Question

这是我的活动，我需要在数据库中添加类别，我使用Android提示用户输入对话框添加该catgeory.I参考此链接的对话框。 http://www.mkyong.com/android/android-prompt-user-input-dialog-example/

但现在当我按下对话框中的ADD（OK）按钮时，它会给出空指针异常。

public class budget extends Activity {

    int selected_id;
    ListView rldlist = null;
    DBhelper helper;
    String budget;
    TextView menubtn;
    Context context = this;
    EditText txr;
    Button btn1;
   SQLiteDatabase db;


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.budget);

        helper = new DBhelper(this);
        txr = (EditText) findViewById(R.id.add);
        btn1 = (Button) findViewById(R.id.button);


        menubtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {

                // get prompts.xml view
                LayoutInflater li = LayoutInflater.from(context);
                View promptsView = li.inflate(R.layout.addcategory, null);

                AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(
                        context);

                // set prompts.xml to alertdialog builder
                alertDialogBuilder.setView(promptsView);



                // set dialog message
                alertDialogBuilder
                        .setCancelable(false)
                        .setPositiveButton("ADD",
                                new DialogInterface.OnClickListener() {
                                    public void onClick(DialogInterface dialog, int id) {
                                        ContentValues value = new ContentValues();
                                        value.put(DBhelper.Name, txr.getText().toString());
                                        db = helper.getWritableDatabase();

                                        if (helper.checkIdExist(txr.getText().toString())) {
                                            db.insert(DBhelper.TABLE1, null, value);
                                            db.close();
                                        } else {
                                            Toast.makeText(getApplicationContext(), "Duplicate Category Name!", Toast.LENGTH_LONG).show();
                                        }

                                    }
                                })
                        .setNegativeButton("Cancel",
                                new DialogInterface.OnClickListener() {
                                    public void onClick(DialogInterface dialog, int id) {
                                        dialog.cancel();
                                    }
                                });

                // create alert dialog
                AlertDialog alertDialog = alertDialogBuilder.create();

                // show it
                alertDialog.show();

            }
        });


    }

这是我的DB类..

public class DBhelper extends SQLiteOpenHelper{

    static final String DATABASE = "wedding9.db";
    static final int VERSION = 9;
    static final String TABLE1 = "Category";
    static final String TABLE2 = "Budget";
    static final String TABLE3 = "Expenses";

    static final String C_ID = "_id";
    static final String Name = "name";
    static final String B_ID = "_id";
    static final String Description = "description";
    static final String Amount = "amount";

    public static final String ID1="_id";
    public static final String DATE_T1="date1";
    public static final String CATEGORY="category";
    public static final String DETAIL="detail";
    public static final String AMOUNT1="amount1";
    public static final String STATUS="status";
    public static final String EX_YEAR="exyear";
    public static final String EX_MONTH="exmonth";

    public DBhelper(Context context){
        super(context, DATABASE, null, VERSION);
    }

    public void onCreate(SQLiteDatabase db){

        db.execSQL("CREATE TABLE " + TABLE1+ "(" +C_ID
                + " INTEGER PRIMARY KEY AUTOINCREMENT," +Name+ " text unique not null)");

        db.execSQL("CREATE TABLE " + TABLE2+ "(" +B_ID
                + " INTEGER PRIMARY KEY AUTOINCREMENT," +Description+ " text,"
                +Amount+ " text, FOREIGN KEY ("+Description+") REFERENCES "+TABLE1+"("+Name+"));");

        db.execSQL("CREATE TABLE " + TABLE3 + " ( "
                + ID1 + " INTEGER PRIMARY KEY AUTOINCREMENT, "
                + DATE_T1 + " text, "
                + CATEGORY + " text, "
                + DETAIL + " text, "
                + STATUS + " text, "
                + EX_YEAR + " text, "
                + EX_MONTH + " text, "
                + AMOUNT1 + " text, FOREIGN KEY ("+CATEGORY+") REFERENCES "+TABLE1+"("+Name+"));");



    }

    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        db.execSQL("drop table " + TABLE1);
        onCreate(db);
    }

    public ArrayList<category> getCategories(){
        ArrayList<category> arrayList = new ArrayList<category>();
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.query(DBhelper.TABLE1, null, null, null, null, null, null);
        while (c.moveToNext()) {
            category cat = new category(c.getInt(0),c.getString(1));
           arrayList.add(cat);

        }

        return arrayList;
    }

    public boolean checkIdExist(String name) {
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.query(DBhelper.TABLE1, null, null, null, null, null, null);
        while (c.moveToNext()) {
            if(c.getString(1).equals(name))
                return false;
        }

        return true;
    }

}

10-25 13:56:21.179  16248-16248/com.example.username.weddingplanning
E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.example.username.weddingplanning.budget$1$2.onClick(budget.java:83)
            at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:185)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:176)
            at android.app.ActivityThread.main(ActivityThread.java:5299)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

下面：

Context context = this;

在创建可能导致问题的context之前初始化Activity。

在context方法中初始化onCreate：

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.budget);
        context=this;//<<<<
        ....
}

如果add EditText位于addcategory布局内，则使用提示视图初始化它，如onClick方法：

View promptsView = li.inflate(R.layout.addcategory, null);
txr = (EditText)promptsView. findViewById(R.id.add);
AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(
                        context);

Android提示用户输入对话框中的空指针异常

1 个答案: