android studio中ListView与SQLite的结合使用

public class Db extends SQLiteOpenHelper {
    public Db(Context context) {
        super(context, "db", null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE user2(" +
                "_id INT DEFAULT 0," +
                "name TEXT DEFAULT ""," +
                "sex TEXT DEFAULT "")");
    }

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

    }
}

public class MainActivity extends AppCompatActivity {
    private SimpleCursorAdapter mAdapter;
    private ListView listView;

    private Button btn_add;
    private Button btn_delete;

    private Button btn_back;
    private EditText et_id;
    private EditText et_name;
    private EditText et_sex;

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

        btn_add = (Button) findViewById(R.id.btn_add);
        btn_delete = (Button) findViewById(R.id.btn_delete);
        listView = (ListView) findViewById(R.id.list);

        btn_add.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                addData();
                refleshListView();
            }
        });

        btn_delete.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                deleteData();
                refleshListView();
            }
        });

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
//                Toast.makeText(MainActivity.this,""+position,Toast.LENGTH_SHORT).show();
                int positions = position + 1;
                Db db = new Db(MainActivity.this);
                SQLiteDatabase dbWrite = db.getWritableDatabase();
                dbWrite.delete("user2","_id=?",new String[]{""+positions});
                dbWrite.close();
                refleshListView();
            }
        });
    }

    public void addData() {
        btn_back = (Button) findViewById(R.id.btn_back);
        et_id = (EditText) findViewById(R.id.et_id);
        et_name = (EditText) findViewById(R.id.et_name);
        et_sex = (EditText) findViewById(R.id.et_sex);

        btn_add = (Button) findViewById(R.id.btn_add);
        btn_delete = (Button) findViewById(R.id.btn_delete);
        listView = (ListView) findViewById(R.id.list);

        btn_add.setVisibility(View.GONE);
        btn_delete.setVisibility(View.GONE);
        listView.setVisibility(View.GONE);

        btn_back.setVisibility(View.VISIBLE);
        et_id.setVisibility(View.VISIBLE);
        et_name.setVisibility(View.VISIBLE);
        et_sex.setVisibility(View.VISIBLE);

        btn_back.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                btn_add.setVisibility(View.VISIBLE);
                btn_delete.setVisibility(View.VISIBLE);
                listView.setVisibility(View.VISIBLE);

                btn_back.setVisibility(View.GONE);
                et_id.setVisibility(View.GONE);
                et_name.setVisibility(View.GONE);
                et_sex.setVisibility(View.GONE);

                String id = et_id.getText().toString(),
                        name =et_name.getText().toString(),
                        sex = et_sex.getText().toString();


                Db db = new Db(MainActivity.this);
                SQLiteDatabase dbWrite = db.getWritableDatabase();

                ContentValues cv = new ContentValues();
                cv.put("_id", id);
                cv.put("name", name);
                cv.put("sex", sex);
                dbWrite.insert("user2", null, cv);

                dbWrite.close();

                refleshListView();
            }
        });

//        String id = et_id.getText().toString(),
//               name =et_name.getText().toString(),
//               sex = et_sex.getText().toString();
//
//
//        Db db = new Db(MainActivity.this);
//        SQLiteDatabase dbWrite = db.getWritableDatabase();
//
//        ContentValues cv = new ContentValues();
//        cv.put("_id",id);
//        cv.put("name", name);
//        cv.put("sex", sex);
//        dbWrite.insert("user2", null, cv);
//
//        dbWrite.close();
    }

    public void deleteData() {
        Db db = new Db(MainActivity.this);
        SQLiteDatabase dbWrite = db.getWritableDatabase();
        dbWrite.delete("user2",null,new String[]{});
        dbWrite.close();
    }

    public void refleshListView() {
        Db db = new Db(MainActivity.this);
        SQLiteDatabase dbRead = db.getReadableDatabase();
        Cursor c = dbRead.query("user2", null, null, null, null, null, null);

        mAdapter = new SimpleCursorAdapter(MainActivity.this, R.layout.user_info, c,
                new String[]{"_id","name","sex"}, new int[]{R.id._id,R.id._name,R.id._sex});

        listView.setAdapter(mAdapter);

        dbRead.close();
    }

}

<LinearLayout xmlns:andro>
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">
    <TextView
        android:id="@+id/_id"
        android:textSize="20sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/_name"
        android:textSize="20sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/_sex"
        android:textSize="20sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

<LinearLayout xmlns:andro>
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/btn_add"
        android:text="Add"
        android:textSize="20sp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
    <Button
        android:id="@+id/btn_delete"
        android:text="Delete"
        android:textSize="20sp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <ListView
        android:id="@+id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

    <Button
        android:id="@+id/btn_back"
        android:text="确定"
        android:textSize="20sp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <EditText
        android:id="@+id/et_id"
        android:visibility="gone"
        android:layout_width="match_parent"
        android:layout_height="40dp" />
    <EditText
        android:id="@+id/et_name"
        android:visibility="gone"
        android:layout_width="match_parent"
        android:layout_height="40dp" />
    <EditText
        android:id="@+id/et_sex"
        android:visibility="gone"
        android:layout_width="match_parent"
        android:layout_height="40dp" />
</LinearLayout>