Spot the Bug #99

The key that changed after it was stored

A hard Java puzzle in the collections contracts family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.

hardCollections contractsbuggy line: 15

1import java.util.*;

3public class Mutable {

4 static class Key {

5 int id;

6 Key(int id) { this.id = id; }

7 @Override public boolean equals(Object o) { return o instanceof Key k && k.id == id; }

8 @Override public int hashCode() { return id; }

9 }

11 public static void main(String[] args) {

12 Map<Key, String> map = new HashMap<>();

13 Key k = new Key(1);

14 map.put(k, "one");

15 k.id = 2;

16 System.out.println(map.get(k));

17 }

18}

Highlighted: line 15. Play this one as a puzzle without the answer.

What is wrong

The entry was stored in the bucket for hash 1. After changing the key, lookups hash to 2, so get returns null, and the entry can no longer be found or removed, which is a memory leak. Keys must not change while in a map.

The fix

static final class Key { final int id; ... } // immutable key

How to avoid it next time

Use immutable objects (strings, records, enums) as map keys and set elements.

Want the background? Read How HashMap works in the Java concepts library.

About collections contracts

Collection classes come with contracts about ordering, mutability and key behavior. Fixed-size lists, views of other collections and mutable map keys break those contracts without compile errors.

In code review: Check what the factory method returns, whether the result is modified later, and whether objects used as keys can change after insertion.

More puzzles in this family

#97: A list that cannot grow (easy)
#98: An immutable list with a missing element (medium)
#100: The employee who disappeared from the set (medium)
#101: A list of numbers with one element (medium)
#102: Ten sorts before nine (easy)

🐞 Play today's puzzle All 132 bugs ← #98 #100 →

Frequently asked questions

What is the bug in "The key that changed after it was stored"?

How do I fix it?

Replace it with: static final class Key { final int id; ... } // immutable key. Use immutable objects (strings, records, enums) as map keys and set elements.

How do I catch collections contracts in code review?

Check what the factory method returns, whether the result is modified later, and whether objects used as keys can change after insertion.

Is there a new puzzle every day?

Yes. The daily Spot the Bug puzzle picks one of these snippets for each UTC day, with the difficulty set by the weekday. Your streak is stored only in your browser.

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