android– 自定义SimpleCursorAdapter,数据库查询和NullPointerException

概述我正在尝试从数据库中填充ListView,并使用花哨的删除按钮调整每一行.所以我创建了列表Activity和自定义SimpleCursorAdapter.这是主要的ListView活动：publicclassEditEntriesextendsListActivity{Cursorcursor;DBAdapterdb=newDBAdapter(this);priv

我正在尝试从数据库中填充ListVIEw,并使用花哨的删除按钮调整每一行.所以我创建了列表Activity和自定义SimpleCursorAdapter.

这是主要的ListVIEw活动：

    public class EditEntrIEs extends ListActivity {    Cursor cursor;    DBAdapter db = new DBAdapter(this);    private static String[] FROM = { _ID, score_ColUMN,            "date(time, 'localtime')", };    private static int[] TO = { R.ID.rowID, R.ID.Title, R.ID.time, };    public voID onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentVIEw(R.layout.entrIEs_List);        try {            cursor = getEvents();            showEvents(cursor);        } finally {            db.close();        }    }    private voID showEvents(Cursor cursor) {        // Set up data binding        SMSimpleCursorAdapter adapter = new SMSimpleCursorAdapter(this,                R.layout.entrIEs_List_item, cursor, FROM, TO);        setlistadapter(adapter);    }    private Cursor getEvents() {        db.open();        cursor = db.getAllscores();        startManagingCursor(cursor);        return cursor;    }    voID delRow() {        // this is method for deleting row by _ID        db.open();        db.deletescore(3); // here will be TAG with _ID from adapter,        db.close();        // for Now I just use hardcoded _ID    }}

这是自定义的SimpleCursorAdapter：

    public class SMSimpleCursorAdapter extends SimpleCursorAdapter{    Cursor c;    Context context;    Activity activity;    public SMSimpleCursorAdapter(Context context, int layout, Cursor c,            String[] from, int[] to) {        super(context, layout, c, from, to);        this.c = c;        this.context=context;        this.activity=(Activity) context;    }    EditEntrIEs dbDel = new EditEntrIEs(); //from prevIoUs code sample    @OverrIDe    public VIEw getVIEw(int position, VIEw convertVIEw, VIEwGroup parent) {        if(convertVIEw == null)            convertVIEw = VIEw.inflate(context, R.layout.entrIEs_List_item, null);        VIEw row = convertVIEw;        c.movetoposition(position);        TextVIEw score = (TextVIEw) convertVIEw.findVIEwByID(R.ID.Title);        TextVIEw time = (TextVIEw) convertVIEw.findVIEwByID(R.ID.time);        TextVIEw ID = (TextVIEw) convertVIEw.findVIEwByID(R.ID.rowID);        ID.setText(c.getString(0));        time.setText(c.getString(2));        score.setText(c.getString(1));        String daTag = c.getString(1);        Imagebutton delbutton = (Imagebutton) convertVIEw.findVIEwByID(R.ID.delbutton);        delbutton.setFocusable(true);        delbutton.setClickable(true);        //delbutton.setTag(daTag);        delbutton.setonClickListener(new OnClickListener() { //Click Listener fro delete button             @OverrIDe             public voID onClick(VIEw vIEw) {                 dbDel.delRow(); //this is "delete row" method from previos code                    sample             }             });        return(row);    }}

当我点击删除按钮时,所有这些东西都给了我NullPointerException.
我是androID的新手,并且假设我错过了一些明显的东西.

logcat的：

ERROR/AndroIDRuntime(14027): FATAL EXCEPTION: main

ERROR/AndroIDRuntime(14027): java.lang.NullPointerException

ERROR/AndroIDRuntime(14027): at androID.content.Contextwrapper.openorCreateDatabase(Contextwrapper.java:203)

ERROR/AndroIDRuntime(14027): at androID.database.sqlite.sqliteOpenHelper.getWritableDatabase(sqliteOpenHelper.java:98)

ERROR/AndroIDRuntime(14027): at namespace.DBAdapter.open(DBAdapter.java:66)

ERROR/AndroIDRuntime(14027): at namespace.EditEntrIEs.delRow(EditEntrIEs.java:50)

ERROR/AndroIDRuntime(14027): at namespace.SMSimpleCursorAdapter$1.onClick(SMSimpleCursorAdapter.java:64)

ERROR/AndroIDRuntime(14027): at androID.vIEw.VIEw.performClick(VIEw.java:2414)

ERROR/AndroIDRuntime(14027): at androID.vIEw.VIEw$PerformClick.run(VIEw.java:8838)

ERROR/AndroIDRuntime(14027): at androID.os.Handler.handleCallback(Handler.java:587)

ERROR/AndroIDRuntime(14027): at androID.os.Handler.dispatchMessage(Handler.java:92)

ERROR/AndroIDRuntime(14027): at androID.os.Looper.loop(Looper.java:123)

ERROR/AndroIDRuntime(14027): at androID.app.ActivityThread.main(ActivityThread.java:4680)

ERROR/AndroIDRuntime(14027): at java.lang.reflect.Method.invokeNative(Native Method)

ERROR/AndroIDRuntime(14027): at java.lang.reflect.Method.invoke(Method.java:521)

ERROR/AndroIDRuntime(14027): at com.androID.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)

ERROR/AndroIDRuntime(14027): at com.androID.internal.os.ZygoteInit.main(ZygoteInit.java:616)

ERROR/AndroIDRuntime(14027): at dalvik.system.NativeStart.main(Native Method)

解决方法:

问题出在您的SMSimpleCursorAdapter代码中：

EditEntrIEs dbDel = new EditEntrIEs(); //from prevIoUs code sample

您创建一个新对象但它不是托管活动(例如,不会调用onCreate方法).当您尝试第二次(从您的适配器)创建它时,NPE可能来自您的DBAdapter.

快速解决：

EditEntrIEs dbDel; //from prevIoUs code samplepublic SMSimpleCursorAdapter(Context context, int layout, Cursor c,        String[] from, int[] to) {    super(context, layout, c, from, to);    this.c = c;    this.context = context;    this.activity = (Activity) context;    this.dbDel = (EditEntrIEs) context;}

如果您不想只是快速修复,以下将是一个更好的解决方案：

>将delRow,getEvents方法移动到DBAdapter类中
>修改SMSimpleCursorAdapter的构造函数以提供DBAdapter类(在ListActivity中创建).

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