Java Academy · Lesson

Implementing hashCode

Write correct hash functions.

Goals of a Good hashCode

A good hashCode() should:

Return the same value for equal objects (the contract).
Spread unequal objects across many different values.
Be cheap to compute.

A poor hashCode that returns a constant still satisfies the contract but turns the map into a slow linked list.

public class Main {
    public static void main(String[] args) {
        // Legal but terrible: every object collides
        System.out.println("constant hashCode is legal but kills performance");
    }
}

Objects.hash for the Common Case

The simplest correct approach is Objects.hash(field1, field2, ...).

It handles nulls and combines fields with a standard algorithm. Use the same fields you compare in equals.

import java.util.Objects;

public class Main {
    static class User {
        final String name; final int age;
        User(String name, int age) { this.name = name; this.age = age; }
        @Override public int hashCode() { return Objects.hash(name, age); }
    }
    public static void main(String[] args) {
        User a = new User("Ada", 36);
        User b = new User("Ada", 36);
        System.out.println(a.hashCode() == b.hashCode());
    }
}

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