PowerPoint Presentation - Day 3: The Command and Visitor Patterns

List Sort

abstract class List {

abstract Object getFirst();

abstract List getRest();

abstract List insert(Object e);

abstract List sort();

abstract String toStringHelp();

abstract List sort();

}

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");

}

List sort() { return Empty.ONLY; }

List insert(Object e) { return new Cons(e,Empty.ONLY); }

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

String toStringHelp() { return ""; }

}