java - Android while !c.isAfterLast() loop not being entered

Question

问题 按照教程，每次按下按钮时都尝试将一些简单的字符串放入数据库中。我相信“用户名”被输入到数据库中，而调试它并没有做任何有趣的事情。我的问题是当我想调用 databaseToString() 时，我将 Cursor 设置为第一个条目。但随后它甚至没有输入 while(!c.isAfterLast())。到底是怎么回事？我希望它 dbString 几乎等于明文形式的数据库，但这并没有发生。谢谢

尝试

• 检查 SO/Google/API 指南/thenewboston 源代码
• 尝试查看我的 SQLite 数据库，但我不知道该怎么做，所以我做不到
• 如果您注释掉 databaseToString，那么我猜它会运行，但它只是停在那里。
• 当我设置 Text.setText(dbString); 时它也会崩溃，但这完全不相关

LogCat 它不会崩溃，如果需要我会发布。

SearchActivity.Java 单击按钮时调用插入

public class SearchActivity extends ActionBarActivity {

    EditText Input;
    TextView Text;
    Database dbHandler;

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

        Input = (EditText) findViewById(R.id.Input);
        dbHandler = new Database(this, null, null, 1);
        // printDatabase();

    }

    // Add a user to database
    public void insert(View view) {
        Users user = new Users(Input.getText().toString());
        dbHandler.addUser(user);
        printDatabase();
    }


    public void printDatabase() {
        // dbString is returning null
        String dbString = dbHandler.databaseToString();
        //Technically it starts to crash right at this line
        Text.setText(dbString);
        Input.setText("");
    }    
}

用户.java

public class Users {
    private int _id;
    private String _username;

    public Users() {

    }

    public Users(String username){
        this._username = username;
    }

    public String get_username() {
        return _username;
    }

    public void set_username(String username) {
        this._username = username;
    }

    public int get_id() {
        return _id;
    }

    public void set_id(int id) {
        this._id = id;
    }
}

Database.java 又名处理程序。查看 databaseToString() 并且由于某种原因没有进入 while 循环。

public class Database extends SQLiteOpenHelper {
    // Version number to upgrade database version
    // each time if you Add, Edit table, you need to change
    // the version number
    private static final int DATABASE_VERSION = 2;
    private static final String DATABASE_NAME = "users.db";
    private static final String TABLE_USERS = "users";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_USERNAME = "username";


    public Database(Context context, String DATABASE_NAME, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // There must be a ',' in between every column definition
        String query = "CREATE TABLE " + TABLE_USERS + "(" +
                COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                COLUMN_USERNAME + " TEXT " +
                ");";
        db.execSQL(query);
    }

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

    //Add a new row to the db
    public void addUser(Users user){
        ContentValues values = new ContentValues();
        values.put(COLUMN_USERNAME, user.get_username());
        SQLiteDatabase db = getWritableDatabase();
        db.insert(TABLE_USERS, null, values);
        db.close();
    }

    public String databaseToString() {
        String dbString = "";
        SQLiteDatabase db = getWritableDatabase();
        String query = "SELECT * FROM " + TABLE_USERS + " WHERE 1";

        // Cursor point to a location in your results
        Cursor c = db.rawQuery(query, null);
        // Move to the first row in your results
        c.moveToFirst();

        while(!c.isAfterLast()) {
            if(c.getString(c.getColumnIndex("username")) != null) {
                dbString += c.getString(c.getColumnIndex("username"));
                dbString += "\n";
            }
        }
        db.close();
        return dbString;
    }
}

Answer 1

修复 NullPointerException:

首先，要修复 NullPointerException，请在 onCreate() 中初始化 Text:

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

    Input = (EditText) findViewById(R.id.Input);
    Text = (TextView) findViewById(R.id.Text); //added
    dbHandler = new Database(this, null, null, 1);
    // printDatabase();

}

调试技巧:

为了确保插入数据库成功，捕获insert()的返回值:

long retVal = db.insert(TABLE_USERS, null, values);

然后，为了验证 Cursor 中是否存在数据，获取 Cursor 的计数:

Cursor c = db.rawQuery(query, null);
int count = c.getCount(); //added

解决方案:

最后，这里要补充几点。首先，您需要推进 Cursor 以避免无限循环。此外，您应该检查 moveToFirst() 的返回值，如果返回 false，则什么也不做。最后，您应该在使用完 Cursor 后将其关闭，否则会发生内存泄漏。

public String databaseToString() {
    String dbString = "";
    SQLiteDatabase db = getWritableDatabase();
    String query = "SELECT * FROM " + TABLE_USERS + " WHERE 1";

    // Cursor point to a location in your results
    Cursor c = db.rawQuery(query, null);
    // Move to the first row in your results
    //check that moveToFirst returns true
    if (c.moveToFirst()){

      while(!c.isAfterLast()) {
        if(c.getString(c.getColumnIndex("username")) != null) {
            dbString += c.getString(c.getColumnIndex("username"));
            dbString += "\n";
        }
        c.moveToNext(); //added
      }

    }
    c.close(); //added
    db.close();
    return dbString;
}