PowerPoint Presentation - Day 3: The Command and Visitor Patterns

List

abstract class List {

abstract Object getFirst();

abstract List getRest();

abstract String toStringHelp();

}

class Empty extends List {

static Empty ONLY = new Empty(); // singleton pattern

private Empty() {}

Object getFirst() { throw new IllegalArgumentException(

"first requires a non Empty List");

}

List getRest() { throw new IllegalArgumentException(

"rest requires a non Empty List");

}

public String toString() { return "()"; }

String toStringHelp() { return ""; }

}

class Cons extends List {

Object first;

List rest;

Cons(Object f, List r) {

first = f;

rest = r;

}

Object getFirst() { return first; }

List getRest() { return rest; }

public String toString() { return "(" + first + rest.toStringHelp() + ")"; }

String toStringHelp() { return " " + first + rest.toStringHelp(); }

}