Question

APP AIM 当我点击登录按钮时，它应该保存姓名＆amp;密码在数据库中，然后它应该显示一个新的活动，在点击时显示它应该从数据库中获取数据并在提供的Textview中显示它。当单击下一个按钮时，它应该在textview中显示下一个数据行。

请求我是Android的新手请帮帮我。谢谢。

主要活动

public class MainActivity extends ActionBarActivity {

DatabaseHelper myDb = new DatabaseHelper(getBaseContext());
EditText et_Username, et_Password;
Button bt_Login;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //INITIALISING VIEWS
    et_Username = (EditText) findViewById(R.id.etUsername);
    et_Password = (EditText) findViewById(R.id.etPassword);
    bt_Login = (Button) findViewById(R.id.btLogin);

    //IMPLEMENTING ON CLICK LISTENER
    bt_Login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String name,pass;
            name = et_Username.getText().toString();
            pass = et_Password.getText().toString();
            myDb.Insert(name, pass);
            Intent i = new Intent(MainActivity.this,DataActivity.class);
            startActivity(i);
        }
    });

}

}

DATAHELPER

公共类DatabaseHelper扩展了SQLiteOpenHelper {

public static final String DATABASE_NAME = "mydatabase.db";
public static final String TABLE_NAME = "Details";
public static final String COL_1 = "NAME";
public static final String COL_2 = "PASSWORD";



public DatabaseHelper(Context context){
    super(context, DATABASE_NAME, null, 1);
}


@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("CREATE TABLE " + TABLE_NAME + " (NAME TEXT PRIMARY KEY, PASSWORD TEXT) ");

}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
    onCreate(db);
}

public boolean Insert(String name, String password){
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues cv = new ContentValues();
    cv.put(COL_1, name);
    cv.put(COL_2, password);
    long res = db.insert(TABLE_NAME, null, cv);
    if(res == -1)
        return false;
    else
        return true;
  }

public Cursor getAllData()
{
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
    return cursor;
}

}

数据活动

public class DataActivity extends Activity {

DatabaseHelper myDb = new DatabaseHelper(getBaseContext());


TextView tv_Name,tv_Password;
Button bt_Ok,bt_Next;
String showName,showPass;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_data);

    //Initialising Views
    tv_Name = (TextView) findViewById(R.id.tvShowUsername);
    tv_Password = (TextView) findViewById(R.id.tvShowPassword);
    bt_Ok = (Button) findViewById(R.id.btOk);
    bt_Next = (Button) findViewById(R.id.btNxt);

    bt_Ok.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            Cursor cursor = myDb.getAllData();
            cursor.moveToFirst();
            if(cursor != null){

                        showName = cursor.getString(0);
                        showPass = cursor.getString(1);
                        show(showName,showPass);

            }

        }
    });

    bt_Next.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            Cursor cursor = myDb.getAllData();
            cursor.moveToNext();
            if(cursor!=null){
                showName = cursor.getString(0);
                showPass = cursor.getString(1);
                show(showName,showPass);
            }

        }
    });

 }

public void show(String n,String p){
    tv_Name.setText(n);
    tv_Password.setText(p);
}

}

异常

9-16 13:56:09.939    9508-9514/? E/jdwp﹕ Failed sending reply to debugger: Broken pipe
09-16 13:56:19.679    9508-9508/com.test.anu.test E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.test.anu.test, PID: 9508
    java.lang.NullPointerException
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
            at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
            at com.test.anu.test.DatabaseHelper.Insert(DatabaseHelper.java:39)
            at com.test.anu.test.MainActivity$1.onClick(MainActivity.java:36)
            at android.view.View.performClick(View.java:4438)
            at android.view.View$PerformClick.run(View.java:18422)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

你的问题是，getBaseContext（）在调用onCreate（）之前返回null，你的活动应该如下所示，

public class MainActivity extends ActionBarActivity {

    DatabaseHelper myDb;
    EditText et_Username, et_Password;
    Button bt_Login;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myDb = new DatabaseHelper(getBaseContext());
        //INITIALISING VIEWS
        et_Username = (EditText) findViewById(R.id.etUsername);
        et_Password = (EditText) findViewById(R.id.etPassword);
        bt_Login = (Button) findViewById(R.id.btLogin);

        //IMPLEMENTING ON CLICK LISTENER
        bt_Login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String name,pass;
                name = et_Username.getText().toString();
                pass = et_Password.getText().toString();
                myDb.Insert(name, pass);
                Intent i = new Intent(MainActivity.this,DataActivity.class);
                startActivity(i);
            }
        });

    }
    }

Answer 2

在Activity的 oncreate 方法中初始化您的db类。

DatabaseHelper myDb = new DatabaseHelper(getBaseContext());

Android测试应用

2 个答案: