printer.rb

class Printer
  def initialize
  end
  def output(bank)
    bank.transaction_history.each do |transaction|
      puts transaction.join( ' || ')
  end
  end
end

printer_spec.rb

require 'printer'
require 'bank'

describe Printer do
  let(:printer) { described_class.new }
  it 'will print something as expected' do
    expect(STDOUT).to receive(:puts).with("date || credit || debit || balance\n22/05/2018 || 10.00 ||   || 10.00\n");
    bank = Bank.new
    bank.transaction_history = [["date || credit || debit || balance"],["22/05/2018", '10.00', " ", '10.00']]
    printer.output(bank);
  end
end