java - Android 中的自定义 ListView 在更新 Android 数据库中的数据时出错

Question

我试图在自定义 ListView 中添加复选框和一些 TextView ( TextView 从数据库获取值)，但它给出了 NullPointerException 错误。我不知道为什么以及我的代码有什么问题。下面是我的代码。

我的 MainActivity 类:

public class Classes extends Activity {

ImageView imageViewNewClass;
ListView mListView;
String[] stg1;
List<String[]> names2 = null;
DataManipulatorClass dataManipulator;
CustomAdapter customAdapter;
public Classes classes = null;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.classes);

    imageViewNewClass = (ImageView) findViewById(R.id.newclass);
    mListView = (ListView) findViewById(R.id.displaydata);

    imageViewNewClass.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Intent intent = new Intent(Classes.this, Class_Create.class);
            startActivity(intent);
        }
    });

    Resources res =getResources();
    classes = this;

dataManipulator = new DataManipulatorClass(this);
    names2 = dataManipulator.selectAll();

    stg1 = new String[names2.size()];
    int x = 0;
    String stg = null;

    for (String[] name : names2) {
        stg = "Class Name : " + name[1];
        stg1[x] = stg;
        x++;
    }
    customAdapter= new CustomAdapter( classes, stg1,res );
    mListView.setAdapter( customAdapter );
    customAdapter.notifyDataSetChanged();

    mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View item,
                int position, long id) {
            Toast.makeText(getApplicationContext(),
                    "Listview item clicked", Toast.LENGTH_LONG).show();
        }
    });
}
}

自定义适配器类.java

public class CustomAdapter extends BaseAdapter {

/*********** Declare Used Variables *********/
private Activity activity;
private String[] data;
private static LayoutInflater inflater = null;
public Resources res;
int i = 0;

String[] stg1;
List<String[]> names2 = null;
DataManipulatorClass dataManipulator;

/************* CustomAdapter Constructor *****************/
public CustomAdapter(Activity a, String[] stg1, Resources resLocal) {

    /********** Take passed values **********/
    activity = a;
    data = stg1;
    res = resLocal;

    /*********** Layout inflator to call external xml layout () ***********/
    inflater = (LayoutInflater) activity
            .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

}

/******** What is the size of Passed Arraylist Size ************/
public int getCount() {
    if (data.length <= 0)
        return 1;
    return data.length;
}

public Object getItem(int position) {
    return position;
}

public long getItemId(int position) {
    return position;
}

public static class ViewHolder {
    public CheckBox checkBox;
    public TextView textView;
}

public View getView(int position, View convertView, ViewGroup parent) {

    View vi = convertView;
    ViewHolder holder;

    if (convertView == null) {
        vi = inflater.inflate(R.layout.check, null);
        holder = new ViewHolder();
        holder.checkBox = (CheckBox) vi.findViewById(R.id.checkBox1);
        holder.textView = (TextView) vi.findViewById(R.id.selection);
        vi.setTag(holder);
    } else
        holder = (ViewHolder) vi.getTag();
    if (data.length <= 0) {
        holder.textView.setText("No Data");
    } else {

        dataManipulator = new DataManipulatorClass(this);
        names2 = dataManipulator.selectAll();

        stg1 = new String[names2.size()];
        int x = 0;
        String stg = null;

        for (String[] name : names2) {
            stg = "Class Name : " + name[1];
            stg1[x] = stg;
            x++;
        }

        holder.textView.setText(stg1[x]);
    }
    return vi;
}

public void onClick(View v) {
    Log.v("CustomAdapter", "=====Row button clicked=====");
}
}

DataManipulator.Java

public class DataManipulatorClass {

private static final String DATABASE_NAME = "mydatabaseclass.db";
private static final int DATABASE_VERSION = 1;
static final String TABLE_NAME = "newtableclass";
private static Context context;
static SQLiteDatabase db;

private SQLiteStatement insertStmt;

private static final String INSERT = "insert into " + TABLE_NAME
        + "(classname) values (?)";

public DataManipulatorClass(Context context) {
    DataManipulatorClass.context = context;
    OpenHelper openHelper = new OpenHelper(DataManipulatorClass.context);
    DataManipulatorClass.db = openHelper.getWritableDatabase();
    this.insertStmt = DataManipulatorClass.db.compileStatement(INSERT);
}

public DataManipulatorClass(CustomAdapter customAdapter) {
    OpenHelper openHelper = new OpenHelper(DataManipulatorClass.context);
    DataManipulatorClass.db = openHelper.getWritableDatabase();
    this.insertStmt = DataManipulatorClass.db.compileStatement(INSERT);
}

public long insert(String classname) {
    this.insertStmt.bindString(1, classname);
    return this.insertStmt.executeInsert();
}

public void deleteAll() {
    db.delete(TABLE_NAME, null, null);
}

public List<String[]> selectAll() {
    List<String[]> list = new ArrayList<String[]>();
    Cursor cursor = db.query(TABLE_NAME,
            new String[] { "id", "classname" }, null, null, null, null,
            "classname asc");
    int x = 0;
    if (cursor.moveToFirst()) {
        do {
            String[] b1 = new String[] { cursor.getString(0),
                    cursor.getString(1) };
            list.add(b1);
            x = x + 1;
        } while (cursor.moveToNext());
    }
    if (cursor != null && !cursor.isClosed()) {
        cursor.close();
    }
    cursor.close();
    return list;
}

public void delete(int rowId) {
    db.delete(TABLE_NAME, null, null);
}

private static class OpenHelper extends SQLiteOpenHelper {
    OpenHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE "
                + TABLE_NAME
                + " (id INTEGER PRIMARY KEY, classname TEXT)");
    }

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

在类类中，我有一个 ListView ，我想在其中显示数据，我的 logcat 在这里 http://i.share.pho.to/9739fdca_o.png

mListView.setAdapter( customAdapter ) 出现错误；将 ListView 中的适配器设置为自定义适配器时的这一行...

提前致谢...

Answer 1

public class Classes extends Activity {

ImageView imageViewNewClass;
ListView mListView;
String[] stg1; // Never Initialized in code before passing to Adapter

//stg1，这个变量你从未在你的 Activity 中初始化过，你将它传递给你的自定义适配器，这就是为什么listview在访问数据计数时抛出NPE vai适配器。

public int getCount() { // Will always be Null, as data is null
    if (data.length <= 0)
        return 1;
    return data.length;
}

只需在 Activity 中传递一个非空变量实例，您的问题就会得到解决。