h = Hash.new('java')
h[:a] = 33
h.default() #=> 'java'
h.default(:a) #=> 'java'
h.default(:b) #=> 'java'
按 Ruby 文档说法，default() 不提供参数，或参数 (key) 不存在时，返回默认值。key 存在，返回键对应的值。
可结果却不一样，这是 bug 吗？ruby-2.0.0-p648, ruby-2.3 都是一样的结果
If obj is specified, this single object will be used for all default values
所以要用 block 的形式：
Hash.new { |h, k| h[k] = 'java' }
[1] pry(main)> h = Hash.new('java')
=> {}
[2] pry(main)> h[:a] = 33
=> 33
[3] pry(main)> h
=> {:a=>33}
[4] pry(main)> h.default(:a)
=> "java"
[5] pry(main)> h.default
=> "java"
[6] pry(main)> h[:b]
=> "java"
[7] pry(main)> h[:a]
=> 33
[8] pry(main)> h[:c]
=> "java"
Supplemental notes
To clarify: When called with no argument, it returns the default value. When called with a hash key it returns the corresponding value for that key if the hash contains that key. Only if the key does not exist does it return the default value.
官方原文，。。。。郁闷
