Difference between Hash.new(arg) and Hash.new(&block)

hash_constructor_differences

hash = Hash.new("")              # => {}
hash[1] << "Hello"               # => "Hello"
hash[2]                          # => "Hello"
hash = Hash.new{|h,k| h[k] = ""} # => {}
hash[1] << "Hello"               # => "Hello"
hash[2]                          # => ""