Visitor pattern AP 2015 Simon's example

Visitor pattern.java

// MyElement only implements the accept method and this just called the visit method of MyVisitor
public interface MyElement {
	public void accept(MyVisitor visitor);
}

import java.util.Calendar;
public class Human implements MyElement {
	public Calendar dOB;
	public Human(Calendar d) {
		dOB = d;
	}
	public void accept(MyVisitor visitor) {
		visitor.visit(this);
	}
}

public class Dog implements MyElement {
	public Integer ageYears;
	public Dog(Integer a) {
		ageYears = a;
	}
	public void accept(MyVisitor visitor) {
		visitor.visit(this);
	}
}

// MyVisitor forces subclasses to implement methods for each of the original objects
public interface MyVisitor {
	public void visit(Dog dog);
	public void visit(Human human);
}

import java.util.GregorianCalendar;
public class CompAgeDaysVisitor implements MyVisitor {
	public void visit(Human human) {
		// Converting dates to differences in days
		GregorianCalendar today = new GregorianCalendar();
		long diffSeconds = (today.getTimeInMillis()) - human.dOB.getTimeInMillis())/1000;
		Integer ageDays = (int) diffSeconds/(60*60*24);
		System.out.println("This human is " + ageDays + " days old.");
	}

	public void visit(Dog dog) {
		Integer ageDays = dog.ageYears * 365;
		System.out.println("This dog is " + ageDays + " days old.");
	}
}

import java.util.*;
public class TestVisitor {
	public static void main(String[] args) {
		
		// set the age of the dog in years
		Dog d = new Dog(5);

		// set the age of the human as dOB from the Greg cal
		Calendar cal = new GregorianCalendar();
		cal.set(1995,5,12);
		Human h = new Human(cal);

		CompAgeDaysVisitor c = new CompAgeDaysVisitor();

		d.accept(c);
		h.accept(c);

	}
}