Method objects with method() in Ruby

Introduction

Method objects let you save a method as a value. This helps you call the method later or pass it around like a regular object.

When you want to store a method to call it multiple times later.

When you want to pass a method as an argument to another method.

When you want to delay running a method until a certain event happens.

When you want to keep your code organized by separating method calls.

When you want to use the same method in different places without repeating code.

Syntax

Ruby

method_object = object.method(:method_name)

The method call returns a Method object representing the method.

You use a symbol (like :method_name) to specify the method name.

Examples

This saves the upcase method of the string "hello" into m. Calling m.call runs upcase and prints HELLO.

Ruby

str = "hello"
m = str.method(:upcase)
puts m.call

This saves the method greet as a Method object and calls it with "Alice".

Ruby

def greet(name)
  "Hi, #{name}!"
end

m = method(:greet)
puts m.call("Alice")

This stores the sum method of the array and calls it to print the total 6.

Ruby

arr = [1, 2, 3]
method_obj = arr.method(:sum)
puts method_obj.call

Sample Program

This program creates a Calculator object, saves its add method as a Method object, then calls it with two numbers and prints the sum.

Ruby

class Calculator
  def add(a, b)
    a + b
  end
end

calc = Calculator.new
add_method = calc.method(:add)

result = add_method.call(5, 7)
puts "The sum is: #{result}"

Important Notes

You can call the method object with call and pass any needed arguments.

Method objects keep the original method's context, so they remember which object they belong to.

Using method objects can make your code more flexible and easier to reuse.

Summary

Method objects let you treat methods like values you can save and pass around.

Use object.method(:name) to get a Method object.

Call the method later with call and arguments.

Practice

1. What does the Ruby expression obj.method(:foo) return?

easy

A. A Method object representing the method foo of obj

B. The result of calling foo on obj

C. A symbol :foo

D. An error because method is not a valid method

Solution

1. Step 1: Understand method usage

   The method method on an object returns a Method object representing the named method.

2. Step 2: Identify the return type

   Calling obj.method(:foo) returns a Method object, not the method's result or a symbol.

3. Final Answer:

   A Method object representing the method foo of obj -> Option A

4. Quick Check:

   obj.method(:foo) returns Method object [OK]

Hint: Remember: method returns a Method object, not the call result [OK]

Common Mistakes:

• Thinking it calls the method immediately
• Confusing with symbol :foo
• Assuming it returns a string or error

2. Which of the following is the correct syntax to get a Method object for the method greet on object person?

easy

A. person.send(:greet)

B. person.method[b greet]

C. person.method(greet)

D. person.method(:greet)

Solution

1. Step 1: Check argument type for method

   The method method expects a symbol representing the method name.

2. Step 2: Validate options

   person.method(:greet) uses a symbol :greet, which is correct. person.send(:greet) calls the method directly, returning its result instead of a Method object. person.method(greet) passes an undefined variable greet, causing error. person.method[b greet] uses brackets, which is invalid syntax.

3. Final Answer:

   person.method(:greet) -> Option D

4. Quick Check:

   Use symbol argument with method [OK]

Hint: Use a symbol like :greet inside method() [OK]

Common Mistakes:

• Confusing method() with send()
• Using brackets instead of parentheses
• Passing undefined variable instead of symbol

3. What is the output of the following Ruby code?

class Calculator
  def add(x, y)
    x + y
  end
end
calc = Calculator.new
adder = calc.method(:add)
puts adder.call(5, 7)

medium

A. 5 7

B. 12

C. add

D. NoMethodError

Solution

1. Step 1: Understand method object creation

   The method(:add) returns a Method object for add on calc.

2. Step 2: Calling the Method object

   Calling adder.call(5, 7) executes add(5, 7), which returns 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   Method object call returns sum 12 [OK]

Hint: Use call on Method object to run method with arguments [OK]

Common Mistakes:

• Expecting method name or arguments printed
• Confusing output with error
• Not using call to invoke method

4. Identify the error in this Ruby code snippet:

class Person
  def name
    "Alice"
  end
end
p = Person.new
m = p.method(name)
m.call

medium

A. Missing quotes around method name in method(name)

B. Cannot call method on an object

C. Method name does not exist

D. Using call on Method object is invalid

Solution

1. Step 1: Check argument to method

   The argument name is used as a variable, but it should be a symbol or string representing the method name.

2. Step 2: Correct usage

   It should be p.method(:name) or p.method('name') to get the Method object.

3. Final Answer:

   Missing quotes around method name in method(name) -> Option A

4. Quick Check:

   Method name must be symbol or string [OK]

Hint: Always pass method name as symbol or string to method() [OK]

Common Mistakes:

• Passing bare method name without quotes or colon
• Thinking method is invalid on objects
• Confusing call usage

5. Given the following code, what will be the output?

class Greeter
  def initialize(name)
    @name = name
  end
  def greet
    "Hello, #{@name}!"
  end
end

g1 = Greeter.new("Bob")
g2 = Greeter.new("Sue")
m1 = g1.method(:greet)
m2 = g2.method(:greet)

puts m1.call
puts m2.call

hard

A. Hello, Sue!\nHello, Sue!

B. Hello, Bob!\nHello, Bob!

C. Hello, Bob!\nHello, Sue!

D. Error: undefined method greet

Solution

1. Step 1: Understand instance variables and method objects

   Each Greeter instance stores its own @name. The method(:greet) returns a Method object bound to that instance.

2. Step 2: Calling Method objects

   Calling m1.call runs greet on g1, printing "Hello, Bob!". Similarly, m2.call prints "Hello, Sue!".

3. Final Answer:

   Hello, Bob! Hello, Sue! -> Option C

4. Quick Check:

   Method objects keep instance context [OK]

Hint: Method objects remember their object, so output matches instance data [OK]

Common Mistakes:

• Assuming method objects lose instance context
• Expecting same output for both calls
• Confusing method object with class method