Are mutable hashmap keys a dangerous practice?

Question

Is it bad practice to use mutable objects as Hashmap keys? What happens when you try to retrieve a value from a Hashmap using a key that has been modified enough to change

Answer 1

It has been noted by many well respected developers such as Brian Goetz and Josh Bloch that :

If an object’s hashCode() value can change based on its state, then we must be careful when using such objects as keys in hash-based collections to ensure that we don’t allow their state to change when they are being used as hash keys. All hash-based collections assume that an object’s hash value does not change while it is in use as a key in the collection. If a key’s hash code were to change while it was in a collection, some unpredictable and confusing consequences could follow. This is usually not a problem in practice — it is not common practice to use a mutable object like a List as a key in a HashMap.