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Uint8List.hashCode #16335
Description
This issue was originally filed by @stevenroose
What steps will reproduce the problem?
Uint8List a = new Uint8List.fromList([0,0]);
Uint8List b = new Uint8List.fromList([0,0]);
print(a.hashCode == b.hashCode);
print(a == b);
What is the expected output?
true
true
What do you see instead?
false
false
Please provide any additional information below.
Java has a method java.util.Arrays.hashcode(byte[])
Why isn't this behaviour adopted? Uint8List resembles an immutable byte array, shouldn't it be comparable and have a hashCode?
Activity
rakudrama commentedon Jan 27, 2014
Uint8List is using object identity for 'get hashCode' and 'operator =='.
This is consistent with all collections in the dart libraries.
Unlike a String, Uint8List is not immutable:
Uint8List a = new Uint8List.fromList([0,0]);
print(a) -> [0, 0];
a[1] = 2;
print(a) -> [0, 2];
A default hashCode that must look at all the elements of the list at each call would be slow.
If you want to use a Uint8List object as a HashMap key, you can create a HashMap with custom equals and hashCode methods:
https://api.dartlang.org/docs/channels/stable/latest/dart_collection/HashMap.html#
Added Area-Library, Library-TypedData, Triaged labels.
DartBot commentedon Jan 27, 2014
This comment was originally written by @stevenroose
Hmm, they are indeed not immutable, should've seen that.
Is there a workaround like in Java?
lrhn commentedon Jan 28, 2014
You can use package:collection/equality to get an external equality and hash code for a list.
Example:
import "package:collection/equality.dart";
import "dart:collection" show HashMap;
import "dart:typed_data";
main() {
const equality = const ListEquality<int>();
Uint8List a = new Uint8List.fromList([0,0]);
Uint8List b = new Uint8List.fromList([0,0]);
print(equality.hash(a) == equality.hash(b));
print(equality.equals(a, b));
// and use it as parameters to hashmap:
var map = new HashMap(equals: equality.equals, hashCode: equality.hash);
}
lrhn commentedon Jan 29, 2014
We don't plan to change equality or hashCode on collections at this point.
It would be a breaking change for any use of them that assumes the current (identity) equality.
Added AsDesigned label.