Three Methods, One Contract, One Silent Failure

Five things this topic established, and where each one was.

What the topic established, and which card it was on
The thing	Where
Every class inherits `equals`, `hashCode` and `toString` with defaults	The Three Methods Every Object Already Has
The default `equals` compares addresses, not contents	Two Identical Orders That Java Says Are Not Equal
Equal objects must have equal hash codes; equal hash codes need not mean equal objects	The Contract Between equals and hashCode
`hashCode` chooses the bucket; `equals` decides what was found in it	Following One Key Into a HashMap
Override the two together, from fields that never change	Why Changing a Key After Putting It In

The one worth carrying: breaking the contract throws nothing. It makes a value you stored impossible to find again and leaves size() insisting it is there, which is why this is worth getting right before anything depends on it.

Java
Object-Oriented Programming
Hash Tables