Why does this Dart code give "The operand can't be null, so the condition is always true. Remove the condition"?

import 'package:sqflite/sqflite.dart';

class DatabaseHelper {
  static final DatabaseHelper instance = DatabaseHelper._privateConstructor();
  static late Database _database;

  DatabaseHelper._privateConstructor();

  Future<Database> get database async {
    if (_database != null) return _database;  // HERE: "The operand can't be null, so the condition is always true. Remove the condition"
    _database = await _initDatabase();
    return _database;
  }

  Future<Database> _initDatabase() async {
    Database _db = await openDatabase('my_db.db');
    return _db;
  }

}

CodePudding user response：

Because checking if _database is null is redundant, since you're declaring it as

static late Database _database;

Database here is an non-nullable type, therefore it can't be null.

Just to make it clear, this is a warning rather than an error.

To refactor your code, declare the _database variable as nullable

static Database? _database;

and your database getter as

Future<Database> get database async {
  final Database? instance = _database;
  if (instance != null) return instance;
  instance = await _initDatabase();
  _database = instance;
  return instance;
}

CodePudding user response：

It is because when you define _database, you specifically defined it as non-nulable:

static late Database _database;

If it was a nullable one, you would add the "?"

static late Database? _database;