ruby中nil?, empty? and blank?

In Ruby, you check with nil? if an object is nil:

article = nil
article.nil?  # => true

empty? checks if an element - like a string or an array f.e. - is empty:

# Array
[].empty?   #=> true
# String
"".empty?   #=> true

Rails adds the method blank? to the Object class:

An object is blank if it‘s false, empty, or a whitespace string. For example, "", " ", nil, [], and {} are blank.

This simplifies

if !address.nil? && !address.empty?

to

if !address.blank?

.nil?

- It is Ruby method
- It can be used on any object and is true if the object is nil.
- "Only the object nil responds true to nil?" - RailsAPI

nil.nil? = true
anthing_else.nil? = false
a = nil
a.nil? = true
“”.nil = false

.empty?

- It is Ruby method
- can be used on strings, arrays and hashes and returns true if:

• String length == 0
• Array length == 0
• Hash length == 0

- Running .empty? on something that is nil will throw a NoMethodError

"".empty = true
" ".empty? = false

.blank?

- It is Rails method
- operate on any object as well as work like .empty? on strings, arrays and hashes.

nil.blank? = true 
[].blank? = true 
{}.blank? = true 
"".blank? = true 
5.blank? == false

- It also evaluates true on strings which are non-empty but contain only whitespace:

"  ".blank? == true"  ".empty? == false

Quick tip: !obj.blank? == obj.present?

activesupport/lib/active_support/core_ext/object/blank.rb, line 17 # (Ruby 1.9) 

def present? 
 !blank?
end