[:IntList]

                                                            
 UML (Unified Modeling Language) class diagram for IntList. 
                                                            

 

/**
 *  Data Definition: IntList := new Empty() + new Cons(int, IntList)
 */abstract class IntList {
    /**
    * throws an IllegalArgumentException on Empty
    * returns first element of non-empty list
    */    abstract int getFirst();
    /**
    * given the elements of this appear in non-descendin order
    * returns this with e inserted in nonde
    */    abstract IntList getRest();
    abstract String toStringHelp();
}
/**
 * Represents the unique empty list using the singleton pattern.
 * This is also an example of the Null Object pattern.
 * Note the use of exceptions to express programming errors in getFirst() and
 * getRest().
 */class Empty extends IntList {
    /**
    * Singleton pattern.
    */    static Empty only = new Empty();
    private Empty() {}
    int getFirst() {
        throw new IllegalArgumentException("An Empty IntList has no first.");
    }
    IntList getRest() {
        throw new IllegalArgumentException("An Empty IntList has no rest.");
    }
    public String toString() {
        return "()";
    }
    String toStringHelp() {
        return "";
    }
}
/**
 * Represents a non-empty list.
 * Note the use of the toStringHelp() "helper" method to "help" compute the
 * String representation of the rest.
 */class Cons extends IntList {
    int first;
    IntList rest;
    Cons(int f, IntList r) {
        first = f;
        rest = r;
    }
    int getFirst() {
        return first;
    }
    IntList getRest() {
        return rest;
    }
    /**
     * no leading space before first
     */    public String toString() {
        return "(" + first + rest.toStringHelp() + ")";
    }
    /**
     * leading space before each elt
     */    String toStringHelp() {
        return " " + first + rest.toStringHelp();
    }
}