How does a set determine that two objects are equal in dart? -

How does a set determine that two objects are equal in dart? -

i don't understand how set determines when 2 objects equal. more specific, when add method of set, adds new object, , when doesn't deed new object, because object in set ?

for example, have objects next class:

class action { final function function; final string description; action(this.function, this.description); call() => function(); tostring() => description; }

now think next set contain 2 elements, 2 of them equal:

void main() { set<action> actions = new set() ..add(new action(() => print("a"), "print a")) ..add(new action(() => print("a"), "print a")) ..add(new action(() => print("b"), "print b")); }

but instead, set contains 3 action objects. see demo. how can create sure equal objects seen equal in set ?

for comprehensive write-up operator== in dart see http://work.j832.com/2014/05/equality-and-dart.html

it checks if equal a == b can override == operator customize behavior. maintain in mind hashcode should overridden when == operator overridden.

class="lang-dart prettyprint-override">class action { @override bool operator==(other) { // dart ensures operator== isn't called null // if(other == null) { // homecoming false; // } if(other is! action) { homecoming false; } homecoming description == (other action).description; } // hashcode must never alter otherwise value can't // found anymore illustration when used key // in hashmaps hence cache after first creation. // if want combine more values in hashcode creation // see http://stackoverflow.com/a/26648915/217408 // 1 attempt, depending on requirements // different handling might more appropriate. // far aware there no right reply // objects members taking part of hashcode , // equality calculation mutable. // see http://stackoverflow.com/a/27609/217408 int _hashcode; @override int hashcode { if(_hashcode == null) { _hashcode = description.hashcode } homecoming _hashcode; } // when key (description) immutable , // fellow member of key can utilize // int hashcode => description.hashcode }

try @ dartpad

dart