseColumns._ID，如下面的 DBHelper 中所用）。还有其他方法，例如在 SELECT （查询）中创建一个 _id 列，例如SELECT rowid AS _id,* FROM the_table.
1 在 activity 的布局中添加（替换 id/name/description/location 的文本视图）一个 ListView，确保给它一个 id，例如:-
<ListView
android:id="@+id/my_listview"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
</ListView>
2 创建一个新布局 mylistview_item.xml，这将有 4 个文本视图，例如
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:id="@+id/textview_id"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_description"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_location"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>
- 这将是用于列表视图中每一行（项目）的布局
3 添加以下class变量：-
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
4 添加以下 int 数组作为 class 变量 :-
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
- 请注意，这些是布局中 TextView 的 ID mylistview_item.xml，适配器将使用它们将数据从列映射到 TextView。
5 类似地为要从中检索数据的列名添加一个字符串数组；但请注意，您 will/may 必须更改列名（以下使用数据库助手中定义的常量（建议定义此类常量并始终使用常量而不是对名称进行硬编码））例如:-
String[] columns_to_list = new String[]{
DBHelper.COL_MYTABLE_ID,
DBHelper.COl_MYTABLE_NAME,
DBHelper.COL_MYTABLE_DESCRIPTION,
DBHelper.COl_MYTABLE_LOCATION
};
6 在activity中添加如下方法：-
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
- 正如方法所说，这是管理 ListView 的。它从 db 获取数据到 Cursor 中，构造（实例化）SimpleCursorAdapter，如果它没有被实例化，然后它将适配器绑定到 ListView。如果适配器已被实例化，它会告诉适配器有一个新的（已更改的）游标（因此列表已刷新）。
7
将以下行添加到 activity 的 onCreate 方法中：-
mList = this.findViewById(R.id.my_listview);
myDb = new DBHelper(this); //<<<<<<<<<< SHOULD ALREADY HAVE SOMETHING LIKE THIS LEAVE AS IT IS
manageListView();
8 使用 :-
覆盖 activity 的 onResume 和 onDestroy 方法
@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
- 目前这还不是必需的，但是很好的做法。
onDestroy 在 activity 被销毁时关闭游标（游标应该在完成时关闭）
- 对主要来说不太重要activity
onResume 被覆盖以调用 manageListView() 方法（尚未添加）并将导致列表视图显示来自另一个 activity return 的最新数据.
我用来测试的activity是:-
public class MainActivity extends AppCompatActivity {
DBHelper myDb;
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
String[] columns_to_list = new String[]{
DBHelper.COL_MYTABLE_ID,
DBHelper.COl_MYTABLE_NAME,
DBHelper.COL_MYTABLE_DESCRIPTION,
DBHelper.COl_MYTABLE_LOCATION
};
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mList = this.findViewById(R.id.my_listview);
myDb = new DBHelper(this);
forTestingAddSomeData();
manageListView();
}
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
private void forTestingAddSomeData() {
if(DatabaseUtils.queryNumEntries(myDb.getWritableDatabase(),DBHelper.TABLE_MYTABLE) < 1) {
myDb.add("Test001","This is a test","Home");
myDb.add("Test002","For Testing","Garage");
myDb.add("Test003","Test using this","Loft");
myDb.add("Test004","Yet again for testing","Cupboard");
}
}
@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
}
- 注意forTestingAddSomeData，调用该方法只是为了添加一些测试数据。逐步指南中省略了添加和使用此方法（以便我可以显示一些数据）。
如果您遇到问题或想使用完整代码，那么 DBHelper class 是 :-
public class DBHelper extends SQLiteOpenHelper {
public static final String DBNAME = "mydb";
public static final int DBVERSION = 1;
public static final String TABLE_MYTABLE = "mytable";
public static final String COL_MYTABLE_ID = BaseColumns._ID;
public static final String COl_MYTABLE_NAME = "name";
public static final String COL_MYTABLE_DESCRIPTION = "description";
public static final String COl_MYTABLE_LOCATION = "location";
SQLiteDatabase mDB;
public DBHelper(Context context) {
super(context, DBNAME, null, DBVERSION);
mDB = this.getWritableDatabase();
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE IF NOT EXISTS " + TABLE_MYTABLE +
"(" +
COL_MYTABLE_ID + " INTEGER PRIMARY KEY," +
COl_MYTABLE_NAME + " TEXT, " +
COL_MYTABLE_DESCRIPTION + " TEXT, " +
COl_MYTABLE_LOCATION + " TEXT" +
")"
);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public long add(String name, String description, String location) {
ContentValues cv = new ContentValues();
cv.put(COl_MYTABLE_NAME,name);
cv.put(COL_MYTABLE_DESCRIPTION,description);
cv.put(COl_MYTABLE_LOCATION,location);
return mDB.insert(TABLE_MYTABLE,null,cv);
}
public Cursor getAllData() {
return mDB.query(TABLE_MYTABLE,null,null,null,null,null,null);
}
}
结果
上面运行的结果是：-
我正在构建一个 Android Studio 应用程序，可以在其中将数据添加到 SQLite 数据库。当我想将数据输出到页面时，它只显示数据库中的最后一个结果（它会覆盖）。我怎样才能防止这种情况发生并输出所有数据？
这是我的代码：
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
TextView textViewID = (TextView) findViewById(R.id.textViewID);
textViewID.setText("Id: " + res.getString(0) + "\n");
TextView textViewName = (TextView) findViewById(R.id.textViewName);
textViewName.setText("Name: " + res.getString(1) + "\n");
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
textViewDescription.setText("Description: " + res.getString(2) + "\n");
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
textViewLocation.setText("Location: " + res.getString(4) + "\n");
}
// show all the data
}
预期的结果是数据库中的所有数据，但它只输出最后一个条目。
编辑
MyActivity.java
package com.example.triptracker;
import android.content.Intent;
import android.database.Cursor;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
public class MomentsActivity extends AppCompatActivity {
private TextView mTextMessage;
private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
= new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.navigation_moments:
Intent intent = new Intent(MomentsActivity.this, MomentsActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivity(intent);
break;
case R.id.navigation_addmoment:
Intent intent2 = new Intent(MomentsActivity.this, AddMomentActivity.class);
intent2.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivity(intent2);
break;
}
return false;
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
myDb = new DatabaseHelper(this);
setContentView(R.layout.activity_moments);
BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
mList = this.findViewById(R.id.my_listview);
manageListView();
}
DatabaseHelper myDb;
EditText editTitle, editDescription, editLocation;
Button btnAddData;
Button btnViewAll;
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
String[] columns_to_list = new String[]{
DatabaseHelper.COL_1,
DatabaseHelper.COL_2,
DatabaseHelper.COL_3,
DatabaseHelper.COL_4
};
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
}
DatabaseHelper.java
package com.example.triptracker;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "triptracker.db";
public static final String TABLE_NAME = "trip_table";
public static final String COL_1 = "trip_id";
public static final String COL_2 = "trip_title";
public static final String COL_3 = "trip_description";
public static final String COL_4 = "trip_image";
public static final String COL_5 = "trip_location";
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table " + TABLE_NAME + " (" + COL_1 + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COL_2 + " TEXT, " + COL_3 + " TEXT, " + COL_4 + " TEXT, " + COL_5 + " TEXT)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("drop table if exists " + TABLE_NAME);
onCreate(db);
}
// TODO: Add image functionality.
public boolean insertData(String title, String description, String location) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(COL_2, title);
contentValues.put(COL_3, description);
// contentValues.put(COL_4, image);
contentValues.put(COL_5, location);
long result = db.insert(TABLE_NAME, null, contentValues);
if (result == -1)
return false;
else
return true;
}
public Cursor getAllData() {
SQLiteDatabase db = this.getWritableDatabase();
Cursor res = db.rawQuery("select * from " + TABLE_NAME, null);
return res;
}
}
输出这些错误：
2019-05-09 14:23:47.959 21253-21253/com.example.triptracker E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.triptracker, PID: 21253
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.triptracker/com.example.triptracker.MomentsActivity}: java.lang.IllegalArgumentException: column '_id' does not exist. Available columns: [trip_id, trip_title, trip_description, trip_image, trip_location]
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.IllegalArgumentException: column '_id' does not exist. Available columns: [trip_id, trip_title, trip_description, trip_image, trip_location]
at android.database.AbstractCursor.getColumnIndexOrThrow(AbstractCursor.java:340)
at android.widget.CursorAdapter.init(CursorAdapter.java:180)
at android.widget.CursorAdapter.<init>(CursorAdapter.java:157)
at android.widget.ResourceCursorAdapter.<init>(ResourceCursorAdapter.java:96)
at android.widget.SimpleCursorAdapter.<init>(SimpleCursorAdapter.java:104)
at com.example.triptracker.MomentsActivity.manageListView(MomentsActivity.java:81)
at com.example.triptracker.MomentsActivity.onCreate(MomentsActivity.java:52)
at android.app.Activity.performCreate(Activity.java:7136)
at android.app.Activity.performCreate(Activity.java:7127)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-05-09 14:23:47.983 21253-21253/com.example.triptracker I/Process: Sending signal. PID: 21253 SIG: 9
原因是对于循环的每次迭代，您都将 TextView 的值设置为该迭代的值，因此您可能只会看到最后一行。
你可以使用：-
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
TextView textViewID = (TextView) findViewById(R.id.textViewID);
textViewID.setText("");
TextView textViewName = (TextView) findViewById(R.id.textViewName);
textViewName.setText("");
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
textViewDescription.setText("");
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
textViewLocation.setText("");
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
textViewId.setText(textViewID.getText().toString()+res.getString(0) + "\n");
textViewName.setText(textViewName.getText().toString()+"Name: " + res.getString(1) + "\n");
textViewDescription.setText(textViewDescription.getText().toString() + "Description: " + res.getString(2) + "\n");
textViewLocation.setText(textViewLocation.getText().toString() + "Location: " + res.getString(4) + "\n");
}
}
或者更好的是 - 循环中的字符串连接效率低下 - 这也只在第一个之后附加一个换行符。
:-
public void viewAll() {
Cursor res = myDb.getAllData();
if (res.getCount() == 0) {
// show error
System.out.println("No Data Found");
return;
}
StringBuilder idsb = new StringBuilder();
StringBuilder namesb = new StringBuilder();
StringBuilder descsb = new StringBuilder();
StringBuilder locsb = new StringBuilder();
String endofline = "";
TextView textViewID = (TextView) findViewById(R.id.textViewID);
TextView textViewName = (TextView) findViewById(R.id.textViewName);
TextView textViewDescription = (TextView) findViewById(R.id.textViewDescription);
TextView textViewLocation = (TextView) findViewById(R.id.textViewLocation);
while (res.moveToNext()) {
System.out.println("Id: " + res.getString(0) + "\n");
System.out.println("Title: " + res.getString(1) + "\n");
System.out.println("Description: " + res.getString(2) + "\n");
System.out.println("Location: " + res.getString(4) + "\n\n");
idsb.append("Id: ").append(res.getString(0)).append(endofline);
namesb.append("Name: ").append(res.getString(1)).append(endofline);
descsb.append("Description: ").append(res.getString(2)).append(endofline);
locsb.append("Location: ").append(res.getString(4)).append(endofline);
if (endofline.length() == 0) {
endofline = "\n";
}
}
textViewId.setText(idsb.toString());
textViewName.setText(namesb.toString());
textViewDescription.setText(descsb.toString());
textViewLocation.setText(locsb.toString());
}
但是，您可能想要使用 ListView 或 RecyclerView。
附加 - 使用 ListView
重要提示
这个例子使用了一个 CursorAdapter，这让生活变得非常轻松，但他们要求存在一个专门命名为 _id 的列（id 不会）。您可能需要更改 table 以便将列命名为 _id（您可以使用常量 BaseColumns._ID，如下面的 DBHelper 中所用）。还有其他方法，例如在 SELECT （查询）中创建一个 _id 列，例如SELECT rowid AS _id,* FROM the_table.
1 在 activity 的布局中添加（替换 id/name/description/location 的文本视图）一个 ListView，确保给它一个 id，例如:-
<ListView
android:id="@+id/my_listview"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
</ListView>
2 创建一个新布局 mylistview_item.xml，这将有 4 个文本视图，例如
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:id="@+id/textview_id"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_description"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/textview_location"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>
- 这将是用于列表视图中每一行（项目）的布局
3 添加以下class变量：-
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
4 添加以下 int 数组作为 class 变量 :-
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
- 请注意，这些是布局中 TextView 的 ID mylistview_item.xml，适配器将使用它们将数据从列映射到 TextView。
5 类似地为要从中检索数据的列名添加一个字符串数组；但请注意，您 will/may 必须更改列名（以下使用数据库助手中定义的常量（建议定义此类常量并始终使用常量而不是对名称进行硬编码））例如:-
String[] columns_to_list = new String[]{
DBHelper.COL_MYTABLE_ID,
DBHelper.COl_MYTABLE_NAME,
DBHelper.COL_MYTABLE_DESCRIPTION,
DBHelper.COl_MYTABLE_LOCATION
};
6 在activity中添加如下方法：-
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
- 正如方法所说，这是管理 ListView 的。它从 db 获取数据到 Cursor 中，构造（实例化）SimpleCursorAdapter，如果它没有被实例化，然后它将适配器绑定到 ListView。如果适配器已被实例化，它会告诉适配器有一个新的（已更改的）游标（因此列表已刷新）。
7
将以下行添加到 activity 的 onCreate 方法中：-
mList = this.findViewById(R.id.my_listview);
myDb = new DBHelper(this); //<<<<<<<<<< SHOULD ALREADY HAVE SOMETHING LIKE THIS LEAVE AS IT IS
manageListView();
8 使用 :-
覆盖 activity 的 onResume 和 onDestroy 方法@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
- 目前这还不是必需的，但是很好的做法。
onDestroy在 activity 被销毁时关闭游标（游标应该在完成时关闭）- 对主要来说不太重要activity
onResume被覆盖以调用manageListView()方法（尚未添加）并将导致列表视图显示来自另一个 activity return 的最新数据.
我用来测试的activity是:-
public class MainActivity extends AppCompatActivity {
DBHelper myDb;
SimpleCursorAdapter mSCA; //Adapts/Handles the data for the listview
ListView mList;
Cursor mCsr;
String[] columns_to_list = new String[]{
DBHelper.COL_MYTABLE_ID,
DBHelper.COl_MYTABLE_NAME,
DBHelper.COL_MYTABLE_DESCRIPTION,
DBHelper.COl_MYTABLE_LOCATION
};
int[] item_layout_ids_for_list = new int[]{
R.id.textview_id,
R.id.textview_name,
R.id.textview_description,
R.id.textview_location
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mList = this.findViewById(R.id.my_listview);
myDb = new DBHelper(this);
forTestingAddSomeData();
manageListView();
}
private void manageListView() {
mCsr = myDb.getAllData();
if (mSCA == null) {
// Builds the Adapter for the List
mSCA = new SimpleCursorAdapter(
this,
R.layout.mylistview_item, mCsr,
columns_to_list,
item_layout_ids_for_list,
0
);
mList.setAdapter(mSCA); // Ties the Adapter to the ListView
} else {
mSCA.swapCursor(mCsr); // Refresh the List
}
}
private void forTestingAddSomeData() {
if(DatabaseUtils.queryNumEntries(myDb.getWritableDatabase(),DBHelper.TABLE_MYTABLE) < 1) {
myDb.add("Test001","This is a test","Home");
myDb.add("Test002","For Testing","Garage");
myDb.add("Test003","Test using this","Loft");
myDb.add("Test004","Yet again for testing","Cupboard");
}
}
@Override
protected void onDestroy() {
super.onDestroy();
mCsr.close();
}
@Override
protected void onResume() {
super.onResume();
manageListView();
}
}
- 注意forTestingAddSomeData，调用该方法只是为了添加一些测试数据。逐步指南中省略了添加和使用此方法（以便我可以显示一些数据）。
如果您遇到问题或想使用完整代码，那么 DBHelper class 是 :-
public class DBHelper extends SQLiteOpenHelper {
public static final String DBNAME = "mydb";
public static final int DBVERSION = 1;
public static final String TABLE_MYTABLE = "mytable";
public static final String COL_MYTABLE_ID = BaseColumns._ID;
public static final String COl_MYTABLE_NAME = "name";
public static final String COL_MYTABLE_DESCRIPTION = "description";
public static final String COl_MYTABLE_LOCATION = "location";
SQLiteDatabase mDB;
public DBHelper(Context context) {
super(context, DBNAME, null, DBVERSION);
mDB = this.getWritableDatabase();
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE IF NOT EXISTS " + TABLE_MYTABLE +
"(" +
COL_MYTABLE_ID + " INTEGER PRIMARY KEY," +
COl_MYTABLE_NAME + " TEXT, " +
COL_MYTABLE_DESCRIPTION + " TEXT, " +
COl_MYTABLE_LOCATION + " TEXT" +
")"
);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public long add(String name, String description, String location) {
ContentValues cv = new ContentValues();
cv.put(COl_MYTABLE_NAME,name);
cv.put(COL_MYTABLE_DESCRIPTION,description);
cv.put(COl_MYTABLE_LOCATION,location);
return mDB.insert(TABLE_MYTABLE,null,cv);
}
public Cursor getAllData() {
return mDB.query(TABLE_MYTABLE,null,null,null,null,null,null);
}
}
结果
上面运行的结果是：-