In what situations can an object be lost from a HashMap?

An object can be lost from a HashMap in Java in the following situations:

• If the object's key overrides the hashCode() and equals() methods incorrectly, for example, changing the state that affects the hash code after placing it in the HashMap. Then the object may become inaccessible for lookup.
• If a weak reference is used (for example, WeakHashMap), and the key or value no longer has strong references, the garbage collector may delete them.
• If the object is explicitly removed via the remove() methods.

Example of a problem with a mutable key:

Map<List<String>, String> map = new HashMap<>();
List<String> key = new ArrayList<>();
key.add("one");
map.put(key, "value");

key.add("two"); // changed the key after adding

// Now, map.get(key) may return null because the hashCode has changed

Therefore, keys in a HashMap should be immutable or not change the state that affects hashCode and equals.