In my testing I encountered, that I was able to make a user with a negative balance. The system should check if the prompted balance is less than 0. I assume a check should be made in the constructor of <b> User.java </b>.
I believe I have resolved the issue by "throwing" an exception when trying to add a user with a negative balance. public User(String s, double d, int i) throws IllegalArgumentException { // not relevant if(d < 0) throw new IllegalArgumentException("Negative balance"); else this.balance = d; // not relevant } Now when trying to create user with negative balance the message "Negative balance" is returned.