null

问题 please dont marked as duplicate , as the question is slightly different ---> null cannot be cast to non-null type kotlin.collections. MutableList Scenerios :- i have been performing delete cart using retrofit.. if atleast one item is present ,it displays in recyclerview 2.if cart is empty ,it crashes with a above error here is my adapter code:- class CartAdapter(context: Context, dataList: MutableList<DataCart?>) : RecyclerSwipeAdapter<CartAdapter.CustomViewHolder>() { //added

问题 I work in an area where memory utilization is very important to us, as we don't run on your classic web browsers / hardware. We use null quite a lot in our application, one thing that has not been clear to me is if null takes up more space than assigning a variable to undefined. Do we know if one or the other is more costly on memory usage? Thanks for help! 回答1: As you can see in this jsperf test, null seems to be slightly faster in the chrome (V8, just like nodejs) which might indicate it

问题 I work in an area where memory utilization is very important to us, as we don't run on your classic web browsers / hardware. We use null quite a lot in our application, one thing that has not been clear to me is if null takes up more space than assigning a variable to undefined. Do we know if one or the other is more costly on memory usage? Thanks for help! 回答1: As you can see in this jsperf test, null seems to be slightly faster in the chrome (V8, just like nodejs) which might indicate it

问题 I know how to initialize other variables such as int or String in constructor but I have no clue how to do it for List and Map. class StackOverFlowQuestion{ StackOverFlowQuestion({this.test='', this.map=?, this.list=?}); String test; Map map; List list; } What shall I replace the question marks with? Thanks. 回答1: The answer depends on whether your default list and map are constant. Let's assume your list is, but map is not. You'd write: class StackOverFlowQuestion { StackOverFlowQuestion({

问题 In flutter, I call a value from Firestore cloud database by using future and try to assigning this value to a variable. Here is my code: import 'package:cloud_firestore/cloud_firestore.dart'; import 'package:flutter/material.dart'; class Gyanpothro extends StatefulWidget { @override _GyanpothroState createState() => _GyanpothroState(); } class _GyanpothroState extends State<Gyanpothro> { Firestore db = Firestore.instance; Future databaseFuture; @override void initState() { databaseFuture = db

问题 In flutter, I call a value from Firestore cloud database by using future and try to assigning this value to a variable. Here is my code: import 'package:cloud_firestore/cloud_firestore.dart'; import 'package:flutter/material.dart'; class Gyanpothro extends StatefulWidget { @override _GyanpothroState createState() => _GyanpothroState(); } class _GyanpothroState extends State<Gyanpothro> { Firestore db = Firestore.instance; Future databaseFuture; @override void initState() { databaseFuture = db

问题 I have dictionary with the following keys and values. I am trying to print the dictionary, but nothing prints where there is a null value. How do I print "null" in the output? Dictionary<string, object> dic1 = new Dictionary<string, object>(); dic1.Add("id", 1); dic1.Add("name", "john"); dic1.Add("grade", null); Console.WriteLine(string.Join(Environment.NewLine, dic1.Select(a => $"{a.Key}: {a.Value}"))); Here is the output I get: id: 1 name: john grade: 回答1: You can use the null-coalescing

问题 I have lately been dipping my feet into the fascinating world of functional programming, largely due to gaining experience in FP platforms like React and reading up on blogs the likes of https://blog.ploeh.dk/. As a primarily imperative programmer, this has been an interesting transition, but I am still trying to get my wet feet under me. I am getting a little tired of using string.IsNullOrEmpty as such. So much of the time I find myself littering my code with expressions such as _ = string

问题 Some while ago, I've been reading through the book SQL and Relational Theory by C. J. Date. The author is well-known for criticising SQL's three-valued logic (3VL). 1) The author makes some strong points about why 3VL should be avoided in SQL, however he doesn't outline how a database model would look like if nullable columns weren't allowed . I've thought on this for a bit and have come up with the following solutions. If I missed other design options, I would like to hear about them! 1)

问题 I want to use unique in groupby aggregation, but I don't want nan in the unique result. An example dataframe: df = pd.DataFrame({'a': [1, 2, 1, 1, pd.np.nan, 3, 3], 'b': [0,0,1,1,1,1,1], 'c': ['foo', pd.np.nan, 'bar', 'foo', 'baz', 'foo', 'bar']}) a b c 0 1.0000 0 foo 1 2.0000 0 NaN 2 1.0000 1 bar 3 1.0000 1 foo 4 nan 1 baz 5 3.0000 1 foo 6 3.0000 1 bar And the groupby : df.groupby('b').agg({'a': ['min', 'max', 'unique'], 'c': ['first', 'last', 'unique']}) It's result is: a c min max unique