ImmutableList Assignment

Motivation

In this assignment we will build a classic data structure from functional programming languages like SML. It is important to think recursively in a functional programming language and so it will serve you well to approach this assignment with a recursive mind set. Further, the fact that this data structure is immutable leads to a very clean solution. If you find yourself going down an overly complicated road, try to imagine how you would build this assignment in the 1950s when hardware companies boasted about supporting floating point numbers.

Code To Investigate

interface ImList<E>

public interface ImList<E> extends Iterable<E> {
	/**
	 * @return the head value if it is present
	 * @throws NoSuchElementException if empty
	 */
	E head();

	/**
	 * @return the tail if it is present
	 * @throws NoSuchElementException if empty
	 */
	ImList<E> tail();

	/**
	 * @return true if empty, false otherwise
	 */
	boolean isEmpty();
}

Code To Implement

Utilities

ImLists

class:	ImLists.java
methods:	nil cons brackets
package:	immutable.list.util.exercise
source folder:	src/main/java

This class is the one required class you must support. It creates 3 static methods which are responsible for returning instances of ImList. There is a suggested path below involving a split implementation: one for empty ImLists and one for non-empty ImLists. For now, investigate this file to see what is required and decide whether or not you want to go the suggested route. Either way, you will want to build the supporting unshared enum and/or classes as you build these public static methods.

ALERT: You do not yet need to support the iterator() method on any instance you create. That will be the subject of a future exercise.

nil

public static <E> ImList<E> nil()

Constructs a new empty list, analogous to the nil constructor for SML list datatype.

cons

public static <E> ImList<E> cons(E head, ImList<E> tail)

Constructs a new list comprised of head::tail, analogous to the :: constructor for SML list datatype.

brackets

public static <E> ImList<E> brackets(E... elements)

Constructs an ImList<E> whose contents are defined by the elements passed in.

The parameter E... arguments is an example usage of varargs in Java. You can treat parameter elements as if it is an E[]. The varargs ... simply allows for more convenient invocations of the brackets method.

In order to build this method, consider added a private static helper method to perform the actual work.

It is suggested that you adopt a recursive approach to solving this problem. What is the base case? What is the recursive case?

Note: Be sure to cons your items onto the list from the back to the front.

Split Implementation Candidate

One possible approach to supporting the public methods on Lists is a split implementation: an enum with a single enum constant to serve for all empty lists and a class for non-empty lists.

ImList<String> a = ImLists.cons("A", ImLists.nil());
ImList<String> ba = ImLists.cons("B", a);
ImList<String> cba = ImLists.cons("C", ba);
ImList<String> dcba = ImLists.cons("D", cba);
ImList<String> edcba = ImLists.cons("E", dcba);
System.out.println(edcba);

outputs:

E::D::C::B::A::[]

produces the data structure

class EmptyImList

class:	EmptyImList.java
methods:	constructor isEmpty head tail
package:	immutable.list.util.exercise
source folder:	src/main/java

class EmptyImList implements ImList<Object>

ALERT: you need NOT implement iterator() now. That will be the subject of a future studio.

constructor

What fields, if any, are required as state for an empty list?

isEmpty

"Hey EmptyImList, are you empty?"

head

Check the behavior of head() dictated by the specification in ImList<E>.

tail

Check the behavior of tail() dictated by the specification in ImList<E>.

class NonEmptyImList<E>

class:	NonEmptyImList.java
methods:	constructor isEmpty head tail
package:	immutable.list.util.exercise
source folder:	src/main/java

final class NonEmptyImList<E> implements ImList<E>

ALERT: you need NOT implement iterator() now. That will be the subject of a future studio.

constructor

What fields, if any, are required as state for a non-empty list?

isEmpty

"Hey NonEmptyImList, are you empty?"

head

Check the behavior of head() dictated by the specification in ImList<E>.

tail

Check the behavior of tail() dictated by the specification in ImList<E>.

Clients

NOTE: Since the constructors on our implementations are package protected, you will need to use nil() and cons(head,tail).

Length

class:	Length.java
methods:	length
package:	immutable.list.clients.exercise
source folder:	src/main/java

public static <E> int length(ImList<E> list)

Compute the length of an arbitrary ImList<E>.

SumProductCountdownFactorial

class:	SumProductCountdownFactorial.java
methods:	sum product countdown factorial
package:	immutable.list.clients.exercise
source folder:	src/main/java

sum

public static int sum(ImList<Integer> xs)

Compute the sum of all the items in an ImList<Integer>.

product

public static int product(ImList<Integer> xs)

Compute the produce all the items in an ImList<Integer>.

countdown

public static ImList<Integer> countdown(int n)

Produce a new list which contains the values from n down to 1. For example, countdown(5) would produce the list [5,4,3,2,1].

factorial

public static int factorial(int n)

Can you implement this method using the methods you have already created?

Concat

class:	Concat.java
methods:	concat
package:	immutable.list.clients.exercise
source folder:	src/main/java

public static <E> ImList<E> concat(ImList<E> xs, ImList<E> ys)

Concatenate one list on to another.

Test

class:	ImListUtilAndClientsTestSuite.java
package:	immutable.list
source folder:	src/test/java

Pledge, Acknowledgments, Citations

file:

exercise-im-list-pledge-acknowledgments-citations.txt

More info about the Honor Pledge