#1 most askedVery commonMediumCore Java & OOP

The equals() and hashCode() contract

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Why must you override hashCode() whenever you override equals()? What breaks if you don't?

🔮 Predict the output

class Point {
    final int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
    @Override public boolean equals(Object o) {
        return o instanceof Point p && p.x == x && p.y == y;
    }
    // hashCode() is NOT overridden
}
public class Main {
    public static void main(String[] args) {
        Set<Point> seen = new HashSet<>();
        seen.add(new Point(1, 2));
        System.out.println(new Point(1, 2).equals(new Point(1, 2)));
        System.out.println(seen.contains(new Point(1, 2)));
    }
}

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☕ Java example · compiled and run to verify it works

record Point(int x, int y) {}  // records generate equals, hashCode and toString
Set<Point> seen = new HashSet<>();
seen.add(new Point(1, 2));
System.out.println(seen.contains(new Point(1, 2)));

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