comment for http://jamesshore.com/Blog/Lets-Play/Episode-98.html - making calculations look nicer (sad, no lambdas in java)

UnderflowOverflowCalculationWithAnonymousClass.java

package com.jamesshore.finances.domain;

// import ....
public class ValidDollars extends Dollars {
	public static final double MAX_VALUE = 1000000000d; // one beeeellion dollars!
	public static final double MIN_VALUE = -1000000000d;

	private double amount;

        //  ... <snip> ...

	private double amount(Dollars dollars) {
		return ((ValidDollars)dollars).amount;
	}

	private boolean outOfRange(double value) {
		return (value > MAX_VALUE) || (value < MIN_VALUE);
	}
	
	private interface DollarsCalculation {
		double perform(double thisAmount, double otherAmount);
	}
	
	private Dollars performCalculation(Dollars dollars, DollarsCalculation calculation)
	{
		// TODO: should InvalidDollars have an error message, taken from .ctor?
		if (!dollars.isValid()) return new InvalidDollars(); // msg: dollars.reason 
		double result = calculation.perform(this.amount, amount(dollars));
		if (outOfRange(result)) return new InvalidDollars(); // msg: overflow/underflow, inline logic here?
		return new ValidDollars(result);		
	}

	public Dollars plus(Dollars dollars) {
		return this.performCalculation(dollars, new DollarsCalculation() {
			public double perform(double thisAmount, double otherAmount)
			{
				return thisAmount + otherAmount;
			}
		});
	}

        //  ... <snip> ...



}