Android 将数据库打印到字符串不起作用
免责声明我是Android新手，我正在学习一个教程 我正在尝试将数据库打印到方法databaseToString下的MyDbHandler文件中的字符串dbString 它不工作，因为dbString保持为空 我还尝试删除while循环并仅使用以下代码：Android 将数据库打印到字符串不起作用,android,sql,database,Android,Sql,Database,免责声明我是Android新手，我正在学习一个教程 我正在尝试将数据库打印到方法databaseToString下的MyDbHandler文件中的字符串dbString 它不工作，因为dbString保持为空 我还尝试删除while循环并仅使用以下代码： dbString += c.getString(c.getColumnIndex("productname")); 这给了我以下运行时错误： FATAL EXCEPTION: main
dbString += c.getString(c.getColumnIndex("productname"));
FATAL EXCEPTION: main
Process: com.example.jarno.a49_savingdata, PID: 10976
java.lang.IllegalStateException: Could not execute method for android:onClick
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: android.database.CursorIndexOutOfBoundsException: Index 0 requested, with a size of 0
at android.database.AbstractCursor.checkPosition(AbstractCursor.java:460)
at android.database.AbstractWindowedCursor.checkPosition(AbstractWindowedCursor.java:136)
at android.database.AbstractWindowedCursor.getString(AbstractWindowedCursor.java:50)
at com.example.jarno.a49_savingdata.MyDbHandler.databaseToString(MyDbHandler.java:65)
at com.example.jarno.a49_savingdata.MainActivity.printDatabase(MainActivity.java:45)
at com.example.jarno.a49_savingdata.MainActivity.addButtonClicked(MainActivity.java:35)
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
caused by: android.database.CursorIndexOutOfBoundsException: Index 0 requested, with a size of 0
package com.example.jarno.a49_savingdata;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class MyDbHandler extends SQLiteOpenHelper {
private static final String TAG = "debug";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "products.db";
public static final String TABLE_PRODUCTS = "products";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_PRODUCTNAME = "productname";
public MyDbHandler(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
super(context, name, factory, version);
}
@Override
public void onCreate(SQLiteDatabase db) {
String query = "CREATE TABLE " + TABLE_PRODUCTS + "(" +
COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
COLUMN_PRODUCTNAME + " TEXT " +
");";
db.execSQL(query);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP_TABLE_IF_EXISTS" + TABLE_PRODUCTS);
onCreate(db);
}
public void addProduct(Products product) {
ContentValues values = new ContentValues();
values.put(COLUMN_PRODUCTNAME, product.get_productname());
SQLiteDatabase db = getWritableDatabase();
db.insert(TABLE_PRODUCTS, null, values);
db.close();
}
public void delProduct(String productName) {
SQLiteDatabase db = getWritableDatabase();
db.execSQL("DELETE FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + "=\"" + productName + "\";" );
}
public String databaseToString() {
String dbString = "";
SQLiteDatabase db = getWritableDatabase();
String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";
// Cursor point to a location in the results
Cursor c = db.rawQuery(query, null);
// Move to the first row in the results
c.moveToFirst();
while(!c.isAfterLast()) {
if(c.getString(c.getColumnIndex("productname")) != null) {
dbString += c.getString(c.getColumnIndex("productname"));
Log.i(TAG, "loop");
dbString += "\n";
c.moveToNext();
}
}
Log.i(TAG, dbString);
db.close();
return dbString;
}
}
package com.example.jarno.a49_savingdata;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "debug";
EditText jarnoInput;
TextView jarnoText;
MyDbHandler dbHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
jarnoInput = (EditText) findViewById(R.id.jarnoInput);
jarnoText = (TextView) findViewById(R.id.jarnoText);
dbHandler = new MyDbHandler(this, null, null, 1);
//printDatabase();
}
public void addButtonClicked(View v) {
Products product = new Products(jarnoInput.getText().toString());
dbHandler.addProduct(product);
printDatabase();
}
public void delButtonClicked(View v) {
String inputText = jarnoInput.getText().toString();
dbHandler.delProduct(inputText);
printDatabase();
}
public void printDatabase() {
String dbString = dbHandler.databaseToString();
jarnoText.setText(dbString);
jarnoInput.setText("");
}
}
package com.example.jarno.a49_savingdata;
public class Products {
private int _id;
private String _productname;
public Products() {
}
public Products(String productname) {
this._productname = productname;
}
public void set_id(int _id) {
this._id = _id;
}
public void set_productname(String _productname) {
this._productname = _productname;
}
public int get_id() {
return _id;
}
public String get_productname() {
return _productname;
}
}
package com.example.jarno.a49_savingdata;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class MyDbHandler extends SQLiteOpenHelper {
private static final String TAG = "debug";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "products.db";
public static final String TABLE_PRODUCTS = "products";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_PRODUCTNAME = "productname";
public MyDbHandler(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
super(context, name, factory, version);
}
@Override
public void onCreate(SQLiteDatabase db) {
String query = "CREATE TABLE " + TABLE_PRODUCTS + "(" +
COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
COLUMN_PRODUCTNAME + " TEXT " +
");";
db.execSQL(query);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP_TABLE_IF_EXISTS" + TABLE_PRODUCTS);
onCreate(db);
}
public void addProduct(Products product) {
ContentValues values = new ContentValues();
values.put(COLUMN_PRODUCTNAME, product.get_productname());
SQLiteDatabase db = getWritableDatabase();
db.insert(TABLE_PRODUCTS, null, values);
db.close();
}
public void delProduct(String productName) {
SQLiteDatabase db = getWritableDatabase();
db.execSQL("DELETE FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + "=\"" + productName + "\";" );
}
public String databaseToString() {
String dbString = "";
SQLiteDatabase db = getWritableDatabase();
String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";
// Cursor point to a location in the results
Cursor c = db.rawQuery(query, null);
// Move to the first row in the results
c.moveToFirst();
while(!c.isAfterLast()) {
if(c.getString(c.getColumnIndex("productname")) != null) {
dbString += c.getString(c.getColumnIndex("productname"));
Log.i(TAG, "loop");
dbString += "\n";
c.moveToNext();
}
}
Log.i(TAG, dbString);
db.close();
return dbString;
}
}
package com.example.jarno.a49_savingdata;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "debug";
EditText jarnoInput;
TextView jarnoText;
MyDbHandler dbHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
jarnoInput = (EditText) findViewById(R.id.jarnoInput);
jarnoText = (TextView) findViewById(R.id.jarnoText);
dbHandler = new MyDbHandler(this, null, null, 1);
//printDatabase();
}
public void addButtonClicked(View v) {
Products product = new Products(jarnoInput.getText().toString());
dbHandler.addProduct(product);
printDatabase();
}
public void delButtonClicked(View v) {
String inputText = jarnoInput.getText().toString();
dbHandler.delProduct(inputText);
printDatabase();
}
public void printDatabase() {
String dbString = dbHandler.databaseToString();
jarnoText.setText(dbString);
jarnoInput.setText("");
}
}
package com.example.jarno.a49_savingdata;
public class Products {
private int _id;
private String _productname;
public Products() {
}
public Products(String productname) {
this._productname = productname;
}
public void set_id(int _id) {
this._id = _id;
}
public void set_productname(String _productname) {
this._productname = _productname;
}
public int get_id() {
return _id;
}
public String get_productname() {
return _productname;
}
}
package com.example.jarno.a49_savingdata;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class MyDbHandler extends SQLiteOpenHelper {
private static final String TAG = "debug";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "products.db";
public static final String TABLE_PRODUCTS = "products";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_PRODUCTNAME = "productname";
public MyDbHandler(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
super(context, name, factory, version);
}
@Override
public void onCreate(SQLiteDatabase db) {
String query = "CREATE TABLE " + TABLE_PRODUCTS + "(" +
COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
COLUMN_PRODUCTNAME + " TEXT " +
");";
db.execSQL(query);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP_TABLE_IF_EXISTS" + TABLE_PRODUCTS);
onCreate(db);
}
public void addProduct(Products product) {
ContentValues values = new ContentValues();
values.put(COLUMN_PRODUCTNAME, product.get_productname());
SQLiteDatabase db = getWritableDatabase();
db.insert(TABLE_PRODUCTS, null, values);
db.close();
}
public void delProduct(String productName) {
SQLiteDatabase db = getWritableDatabase();
db.execSQL("DELETE FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + "=\"" + productName + "\";" );
}
public String databaseToString() {
String dbString = "";
SQLiteDatabase db = getWritableDatabase();
String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";
// Cursor point to a location in the results
Cursor c = db.rawQuery(query, null);
// Move to the first row in the results
c.moveToFirst();
while(!c.isAfterLast()) {
if(c.getString(c.getColumnIndex("productname")) != null) {
dbString += c.getString(c.getColumnIndex("productname"));
Log.i(TAG, "loop");
dbString += "\n";
c.moveToNext();
}
}
Log.i(TAG, dbString);
db.close();
return dbString;
}
}
package com.example.jarno.a49_savingdata;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "debug";
EditText jarnoInput;
TextView jarnoText;
MyDbHandler dbHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
jarnoInput = (EditText) findViewById(R.id.jarnoInput);
jarnoText = (TextView) findViewById(R.id.jarnoText);
dbHandler = new MyDbHandler(this, null, null, 1);
//printDatabase();
}
public void addButtonClicked(View v) {
Products product = new Products(jarnoInput.getText().toString());
dbHandler.addProduct(product);
printDatabase();
}
public void delButtonClicked(View v) {
String inputText = jarnoInput.getText().toString();
dbHandler.delProduct(inputText);
printDatabase();
}
public void printDatabase() {
String dbString = dbHandler.databaseToString();
jarnoText.setText(dbString);
jarnoInput.setText("");
}
}
package com.example.jarno.a49_savingdata;
public class Products {
private int _id;
private String _productname;
public Products() {
}
public Products(String productname) {
this._productname = productname;
}
public void set_id(int _id) {
this._id = _id;
}
public void set_productname(String _productname) {
this._productname = _productname;
}
public int get_id() {
return _id;
}
public String get_productname() {
return _productname;
}
}
Cursor c = db.query(
TABLE_PRODUCTS,
new String[] { "productname" },
"1",
new String[] { "id" },
null,
null,
null
);
// Move to the first row in the results
c.moveToFirst();
if (c.getCount() >= 1) {
while(c.moveToNext()) {
if(c.getString(c.getColumnIndex("productname")) != null) {
dbString += c.getString(c.getColumnIndex("productname"));
Log.i(TAG, "loop");
dbString += "\n";
}
}
}
FATAL EXCEPTION: main
Process: com.example.jarno.a49_savingdata, PID: 9807
java.lang.IllegalStateException: Could not execute method for android:onClick
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.IllegalArgumentException: Cannot bind argument at index 1 because the index is out of range. The statement has 0 parameters.
at android.database.sqlite.SQLiteProgram.bind(SQLiteProgram.java:212)
at android.database.sqlite.SQLiteProgram.bindString(SQLiteProgram.java:166)
at android.database.sqlite.SQLiteProgram.bindAllArgsAsStrings(SQLiteProgram.java:200)
at android.database.sqlite.SQLiteDirectCursorDriver.query(SQLiteDirectCursorDriver.java:47)
at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1318)
at android.database.sqlite.SQLiteDatabase.queryWithFactory(SQLiteDatabase.java:1165)
at android.database.sqlite.SQLiteDatabase.query(SQLiteDatabase.java:1036)
at android.database.sqlite.SQLiteDatabase.query(SQLiteDatabase.java:1204)
at com.example.jarno.a49_savingdata.MyDbHandler.databaseToString(MyDbHandler.java:61)
at com.example.jarno.a49_savingdata.MainActivity.printDatabase(MainActivity.java:45)
at com.example.jarno.a49_savingdata.MainActivity.addButtonClicked(MainActivity.java:35)
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
光标从-1开始，将循环修改为：
if(cursor.getCount() >= 1) {
while(cursor.moveToNext()) {
// Access cursor values here
}
}
如果要在一个使用StringBuilder或StringBuffers中追加所有字符串，则这不是获取所有字符串的正确方法。其他方法在光标移动时逐个获取字符串=null并将它们放入数组中@Soundwave您的查询不正确…请参阅我修改后的答案。谢谢，但我似乎无法理解。我已经实现了对您答案的解释。你能看一下吗？@声波你选择得不正确，请看我修改过的答案。StringBuilder与手动追加字符串与此DB错误无关。是的，这是一个很好的建议，但请保留与回答问题相关的答案。