What does JD.COM mean by sending a text message showing [JD.COM] "empty"?

Null means null pointer, that is, invalid.

Null is a reserved value in calculation, indicating that the pointer does not point to a valid object. Programs usually use null pointers to indicate conditions, such as the end of an unknown length list or the inability to perform certain operations; The use of this null pointer can be compared with the Nothing value in nullable types and option types.

Because Null is not a data value, but a symbol with missing value, using Null as a mathematical operator will produce unknown results. Because Null is not a member of any data field, it is not regarded as a "value", but a marker (or placeholder) of the missing value. Therefore, comparison with Null never leads to true or false, but always becomes in the unknown result of ternary logic.

Extended data:

Different meanings of zero in computer science;

1. null value (SQL)? (NULL), a special flag used to indicate that there are no data values in the database.

2. Empty character, a control character with a value of 0. Many character encodings contain null characters.

3. An empty device is a special device file, which discards all the data written in it, but reports that the write operation is successful, and immediately reads it to obtain EOF.

4. Empty modem, also known as empty modem. This is a communication method that uses RS-232 serial cable to directly connect two data terminal devices (without modem).

5. Empty object pattern, a software design pattern.

6. Empty string, zero-length string.

7. Empty termination string, the length of which is determined by the first empty character.

8. Nullable types (a feature of some static programming languages) allow data types to be set to special NULL values.

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