s = ''
#  s.is_a? String, this is the same!
String === s
 
# Determines whether the object has a public or protected method with the specified name.
s.respond_to? :slice #=>true
 
# Or
String.respond_to? :include #=> false
 
# Passes true as the second argument to check private methods as well.
String.respond_to? :include , true #=> true
 
# Check whether some module includes another
module M1 ; end
module M2 ; include M1 ; end
M2.include? M1 #=> true
 
# Check whether instance variable is defined or not.
d=Date.new
d.instance_variables #=> ["@sg", "@of", "@ajd"]
d.instance_variable_defined? "@of" #=>true
 
# I have to clarify this, a note for the previous post:
# obj.public_methods(all=true) ,returns the list of public methods accessible to obj. If the all parameter is set to false, inherited methods will be excluded.  
# obj.protected_methods(all=true),returns the list of protected methods accessible to obj. If the all parameter is set to false, inherited methods will be excluded.  
# obj.private_methods(all=true),returns the list of private methods accessible to obj. If the all parameter is set to false, inherited methods will be excluded.
# Ex:
d.public_methods #=> [">>", "between?", "h!", "methods", "send", "ctime", "history_write", "year", "h", "taint", "to_enum", "instance_variable_defined?", "history", "ld", "_dump","q", "singleton_methods", "instance_eval", "os?", "wday", "enum_for", "nil?", "succ", "pretty_print_cycle", "asctime", "po", "protected_methods", "instance_exec", "start", "tainted?", "ns?", "handling_jruby_bug", "new_start", "yday", "untaint", "instance_of?", "equal?", "taguri", "pretty_print", "julian?", "step", "amjd","hash", "poc", "private_methods", "jd", "newsg", "taguri=", "history_to_vi", "pretty_print_instance_variables", "ajd", "italy", "kind_of?", "freeze", "eql?", "next", "to_yaml_style", "id", "sg", "public_methods", "hvi", "quiet", "england", "is_a?", "mday", "tap", "type", "ri", "to_yaml_properties", "+", "instance_variables", "__id__", "frozen?", "-", "julian", "pretty_inspect", "to_a", "cwyear", "respond_to?", "upto", "display", "history_do", "day", "method", "class", "verbose", "gregorian?", "downto", "mjd", "strftime", "<=>", "instance_variable_get", "==", "__send__", "leap?", "===", "gregorian", "pretty_print_inspect", "extend", "to_s","cweek", ">=", "v", "mon", "<=", "clone", "to_yaml", "=~", "instance_variable_set", "<", ">", "cwday", "inspect", "day_fraction", "month", "dup", "object_id", "<<"]
 
d.public_methods(false) #=> ["jd", "month", "_dump", "ctime", "ld", "cweek", "succ", "to_s", "step", "day", "gregorian?", "hash", "ajd", "julian", "+", "yday", "taguri", "os?", "strftime", "italy", "downto", "-", "eql?", "sg", "year", "asctime", "leap?", "taguri=", "inspect", "amjd", "cwday", "to_yaml", "mon", "<<", "gregorian", "newsg", ">>", "start", "cwyear", "day_fraction", "mday", "ns?", "mjd", "england", "upto", "wday", "<=>", "julian?", "new_start", "===", "next"]
 
#Check if a method is defined
String.method_defined? :slice! # => true 
 
# Check if public method is defined?
String.public_method_defined? :upcase     # => true
 
# Check if protected method is defined?
String.protected_method_defined? :downcase  # => false 
 
# Check if private method is defined?
String.private_method_defined? :initialize # => true 
 
# Check local variables
local_variables
 
# Check class variables
class One ; @@var1 = 1 ; end
class Two < One ; @@var2 = 2 ; end
One.class_variables   #=> ["@@var1"]
Two.class_variables   #=> ["@@var2", "@@var1"]
 
# Check constants
Math.constants #=> ["PI", "E"]
 
# Check global variables
global_variables #=> ["$fileutils_rb_have_lchmod", "$binding", "$-w", "$:", "$.", "$KCODE", "$-F", "$*", "$stderr", "$,", "$`", "$-p", "$\"", "$$", "$<", "$@", "$-v", "$-i", "$deferr", "$\\", "$=", "$;", "$PROGRAM_NAME", "$stdout", "$&", "$fileutils_rb_have_lchown", "$-d", "$LOAD_PATH", "$-a", "$VERBOSE", "$FILENAME", "$defout", "$-0", "$+", "$0", "$stdin", "$~", "$DEBUG", "$-I", "$_", "$-K", "$>", "$/", "$'", "$-l", "$LOADED_FEATURES", "$?", "$SAFE", "$!"]
 
# How to get the name of the current method?
# Add this snippet of code to your logic somewhere
 
module Kernel
  private
  # Defined in ruby 1.9
  unless defined?(__method__)
    def __method__
      caller[0] =~ /`([^']*)'/ and $1
    end
  end
end

#ObjectSpace.each_object([module]) {|obj| ... } => fixnum
#Calls the block once for each living, nonimmediate object in this Ruby process. If module is specified, calls the block for only those classes or modules that match (or are a subclass of) module. Returns the number of objects found. Immediate objects (Fixnums, Symbols true, false, and nil) are never returned. In the example below, each_object returns both the numbers we defined and several constants defined in the Math module.
 
a = 102.7
b = 95       # Won't be returned
c = 12345678987654321
count = ObjectSpace.each_object(Numeric) {|x| p x }
puts "Total count: #{count}"
 
#=>12345678987654321
#102.7
#2.71828182845905
#3.14159265358979
#2.22044604925031e-16
#1.7976931348623157e+308
#2.2250738585072e-308
#Total count: 7

# Whats is your class?
 
a = 5
b = "Hello"
 
# Whats is your class?
p "Class of a : #{a.class} ,class of b : #{b.class}" #=>"Class of a : Fixnum ,class of b : String"
 
# Whats is your super class?
p "Super class of a : #{a.class.superclass} ,super class of b : #{b.class.superclass}" #=>"Super class of a : Integer ,super class of b : Object"
 
# Is a.class = Fixnum ?
a.instance_of? Fixnum #=> true
 
# Is a of type Integer (does it have Integer in it's class hierarchy)?
a.is_a? Integer #=> true
 
# Or this way:
a.kind_of? Integer #=> true
 
# Introspection, know all the details about classes :
# List all ancestors(modules and classes) of a class
String.ancestors.each{|a| p a}
 
# List all modules included in a class
String.included_modules.each{|m| p m} 
 
# Check class hierarchy
String < Comparable #=> true
String < Integer #=> nil  , strings r not integers
Object < String #=> false , Not all objects are strings
 
# List ancestors of class type
String.ancestors.select{|a| a.class==Class}.each{|c| p c}
 
# List all methods available to an object
b.methods.each{|m| p m}
 
# Get public instance methods
String.public_instance_methods.each{|m| p m}
 
# Get protected instance methods
String.protected_instance_methods.each{|m| p m}
 
# Get private instance methods
String.private_instance_methods.each{|m| p m}
 
# Get class singleton methods
String.singleton_methods{|m| puts m}
 
# Get the instance variables of an object
d = Date.new
d.instance_variables.each{|i| p i}
 
# Get public instance methods
d.public_methods.each{|m| puts m}
 
# Get protected instance methods
d.protected_methods.each{|m| puts m}
 
# Get private instance methods
d.private_methods.each{|m| puts m}
 
# Get instance singleton methods
d.singleton_methods.each{|m| puts m}