intro
- hash code is not Hash key-value data structure.
- hash code is usually a short identifier of an object.
- the short identifier is usually a numeric value, so it is faster.
- eql?() method compare with two objects' hash codes.
- hash() method return the object's hash code.
- better overwrite ==, hash, and eql? methods.
- Array#uniq, Hash#lookups compare with hash code.
