Android：尝试在空对象引用[duplicate]上调用虚方法'void android.database.sqlite.SQLiteDatabase.execSQL（java.lang.String）'

Question

这个问题在这里已有答案：

• What is a NullPointerException, and how do I fix it? 12个答案

我收到这个错误：

无法启动活动ComponentInfo {amsi.dei.estg.ipleiria.pt.ima / amsi.dei.estg.ipleiria.pt.ima.ListaEmpresas}：java.lang.NullPointerException：尝试调用虚方法'void android.database。我的DBHelper中这一行上的空对象引用上的sqlite.SQLiteDatabase.execSQL（java.lang.String）：

错误在这一行：

database.execSQL(createEmpresasTable);

我的DBHelper：

public class ImaBDHelper extends SQLiteOpenHelper {

private static final String DB_NAME = "IMA";
private static final int DB_VERSION = 1;

private static final String EMPRESAS_TABLE_NAME = "empresas";
private static final String NOME_EMPRESA = "nome";
private static final String DESCRICAO_EMPRESA = "descricao";
private static final String LOCALIDADE_EMPRESA = "localidade";
private static final String AREA_EMPRESA = "area";

private final SQLiteDatabase database;

public ImaBDHelper(Context context)
{
    super(context, DB_NAME, null, DB_VERSION);

    this.database = getWritableDatabase();
}


@Override
public void onCreate(SQLiteDatabase db) {
    String createEmpresasTable = "CREATE TABLE " + EMPRESAS_TABLE_NAME +
            "(id INTENGER PRIMARY KEY AUTOINCREMENT," +
            NOME_EMPRESA + " TEXT NOT NULL," +
            DESCRICAO_EMPRESA + " TEXT," +
            LOCALIDADE_EMPRESA + " TEXT NOT NULL," +
            AREA_EMPRESA + " TEXT NOT NULL," + ")";
database.execSQL(createEmpresasTable);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    String sql = "DROP TABLE IF EXISTS " + EMPRESAS_TABLE_NAME;
    database.execSQL(sql);
    this.onCreate(database);
}

其余的只是创建，读取，更新和删除的方法。

很抱歉，如果这是重复但我在搜索中找不到这个确切问题的答案。先感谢您！

Answer 1

使用onCreate(SQLiteDatabase db)签名值代替private final SQLiteDatabase database;执行查询，如下所示

@Override
public void onCreate(SQLiteDatabase db) {
    String createEmpresasTable = "CREATE TABLE " + EMPRESAS_TABLE_NAME +
            "(id INTENGER PRIMARY KEY AUTOINCREMENT," +
            NOME_EMPRESA + " TEXT NOT NULL," +
            DESCRICAO_EMPRESA + " TEXT," +
            LOCALIDADE_EMPRESA + " TEXT NOT NULL," +
            AREA_EMPRESA + " TEXT NOT NULL," + ")";
db.execSQL(createEmpresasTable);// change here
}

像这样也用在你的onUpgrade（）