每当我包含updateUI（）方法时，我的应用程序崩溃了，但是当我排除它时它不会崩溃，请解释我为什么？并且请解释使用ArrayList和ArrayAdapter更新我的ListView的更好方法，因为我＆＃ 39;我对使用SimpleCursorAdapter感到困惑，提前问候......
我的MainActivity
package org.example.tasklist;
import java.util.ArrayList;
import android.app.Activity;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
public class MainActivity extends Activity {
HelperMethods localhelpermethods;
HelperMethods.OpenHelper localopenhelper;
SQLiteDatabase localdatabase;
ArrayList<String> task_list;
ListView listView1;
EditText editText1;
ArrayAdapter<String> aa;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Context activitycontext = getApplicationContext();
localhelpermethods = new HelperMethods(activitycontext);
task_list = new ArrayList<String>();
listView1 = (ListView)findViewById(R.id.listView1);
editText1 = (EditText)findViewById(R.id.editText1);
Button button1 = (Button)findViewById(R.id.button1);
aa = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,task_list);
listView1.setAdapter(aa);
updateUI();
button1.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View v)
{
String task;
localhelpermethods.open_database();
task = editText1.getText().toString();
localhelpermethods.add_task(task);
editText1.setText("");
updateUI();
}
});//working fine, don't know whether the task is being stored in the database.
}
public void updateUI()
{
task_list = localhelpermethods.getAllTasks();
aa = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,task_list);
listView1 = (ListView)findViewById(R.id.listView1);
listView1.setAdapter(aa);
aa.notifyDataSetChanged();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
My HelperMethods.java
package org.example.tasklist;
import java.util.ArrayList;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class HelperMethods
{
public static final String DATABASE_NAME = "TASK_DATABASE";
public final String DATABASE_TABLE = "TASKS";
public static final int DATABASE_VERSION = 1;
//Columns in the table.
public final String KEY_ID = "_id";
public final String KEY_TASK = "task";
private static String CREATE_DATABASE = "create table TASKS (_id integer primary key autoincrement," + "task text not null);";
private final Context localcontext;
private OpenHelper localopenhelper;
private SQLiteDatabase localdatabase;
public HelperMethods(Context ctx)
{
this.localcontext = ctx;
localopenhelper = new OpenHelper(localcontext);
}//correct.
//correct
public class OpenHelper extends SQLiteOpenHelper
{
Context openhelpercontext;
public OpenHelper(Context context)
{
super(context,DATABASE_NAME,null,DATABASE_VERSION);
this.openhelpercontext = context;
}
@Override
public void onCreate(SQLiteDatabase database)
{
database.execSQL(CREATE_DATABASE);
}
@Override
public void onUpgrade(SQLiteDatabase database, int oldversion, int newversion)
{
if(newversion > oldversion)
{
database.execSQL("DROP TABLE IF EXISTS tasks" );
onCreate(database);
}
}//correct
}
public HelperMethods open_database() throws SQLException
{
localdatabase = localopenhelper.getWritableDatabase();
return this;
}//correct.
public void close_database()
{
localopenhelper.close();
}//correct.
public long add_task(String task)
{
ContentValues cv = new ContentValues();
cv.put(KEY_TASK, task);
return localdatabase.insert("TASKS", null, cv);
}//correct
public ArrayList<String> getAllTasks()
{
ArrayList<String> task_list = new ArrayList<String>();
localopenhelper.getReadableDatabase();
String[] columns = new String[] {KEY_TASK};
Cursor cursor = localdatabase.query(DATABASE_TABLE, columns, null, null, null, null, null, null);
while(cursor.moveToNext())
{
task_list.add(cursor.getString(cursor.getColumnIndex(KEY_TASK)));
}
return task_list;
}//correct but not verified.
public void deleteAllTasks()
{
localdatabase.delete(DATABASE_TABLE, KEY_TASK, null);
}
}
我的错误logcat
05-03 11:12:29.490: E/AndroidRuntime(2714): FATAL EXCEPTION: main
05-03 11:12:29.490: E/AndroidRuntime(2714): Process: org.example.tasklist, PID: 2714
05-03 11:12:29.490: E/AndroidRuntime(2714): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.example.tasklist/org.example.tasklist.MainActivity}: java.lang.NullPointerException
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread.access$700(ActivityThread.java:135)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.os.Handler.dispatchMessage(Handler.java:102)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.os.Looper.loop(Looper.java:137)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread.main(ActivityThread.java:4998)
05-03 11:12:29.490: E/AndroidRuntime(2714): at java.lang.reflect.Method.invokeNative(Native Method)
05-03 11:12:29.490: E/AndroidRuntime(2714): at java.lang.reflect.Method.invoke(Method.java:515)
05-03 11:12:29.490: E/AndroidRuntime(2714): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
05-03 11:12:29.490: E/AndroidRuntime(2714): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
05-03 11:12:29.490: E/AndroidRuntime(2714): at dalvik.system.NativeStart.main(Native Method)
05-03 11:12:29.490: E/AndroidRuntime(2714): Caused by: java.lang.NullPointerException
05-03 11:12:29.490: E/AndroidRuntime(2714): at org.example.tasklist.HelperMethods.getAllTasks(HelperMethods.java:88)
05-03 11:12:29.490: E/AndroidRuntime(2714): at org.example.tasklist.MainActivity.updateUI(MainActivity.java:68)
05-03 11:12:29.490: E/AndroidRuntime(2714): at org.example.tasklist.MainActivity.onCreate(MainActivity.java:45)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.Activity.performCreate(Activity.java:5243)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-03 11:12:29.490: E/AndroidRuntime(2714): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
05-03 11:12:29.490: E/AndroidRuntime(2714): ... 11 more
答案 0 :(得分：0)
从您的logcat ...
05-03 11:12:29.490: E/AndroidRuntime(2714): Caused by: java.lang.NullPointerException
05-03 11:12:29.490: E/AndroidRuntime(2714): at org.example.tasklist.HelperMethods.getAllTasks(HelperMethods.java:88)
您在NullPointerException课程的第88行获得HelperMethods。查看您的代码，最可能的行是......
while(cursor.moveToNext())
...这表明您的查询为null返回Cursor。尝试拨打Cursor之前，请尝试检查moveToNext()。实施例...
if (cursor != null)
while (cursor.moveToNext())
{
task_list.add(cursor.getString(cursor.getColumnIndex(KEY_TASK)));
}