Mutable keys

Definition

If a field used by hashCode changes after the object is stored in a HashMap or HashSet, the object sits in the wrong bucket and can no longer be found. Use immutable objects (or records) as keys.

Example code

A key that changes after it was stored

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

class Tag {
    String name;
    Tag(String name) { this.name = name; }
    @Override public boolean equals(Object o) { return o instanceof Tag t && t.name.equals(name); }
    @Override public int hashCode() { return Objects.hash(name); }
}

record SafeTag(String name) { }       // a record is immutable: safe as a key

class Demo {
    public static void main(String[] args) {
        Set<Tag> tags = new HashSet<>();
        Tag java = new Tag("java");
        tags.add(java);

        java.name = "kotlin";                                           // the key changes after storing
        System.out.println(tags.contains(java));                        // false: now looks in another bucket
        System.out.println(tags.size());                                // still stored, but unreachable

        Set<SafeTag> safe = new HashSet<>();
        safe.add(new SafeTag("java"));
        System.out.println(safe.contains(new SafeTag("java")));
    }
}

// Output:
// false
// 1
// true