我是Android应用程序开发的新手。并尝试使用少量代码来保留listView数据，即使屏幕方向已更改。
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<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Enter Name"
tools:layout_editor_absoluteX="43dp"
tools:layout_editor_absoluteY="1060=dp" />
<EditText
android:id="@+id/editText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/textView"
android:ems="10"
android:hint="Name"
android:inputType="textPersonName" />
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/editText"
android:text="Add Name" />
<Button
android:id="@+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/editText"
android:layout_toRightOf="@+id/button1"
android:text="View Names" />
<ListView
android:id="@+id/listView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_below="@id/button1"/>
</RelativeLayout>
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package com.example.providerexample;
import androidx.appcompat.app.AppCompatActivity;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.os.Parcelable;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.Toolbar;
import java.util.ArrayList;
import java.util.List;
import static android.widget.Toast.makeText;
public class MainActivity extends AppCompatActivity {
String tag = getClass().getSimpleName();
TextView textView;
EditText editText;
Button button1;
Button button2;
ListView listView;
// SQLiteDatabase db;
DataBaseHandler dbHandler;
List<String> newList = new ArrayList<String>();
ArrayAdapter listAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = (TextView) findViewById(R.id.textView);
editText = (EditText) findViewById(R.id.editText);
button1 = (Button) findViewById(R.id.button1);
button2 = (Button) findViewById(R.id.button2);
listView = (ListView) findViewById(R.id.listView);
// db = new SQLiteDatabase();
dbHandler = new DataBaseHandler(this);
button1.setOnClickListener(new View.OnClickListener(){
//Add the name to the local database
public void onClick(View v1) {
//dbHandler.onCreate(db);
Toast.makeText(getApplicationContext(), "Entering", Toast.LENGTH_LONG).show();
Log.v(tag,"Entering Button1 onClick");
dbHandler.addData(getApplicationContext(),editText.getText().toString());
Toast.makeText(getApplicationContext(), "Added", Toast.LENGTH_LONG).show();
Log.v(tag,"Exiting Button1 onClick");
}
});
button2.setOnClickListener(new View.OnClickListener(){
//View the name from the local database
public void onClick(View v2) {
Log.v(tag,"Entering Button2 onClick");
//Toast.makeText(getApplicationContext(), "View Name", Toast.LENGTH_LONG).show();
newList = dbHandler.getNameList();
listAdapter = new ArrayAdapter(getApplicationContext(),android.R.layout.simple_list_item_1,newList);
listView.setAdapter(listAdapter);
Log.v(tag,"Exiting Button2 onClick");
}
});
}
@Override
protected void onSaveInstanceState(Bundle outState){
super.onSaveInstanceState(outState);
Log.v(tag,"Entering onSaveInstanceState");
outState.putParcelable("listView.state", listView.onSaveInstanceState());
}
@Override
protected void onRestoreInstanceState(Bundle savedInstanceState){
super.onRestoreInstanceState(savedInstanceState);
Log.v(tag,"Entering onRestoreInstanceState");
Parcelable listViewState = savedInstanceState.getParcelable("listView.state");
listView.onRestoreInstanceState(listViewState);
}
}
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package com.example.providerexample;
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;
import java.util.ArrayList;
import java.util.List;
public class DataBaseHandler extends SQLiteOpenHelper {
String tag = getClass().getSimpleName();
//DATABASE VERSION
private static final int DATABASE_VERSION = 1;
//DATABASE NAME
private static final String DATABASE_NAME = "MyDB";
//TABLE NAME
private static final String TABLE_NAME = "Name";
//COLUMN NAME
private static final String KEY_ID = "id";
private static final String KEY_NAME = "ColName";
String NameList;
//Constructor
public DataBaseHandler(Context context){
/**
* Create a helper object to create, open, and/or manage a database.
* This method always returns very quickly. The database is not actually
* created or opened until one of {@link #getWritableDatabase} or
* {@link #getReadableDatabase} is called.
*
* @param context to use for locating paths to the the database
* @param name of the database file, or null for an in-memory database
* @param factory to use for creating cursor objects, or null for the default
* @param version number of the database (starting at 1); if the database is older,
* {@link #onUpgrade} will be used to upgrade the database; if the database is
* newer, {@link #onDowngrade} will be used to downgrade the database
*/
super(context,DATABASE_NAME,null,DATABASE_VERSION);
Log.v(tag,"Exiting DataBaseHandler");
}
//Create Table
@Override
public void onCreate(SQLiteDatabase db){
Log.v(tag,"Entering onCreate");
String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_NAME + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT" + ")";
db.execSQL(CREATE_CONTACTS_TABLE);
Log.v(tag,"Exiting onCreate");
}
@Override
public void onUpgrade(SQLiteDatabase db,int oldVersion, int newVersion){
Log.v(tag,"Entering onUpgrade");
String CONTENT = "DROP TABLE IF EXISTS" + TABLE_NAME;
db.execSQL(CONTENT);
onCreate(db);
Log.v(tag,"Exiting onUpgrade");
}
//Insert Data
public void addData(Context context, String name){
Log.v(tag,"Entering addData");
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_NAME,name);
db.insert(TABLE_NAME,null,values);
db.close();
Log.v(tag,"Exiting addData");
}
//List the data
public List<String> getNameList(){
String selectQuery = "SELECT *FROM " + TABLE_NAME;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery,null);
List<String> contentList = new ArrayList<String>();
if(cursor.moveToFirst()){
do {
contentList.add(cursor.getString(0));
} while(cursor.moveToNext());
}
return contentList;
}
}
当我将屏幕方向从纵向更改为水平或反之时，listView数据会丢失。请帮忙。