การสร้าง hashCode

เป้าหมายของ hashCode ที่ดี

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 สำหรับกรณีทั่วไป

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());
    }
}

ตัวคูณ 31 แบบดั้งเดิม

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() {
            int result = 17;
            result = 31 * result + (name == null ? 0 : name.hashCode());
            result = 31 * result + age;
            return result;
        }
    }
    public static void main(String[] args) {
        System.out.println(new User("Ada", 36).hashCode());
    }
}

การคำนวณแฮชของชนิดข้อมูลพื้นฐาน

public class Main {
    public static void main(String[] args) {
        long id = 4_000_000_000L;
        int longHash = (int) (id ^ (id >>> 32));
        System.out.println("long hash: " + longHash);
        System.out.println("double hash: " + Double.hashCode(3.14));
        System.out.println("bool hash: " + Boolean.hashCode(true));
    }
}

การคำนวณแฮชของอาร์เรย์

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] a = {1, 2, 3};
        int[] b = {1, 2, 3};
        System.out.println("identity equal: " + (a.hashCode() == b.hashCode()));
        System.out.println("content equal: " + (Arrays.hashCode(a) == Arrays.hashCode(b)));
    }
}

ทำให้ equals และ hashCode สอดคล้องกัน

import java.util.Objects;

public class Main {
    static class Coord {
        final int x, y;
        Coord(int x, int y) { this.x = x; this.y = y; }
        @Override public boolean equals(Object o) {
            return o instanceof Coord c && c.x == x && c.y == y;
        }
        @Override public int hashCode() { return Objects.hash(x, y); }
    }
    public static void main(String[] args) {
        Coord a = new Coord(3, 4), b = new Coord(3, 4);
        System.out.println(a.equals(b) && a.hashCode() == b.hashCode());
    }
}

การเก็บค่าแฮชไว้ใช้ซ้ำ

import java.util.Objects;

public class Main {
    static final class Key {
        final String a, b;
        private int hash; // 0 until computed
        Key(String a, String b) { this.a = a; this.b = b; }
        @Override public int hashCode() {
            int h = hash;
            if (h == 0) { h = Objects.hash(a, b); hash = h; }
            return h;
        }
    }
    public static void main(String[] args) {
        Key k = new Key("x", "y");
        System.out.println(k.hashCode());
        System.out.println(k.hashCode());
    }
}

การกระจายมีความสำคัญ

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

public class Main {
    record Pair(int a, int b) {}
    public static void main(String[] args) {
        Set<Integer> hashes = new HashSet<>();
        for (int i = 0; i < 100; i++) {
            hashes.add(Objects.hash(i, i * 7));
        }
        System.out.println("distinct hashes: " + hashes.size());
    }
}

ตัวอย่างการกระจายแบบ Bad

public class Main {
    static int badHash(int a, int b) { return a + b; }
    static int goodHash(int a, int b) { return 31 * a + b; }
    public static void main(String[] args) {
        System.out.println("bad (1,2): " + badHash(1, 2) + ", (2,1): " + badHash(2, 1));
        System.out.println("good (1,2): " + goodHash(1, 2) + ", (2,1): " + goodHash(2, 1));
    }
}

ควรใช้ระเบียนสำหรับชนิดข้อมูลค่า

public class Main {
    record Money(long cents, String currency) {}
    public static void main(String[] args) {
        Money a = new Money(1099, "USD");
        Money b = new Money(1099, "USD");
        System.out.println(a.equals(b));
        System.out.println(a.hashCode() == b.hashCode());
    }
}

ประกอบทุกอย่างเข้าด้วยกัน

import java.util.Objects;

public class Main {
    static final class Version {
        final int major, minor, patch;
        Version(int major, int minor, int patch) {
            this.major = major; this.minor = minor; this.patch = patch;
        }
        @Override public boolean equals(Object o) {
            return o instanceof Version v && v.major == major && v.minor == minor && v.patch == patch;
        }
        @Override public int hashCode() { return Objects.hash(major, minor, patch); }
        @Override public String toString() { return major + "." + minor + "." + patch; }
    }
    public static void main(String[] args) {
        Version v = new Version(2, 1, 0);
        System.out.println(v + " hash=" + v.hashCode());
    }
}

ตรวจสอบความเข้าใจ

ทบทวน

import java.util.Objects;

public class Main {
    public static void main(String[] args) {
        System.out.println("hashCode recap done: " + Objects.hash("done"));
    }
}