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(); }

•}

•