会议室数据库查询返回空值(在Kotlin中应为空值安全) [英] Room database query returns null where it should be null-safe in Kotlin

问题描述

当数据库为空时，我有一个没有结果的查询.因此，NULL是正确的返回值.

I have a query that does not have a result, when the DB is empty. Therefore NULL is the correct return value.

但是，Android Studio中的编译器向我发出警告:
条件'maxDateTime！= null'始终为'true'.

However, the compiler in Android Studio gives me the warning:
Condition 'maxDateTime != null' is always 'true'.

如果我调试代码，则空检查会正确执行，因为该值实际上为空.

If I debug the code, the null check performs correctly as the value is actually null.

当我将接口重写为"fun queryMaxServerDate():字符串?"时(注意问号)，编译器警告就会消失.

When I rewrite the interface to 'fun queryMaxServerDate(): String?' (notice the question mark), the compiler warning goes away.

但是'fun queryMaxServerDate():String'是否不应该导致编译错误，因为它可以为null?

But should not 'fun queryMaxServerDate(): String' result in a compilation error since it can be null?

@Dao
interface CourseDao {

    // Get latest downloaded entry
    @Query("SELECT MAX(${Constants.COL_SERVER_LAST_MODIFIED}) from course")
    fun queryMaxServerDate(): String

}

---

// calling function
/**
 * @return Highest server date in table in milliseconds or 1 on empty/error.
 */
fun queryMaxServerDateMS(): Long {
    val maxDateTime = courseDao.queryMaxServerDate()

    var timeMS: Long = 0
    if (maxDateTime != null) { // Warning: Condition 'maxDateTime != null' is always 'true'
        timeMS = TimeTools.parseDateToMillisOrZero_UTC(maxDateTime)
    }
    return if (timeMS <= 0) 1 else timeMS
}

推荐答案

由注释生成的基础代码是 java ，因此，按照:-

The underlying code generated by the annotation is java and thus the exception to null safety as per :-

Kotlin的类型系统旨在从中消除NullPointerException 我们的代码. NPE的唯一可能原因可能是:

Kotlin's type system is aimed to eliminate NullPointerException's from our code. The only possible causes of NPE's may be:

显式调用以抛出NullPointerException();的用法！ 如下所述的运算符；

An explicit call to throw NullPointerException(); Usage of the !! operator that is described below;

关于某些数据不一致 进行初始化，例如:

Some data inconsistency with regard to initialization, such as when:

• 未初始化的此可用 构造函数在某处传递和使用(泄漏")；
• A 超类构造函数调用一个open成员，其实现在 派生类使用未初始化状态；
• Java互操作性:
  • 尝试访问平台类型为null的成员；
  • 用于Java互操作的泛型类型具有不正确的可空性， 例如一段Java代码可能会将空值添加到Kotlin中 MutableList，表示应使用MutableList 与之合作；
  • 由外部Java代码引起的其他问题.
  • An uninitialized this available in a constructor is passed and used somewhere ("leaking this");
  • A superclass constructor calls an open member whose implementation in the derived class uses uninitialized state;
  • Java interoperation:
    • Attempts to access a member on a null reference of a platform type;
    • Generic types used for Java interoperation with incorrect nullability, e.g. a piece of Java code might add null into a Kotlin MutableList, meaning that MutableList should be used for working with it;
    • Other issues caused by external Java code.

    无效安全

    例如在 CourseDao 中为 queryMaxServerDate()生成的代码将类似于:-

    e.g. the generated code for queryMaxServerDate() in CourseDao would be along the lines of :-

      @Override
      public String queryMaxServerDate() {
        final String _sql = "SELECT max(last_mopdified) from course";
        final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 0);
        __db.assertNotSuspendingTransaction();
        final Cursor _cursor = DBUtil.query(__db, _statement, false, null);
        try {
          final String _result;
          if(_cursor.moveToFirst()) {
            final String _tmp;
            _tmp = _cursor.getString(0);
            _result = _tmp;
          } else {
            _result = null;
          }
          return _result;
        } finally {
          _cursor.close();
          _statement.release();
        }
      }
    

    如您所见，没有提取数据(没有第一行)并且返回null.

    As you can see, no data extracted (no first row) and null is returned.

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