Lesson 4.4 · Hashing

Hashing Pitfalls

Arrays as keys, mutable keys, boxed comparisons and missing keys: the bugs that make correct-looking hash code fail.

8 min

Think of it like this

If you change the name on a cloakroom ticket after handing in your coat, the attendant computes a different hook and can't find it. Keys must not change while they're in a map.

1.Four classic traps

Arrays as keys: int[] uses object identity for equals and hashCode, so two arrays with the same numbers are different keys. Use List<Integer> or a String.

Mutable keys: changing an object after putting it in a map changes its hash code, so the map can't find it again.

Boxed comparisons: map.get(a) == map.get(b) compares Integer references; use .equals or unbox.

Missing keys: map.get(k) returns null; unboxing null throws. Use getOrDefault.

Pitfalls.java

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Set<int[]> arrays = new HashSet<>();
        arrays.add(new int[]{1, 2});
        System.out.println(arrays.contains(new int[]{1, 2}));        // false: identity

        Set<List<Integer>> lists = new HashSet<>();
        lists.add(List.of(1, 2));
        System.out.println(lists.contains(List.of(1, 2)));           // true: content
    }
}

Output

false
true

Remember

Never use arrays as keys.
Never change a key while it's in a map.
Compare boxed values with equals; use getOrDefault.

Common mistakes

Set<int[]> for de-duplicating pairs.
count.get(x) + 1 when x might be missing.

Command Palette

Hashing Pitfalls

1.Four classic traps