How do I combine two lists in Dart?

Question

I was wondering if there was an easy way to concatenate two lists in dart to create a brand new list object. I couldn't find anything and something like this:

My list:

list1 = [1, 2, 3]
list2 = [4, 5, 6]

I tried:

var newList = list1 + list2;

I wanted the combined output of:

[1, 2, 3, 4, 5, 6]

Solution 1

You can use:

var newList = new List.from(list1)..addAll(list2);

If you have several lists you can use:

var newList = [list1, list2, list3].expand((x) => x).toList()

As of Dart 2 you can now use +:

var newList = list1 + list2 + list3;

As of Dart 2.3 you can use the spread operator:

var newList = [...list1, ...list2, ...list3];

Solution 2

maybe more consistent~

var list = []..addAll(list1)..addAll(list2);

Solution 3

Dart now supports concatenation of lists using the + operator.

Example:

List<int> result = [0, 1, 2] + [3, 4, 5];

Solution 4

If you want to merge two lists and remove duplicates could do:

var newList = [...list1, ...list2].toSet().toList(); 

Solution 5

Alexandres' answer is the best but if you wanted to use + like in your example you can use Darts operator overloading:

class MyList<T>{
  List<T> _internal = new List<T>();
  operator +(other) => new List<T>.from(_internal)..addAll(other);
  noSuchMethod(inv){
    //pass all calls to _internal
  }
}

Then:

var newMyList = myList1 + myList2;

Is valid :)

View more solutions

Comments

[Florian Loitsch]

An alternative (which allows to easily concatenate many lists): [list1, list2, list3, ...].expand((x) => x).toList();

[Abbas.M]

Old i know but what is the ..addAll() why not just a single dot?

[csga5000]

@Abbas.M .. is for chaining, to do with out the double dot you would have to: list = []; list.addAll(list1); list.addAll(list2); The way I see it, it basically means call this function, but ignore what it returns, and keep operating on the object we were operating on.

[Sébastien]

Note that the + operator cannot be be used on lists of different types. (in this case you get an error like type 'List<Widget>' is not a subtype of type 'List<Image>'. ). The spread operator works great for this use case though.

[Roxx]

Can you please check this question and suggest me something. stackoverflow.com/questions/62228799/…

[Fuad All]

best solution with preventing duplicate items.👍

[visoft]

True, listone will contain the sum. But addAll will return null. Can't use it where you want the result, directly (like list of widgets)

[PJP]

It helps more if you supply an explanation why this is the preferred solution and explain how it works. We want to educate, not just provide code.

[Sassan]

var newList = {...list1, ...list2}.toList();