Lokacin da kake bugi equals() don bayyana daidaitaccen daidaita, dole ne ka bugi hashCode() kuma — saboda tarin da suka dogara da hash (HashMap, ) suna dogara ga yarjejeniya cewa . Karewa da haka yana haifar da kusan kuskure, wanda ba azaman tsofa.
HashSet// by default, equals() compares IDENTITY (same object?), hashCode() is based on memory address
Person p1 = new Person("Ann", 30);
Person p2 = new Person("Ann", 30);
p1.equals(p2); // false by default — different objects, even with same data
Ba tare da bugi ba, abubuwa biyu da suke da kamannin abubuwa "ba su daidai ba" — galibi ba abin da kake so wa abubuwan darajar.
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person p = (Person) o;
return age == p.age && Objects.equals(name, p.name); // compare by CONTENT
}
@Override
public int hashCode() {
return Objects.hash(name, age); // MUST be consistent with equals()
}
Yarjejeniya: idan a.equals(b) gaskiya ne, to a.hashCode() == b.hashCode() DOLE ne gaskiya ne.
Person p1 = new Person("Ann", 30);
Person p2 = new Person("Ann", 30); // equal by our equals()
Set<Person> set = new HashSet<>();
set.add(p1);
set.contains(p2); // ❌ likely FALSE — even though p1.equals(p2)!
Me ya sa haka? HashMap/HashSet suna fara amfani da hashCode() don samun daidai bucket, sannan equals() a cikinsa. Tare da tsohuwar (identity-based) hashCode(), p1 da p2 suna shiga daban-dabai buckets, saboda haka contains kada ma suka kira su — saitin yana tunani suna daban. Wannan yana haifar da abubuwan da ba a gabane: kuje a Set, gazawar kargi, da sauran.
1. equal objects → equal hash codes (REQUIRED for correctness)
2. unequal objects MAY have the same hash (collisions are allowed)
3. hashCode() must be consistent (same object → same code, unchanged)
4. equals() must be reflexive, symmetric, transitive, consistent
record Person(String name, int age) {} // records auto-generate equals/hashCode/toString!
Java records (da IDE jenerewa / Lombok) suna samar da daidaitaccen, dogaraccen equals/hashCode maka.
Yarjejeniya na equals/hashCode ita ce ɗaya daga mahimmantan -- da kuma yawan karewa -- sagogi a Java.
Bugi equals() ba tare da hashCode() ba yana karewa tarin da suka dogara da hash a waɗanda ba azaman tsofa a gano: abubuwan da kake tunani suna daidai suna ɗora "rasa" a HashMap/HashSet (buɗewar kargi da karewa, phantom kuje) saboda suna hash zuwa daban-dabai buckets.
'alal fakondiji cewa waɗannan tarin suna kasancewa sosai, kowane class-darajar da aka yi amfani da shi azaman serke ko saitin jijiya dole ne ya bugi **duka" tare.
Janye fihimta me ya sa (mekanismin bucket-then-equals kargi) -- da amfani da records ko jenerewan lambar don samun daidai -- yana da muhimmanci don daidaitaccen hali da gida kasada kasuwa wanda ta nuna tunani na gaskiya na yadda Java tarin ke aiki.