package classes; /** * * - equals() * for defining equivalence relation (reflexive, transitive, symmetric) * must be consistent until object is modified * used when search inside collection (e.g. contains, remove ...) * tells whether you have found exact match * default implementation in Object class use operator '=' * * - hashCode() * must return consistent integer value for object during application execution * used when inserting object into hashtable, hashmap or hashset * tells which bucket to look in * default implementation in Object class provides memory address * * - Note * if override one of them, then necessary to override the other one too * if override, both must be public * same set of fields should be used to compute both * If a.equals(b), then a.hashCode() and b.hashCode() must be same * same hashcode for two objects doesn't guarantee one equals to other */ public class EqualsAndHashCode { String name; int versionId; @Override public boolean equals(Object obj) { if (obj instanceof EqualsAndHashCode) return this.name.equals(((EqualsAndHashCode)obj).name) && this.versionId == ((EqualsAndHashCode)obj).versionId; return false; } @Override public int hashCode() { return name.hashCode() + versionId * 19; } }