Your task is to write a Java program to simulate your investment
interactions with a brokerage.

When you interact with the brokerage, you will have several options:

Open an account
Purchase shares of a stock
Sell shares of a stock
Display your current portfolio
Close your account

In the simulation, you'll let the brokerage handle your account. They'll
need to keep track of how much money you have left, which stocks you
have purchased, and how many shares of each one, as well as the current
market price of each the stocks. Since this is just a simulation, you'll
also have the ability to tell your brokerage to let time pass, and have
the value of your stocks change over time.