account.rb

class Account
  attr_reader :balance

  def initialize(balance)
    @balance = balance
  end

  def receive(amount)
    @balance += amount
  end

  def transfer1(account_to,  amount)
    if (@balance - amount) < 0
      raise "You do not have enough money."
    else
      sleep 1
      @balance -= amount
      account_to.receive(amount)
     end
  end

  def transfer2(account_to,  amount)
    Thread.exclusive do
      if (@balance - amount) < 0
        raise "You do not have enough money."
      else
        sleep 1
        @balance -= amount
        account_to.receive(amount)
      end
    end
  end

  require "thread"
  def transfer3(account_to, amount)
    @semaphore ||= Mutex.new
    @semaphore.synchronize do
      if (@balance - amount) < 0
        raise "You do not have enough money."
      else
        sleep 1
        @balance -= amount
        account_to.receive(amount)
      end
    end
  end

end

output.txt

:w ! ruby
Loaded suite -
Started
F..
Finished in 3.005404 seconds.

  1) Failure:
test_transfer1(TestThreads) [-:66]:
<1> expected but was
<-98>.

3 tests, 5 assertions, 1 failures, 0 errors, 0 skips

test.rb

require "test/unit"

class TestThreads < Test::Unit::TestCase

  def setup
    @account_from = Account.new 100
    @account_to = Account.new 0
    @amount = 99
  end

  def test_transfer1
    threads = []
    threads << Thread.new { @account_from.transfer1(@account_to, @amount) }
    threads << Thread.new { @account_from.transfer1(@account_to, @amount) }
    threads.each(&:join)

    assert_equal 1, @account_from.balance
  end

  def test_transfer2
    threads = []
    threads << Thread.new { @account_from.transfer2(@account_to, @amount) }
    threads << Thread.new { @account_from.transfer2(@account_to, @amount) }
    assert_raise RuntimeError do
      threads.each(&:join)
    end

    assert_equal 1, @account_from.balance
  end

  def test_transfer3
    threads = []
    threads << Thread.new { @account_from.transfer3(@account_to, @amount) }
    threads << Thread.new { @account_from.transfer3(@account_to, @amount) }
    assert_raise RuntimeError do
      threads.each(&:join)
    end

     assert_equal 1, @account_from.balance
   end

end