HashCode

HashCode returns an int hash value used by hash-based collections (HashMap, HashSet, etc.) to store and retrieve objects efficiently.

String s = "hello";
int hash = s.hashCode(); // hash value

Relationship with equals

If two objects are .equals(), they must have the same hashCode()
But two objects with same hashCode() may not be .equals()

if (a.equals(b)) {
    a.hashCode() == b.hashCode(); // must be true
}

Example

public class Person {
    String name;
    
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (!(obj instanceOf Person)) return false;
        Person p = (Person) obj;
        return this.name.equals(p.name);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(name); // consistent with equals()
    }
}

Tip

When working with HashMap or set - always keep in mind the equals() + hashCode() contract.

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Parent: _Object Methods