Скачать или смотреть Never Mutate Keys Used in HashMap

When you use a mutable object as a key inside a HashMap, changing the object's fields after inserting it into the map can break retrieval.

In the example below, class A uses its id in hashCode(). The key is inserted into the map, but later the id is mutated:

class A {
int id;
public int hashCode() { return id; }
}

A a = new A();
map.put(a, "value"); // key stored using current hash (id = 0)
a.id = 10; // key mutated → hashCode now returns 10

System.out.println(map.get(a));

❌ Bug: The value cannot be retrieved

HashMap stores the key in a bucket based on the old hash.
Once you mutate the key, its hash changes → the map looks in the wrong bucket → returns null.

🔍 Key Takeaway

Never mutate fields that participate in hashCode() or equals() after using the object as a Map key or Set element.

Prefer immutable objects for keys.

This is a classic Java bug that even experienced developers sometimes overlook.