$ macirb
irb(main):001:0> a = NSMutableArray.alloc.init
=> []
irb(main):002:0> a.blank?
NoMethodError: undefined method `blank?' for []:NSMutableArray
	from (irb):2:in `<main>'
	from core:in `eval:'
	from core:in `evaluate:'
irb(main):003:0> class Array; def blank?; length == 0 end end
=> nil
irb(main):004:0> a.blank?
=> true
irb(main):005:0> a << 1
=> [1]
irb(main):006:0> a.blank?
=> false

$ jirb
irb(main):001:0> a = java.util.ArrayList.new
=> #<Java::JavaUtil::ArrayList:0x4df8b14>
irb(main):002:0> a.blank?
NoMethodError: undefined method `blank?' for #<Java::JavaUtil::ArrayList:0x4df8b14>
	from (irb):3
irb(main):003:0> class Array; def blank?; length == 0 end end
=> nil
irb(main):004:0> a.blank?
NoMethodError: undefined method `blank?' for #<Java::JavaUtil::ArrayList:0x4df8b14>
	from (irb):5