Cash register allowing all coins
You really shouldn't use double for currency. It's probably fine for this, but in a real world application, the accuracy problems you'd experience would be terrible to deal with. It's recommended that you use the currency class for money.



Also, just a small note. I like your brace style, but most Java folks use a different convention. Consistency within a code base is most important, but folks get kind of crazy about this kind of trivial stuff. Best to do as the Romans when in Rome.

public Coin(double value){
    this.value = value;
}