สัญญาของ equals/hashCode

สองเมธอด หนึ่งสัญญา

public class Main {
    public static void main(String[] args) {
        Object a = new Object();
        Object b = new Object();
        System.out.println(a.equals(b));
        System.out.println(a.hashCode() == b.hashCode());
    }
}

กฎหลัก

public class Main {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "hel" + "lo";
        System.out.println(s1.equals(s2));
        System.out.println(s1.hashCode() == s2.hashCode());
    }
}

อ็อบเจ็กต์ที่ไม่เท่ากันอาจมีค่าแฮชร่วมกัน

public class Main {
    public static void main(String[] args) {
        // Equal value, unequal objects, same hash is fine
        System.out.println("FB".hashCode() == "Ea".hashCode());
        System.out.println("FB".equals("Ea"));
    }
}

คลาสที่โอเวอร์ไรด์เฉพาะ equals

public class Main {
    static class Point {
        final int x, y;
        Point(int x, int y) { this.x = x; this.y = y; }
        @Override public boolean equals(Object o) {
            if (!(o instanceof Point p)) return false;
            return x == p.x && y == p.y;
        }
        // BUG: no hashCode override
    }
    public static void main(String[] args) {
        Point a = new Point(1, 2);
        Point b = new Point(1, 2);
        System.out.println("equals: " + a.equals(b));
        System.out.println("same hash: " + (a.hashCode() == b.hashCode()));
    }
}

ข้อบกพร่องใน HashSet

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

public class Main {
    static class Point {
        final int x, y;
        Point(int x, int y) { this.x = x; this.y = y; }
        @Override public boolean equals(Object o) {
            if (!(o instanceof Point p)) return false;
            return x == p.x && y == p.y;
        }
    }
    public static void main(String[] args) {
        Set<Point> set = new HashSet<>();
        set.add(new Point(1, 2));
        set.add(new Point(1, 2));
        System.out.println("size = " + set.size());
    }
}

การแก้ไข

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

public class Main {
    static class Point {
        final int x, y;
        Point(int x, int y) { this.x = x; this.y = y; }
        @Override public boolean equals(Object o) {
            if (!(o instanceof Point p)) return false;
            return x == p.x && y == p.y;
        }
        @Override public int hashCode() { return Objects.hash(x, y); }
    }
    public static void main(String[] args) {
        Set<Point> set = new HashSet<>();
        set.add(new Point(1, 2));
        set.add(new Point(1, 2));
        System.out.println("size = " + set.size());
    }
}

equals ต้องมีสมบัติสะท้อนตัวเอง

public class Main {
    public static void main(String[] args) {
        String x = "java";
        System.out.println(x.equals(x));
    }
}

สมมาตรและถ่ายทอด

public class Main {
    public static void main(String[] args) {
        String a = "x", b = "x", c = "x";
        boolean symmetric = a.equals(b) == b.equals(a);
        boolean transitive = a.equals(b) && b.equals(c) && a.equals(c);
        System.out.println("symmetric: " + symmetric);
        System.out.println("transitive: " + transitive);
    }
}

สอดคล้อง

import java.util.Objects;

public class Main {
    public static void main(String[] args) {
        // Immutable record gives consistent equals/hashCode automatically
        record Id(int value) {}
        Id id = new Id(7);
        System.out.println(Objects.equals(id, new Id(7)));
        System.out.println(id.hashCode() == new Id(7).hashCode());
    }
}

ระเบียนจัดการให้คุณ

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

public class Main {
    record Point(int x, int y) {}
    public static void main(String[] args) {
        Set<Point> set = new HashSet<>();
        set.add(new Point(1, 2));
        set.add(new Point(1, 2));
        System.out.println("size = " + set.size());
    }
}

กับดักคีย์ที่เปลี่ยนแปลงได้

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;

public class Main {
    static class Box {
        int id;
        Box(int id) { this.id = id; }
        @Override public boolean equals(Object o) {
            return o instanceof Box b && b.id == id;
        }
        @Override public int hashCode() { return Objects.hash(id); }
    }
    public static void main(String[] args) {
        Map<Box, String> m = new HashMap<>();
        Box key = new Box(1);
        m.put(key, "value");
        key.id = 99; // mutated after insertion
        System.out.println(m.get(key));
    }
}

ตรวจสอบความเข้าใจ

ทบทวน

import java.util.Objects;

public class Main {
    public static void main(String[] args) {
        System.out.println("Contract recap: " + Objects.hash(1, 2, 3));
    }
}