Question

我有一个方法来搜索特定的项目，但是当我使用这个代码并尝试使用返回对象时，我得到NullPointerException

public Obra buscarInfoObra(String filtro) {
    SQLiteDatabase db = helper.getReadableDatabase();
    Obra obraSelecionada = null;


    Cursor cursor = db.rawQuery("SELECT * FROM " +BancoDadosHelper.TABELA_OBRA+ " WHERE "+BancoDadosHelper.COLUNA_NUM_OBRA + "=" +filtro, null);

    if (cursor.moveToFirst()){
        String numObra = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_NUM_OBRA));
        int anoObra = cursor.getInt(cursor.getColumnIndex(BancoDadosHelper.COLUNA_ANO_OBRA));
        String descObra = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_DESCRICAO_OBRA));
        String jurisdicionado = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_JURISDICIONADO));
        String tipoObra = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_TIPO_OBRA));
        String tipoObjeto = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_TIPO_OBJETO));
        String endereco = cursor.getString(cursor.getColumnIndex(BancoDadosHelper.COLUNA_ENDERECO_OBRA));

        obraSelecionada = new Obra(numObra,anoObra,descObra,jurisdicionado,tipoObra,tipoObjeto,endereco);
    }

    cursor.close();
    db.close();
    return obraSelecionada;

我做错了什么？

[编辑]这不是重复，我知道什么是NullPointerException，但我不明白为什么我得到这个例外。我已经尝试将现有数据直接放在代码上，但仍然得到NullPointer ..

Answer 1

您应该检查光标上的null，然后使用，然后关闭。

public Obra buscarInfoObra(String filtro) {
    SQLiteDatabase db = helper.getReadableDatabase();
    Obra obraSelecionada = null;
    Cursor cursor = db.rawQuery(...);
    if (cursor != null) {
        if (cursor.moveToFirst()) {
            // use cursor
            obraSelecionada  = ...
        }
        cursor.close();
    }
    db.close();
    return obraSelecionada;
}

但是 null游标可以由错误的sql语句组成。你需要用等于

的引号来包装你的等号的字符串

"SELECT * FROM table_name WHERE column_name = 'some value'"

你有一些看起来更像是

的东西

"SELECT * FROM table_name WHERE column_name = some value"

所以这应该修复

Cursor cursor = db.rawQuery("SELECT * FROM " + BancoDadosHelper.TABELA_OBRA + " WHERE " + BancoDadosHelper.COLUNA_NUM_OBRA + "=" + "'" + filtro + "'", null);

侧面提示，您应该编写这样的查询，以避免sql注入

db.query(BancoDadosHelper.TABELA_OBRA, null,
     BancoDadosHelper.COLUNA_NUM_OBRA + "=?",
     new String[] { filtro  },
     null, null, null);

希望有所帮助

Answer 2

尝试这种方式

  public RentMasterModel getRent(int id) {
    SQLiteDatabase db = this.getReadableDatabase();

    Cursor cursor = db.query(TABLE_RENTMASTER, new String[]{KEY_RENTMASTER_ITEMID,
                    KEY_RENTMASTER_RENTHOUR, KEY_RENTMASTER_RENTDAY, KEY_RENTMASTER_RENTWEEK, KEY_RENTMASTER_RENTMONTH}
            , KEY_RENTMASTER_ITEMID + "=?",
            new String[]{String.valueOf(id)}, null, null, null, null);
    if (cursor != null)
        cursor.moveToFirst();

    RentMasterModel rm = new RentMasterModel(Integer.parseInt(cursor.getString(0)),
            cursor.getString(1), cursor.getString(2), cursor.getString(3), cursor.getString(4));
    // return contact
    return rm;
}

这里将游标值添加到bean类

在SQLite上选择返回null

2 个答案: