cs 600.226: data structuresphf/2016/fall/cs226/lectures/13.morelists.pdf · cs 600.226: data...

CS 600.226: Data Structures Michael Schatz Oct 3, 2016 Lecture 13: More Lists

Assignment 4: Due Sunday Oct 2 @ 10pm

Remember: javac –Xlint:all & checkstyle *.java & JUnit Solutions should be independently written!

Part 1: Inheritence

Why Inheritance?

Code Reuse •  Subclass gets to use all methods of

the parent class “for free”

Overriding •  Subclass can have more specific

implementation than the parent class

Design constraints •  Subclasses get all of the features of

the parent, whether you like them or not!

Saying B inherits from A is a very

strong relationship: anytime that A could be used, B

could be instead

Inheritance Types

B isa A

Square isa Rectangle

Single Inheritance

C isa B, B isa A

Square isa Rectangle Rectangle isa Shape

=> Square isa Shape

Multilevel Inheritance

C isa A, C isa B

Mike isa CS Prof Mike isa Bio Prof

Multiple Inheritance

Inheritance versus Encapsulation

B isa A

Square isa Rectangle

Pokemon Trainer

Position pos String name String team List<Pokemon> List<Eggs> Backpack pack

Position

Double lat Double long Double altitude

Backpack

Int capacity List<Items> items

Trainer hasa Position, Trainer hasa Backpack,

Encapsulation is used to hide the values or state of a structured data object inside a class,

Part 2: Lists

List Queue insertFront insertBack

removeBack removeFront

addme.next = first; first = addme; last.next = addme; addme.next = null

first = first.next; ???

removeLast (Singly Linked List)

next null

Oops, just made remove an O(n) operation How might you address this?

front ???

1 next prev

Deque with Doubly Linked List

2 next prev

3 next prev

4 next prev

Very similar to a singly linked list, except each node has a reference to both the next and previous node in the list

A little more overhead, but significantly increased flexibility: supports insertFront, insertBack, removeFront, removeBack,

insertBefore, removeMiddle

Dequeue Interface public interface Dequeue<T> {

boolean empty();int length();

T front();T back();

void insertFront(T t);void insertBack(T t);

void removeFront();void removeBack();

How would you implement a deque with a doubly linked list?

How would you implement a general List<T> class?

public class MyDeque implements Dequeue<T> {

private static class Node<T>{T dataNode<T> next;Node<T> prev;

private Node<T> front;private Node<T> back;

T front() { return front.data; }T back() { return back.data; }

List v1 public interface Node<T> {

void setValue(T t); T getValue();

void setNext(Node<T> n);void setPrev(Node<T> n);

void getNext(Node<T> n);void getPrev(Node<T> n);

public interface List<T> {boolean empty();int length();

Node<T> front() throws EmptyException;Node<T> back() throws EmptyException;

void insertFront(Node<T> t);void insertBack(Node<T> t);

void insertBefore(Node<T> t);void insertAfter(Node<T> t);

void removeFront() throws EmptyException;void removeBack() throws EmptyException;void removeNode(Node<T> t);

Is this a good design?

No! We expose that Lists use Nodes for underlying storage. Worse, client programs may incorrectly edit the next/prev

references

List v2 public class MyList<T> implements List<T> {

private static class Node<T> {T data;Node<T> next;Node<T> prev;

boolean empty() { ... }int length() { ... }

T front() throws EmptyException { ... }T back() throws EmptyException { ... }

void insertFront(T t) { ... }void insertBack(T t) { ... }

void insertBefore(???) { ... }void insertAfter(???) { ... }

void removeFront() throws EmptyException { ... }void removeBack() throws EmptyException { ... }void removeNode(???) { ... }

Better, the nested class encapsulates the storage medium, but restricts what methods can be implemented to very

first/last elements

Some implementations will have public node classes with private next/prev references. However they also have

public setNext(), setPrev() => poor encapsulation!

List v3 public class MyList<T> implements List<T> {

private static class Node<T> {T data;Node<T> next;Node<T> prev;

boolean empty() { ... }int length() { ... }

T front() throws EmptyException { ... }T back() throws EmptyException { ... }void insertFront(T t) { ... }void insertBack(T t) { ... }

void insertBefore(int idx) { ... }void insertAfter(int idx) { ... }

void removeFront() throws EmptyException { ... }void removeBack() throws EmptyException { ... }void removeNode(int idx) { ... }

Slightly better: More flexible, but now insertBefore and removeNode() are

O(n) operations

Node<T> front();Node<T> back();

public interface Position<T> { // empty on purpose}

public interface List<T> {// simplified interfaceint length();

Position<T> insertFront(T t);Position<T> insertBack(T t);void insertBefore(Position<T> t);void insertAfter(Position<T> t);

void removeAt(Position<T> p);}

“I am a position and while you can hold on to me, you can’t do anything else with me!”

Inserting at front or back creates the Position objects.

If you want, you could keep references to the Position

objects even in the middle of the list

Pass in a Position, and it will

remove it from the list

Position<T> insertFront(T t);Position<T> insertBack(T t);void insertBefore(Position<T> t);void insertAfter(Position<T> t);

void removeAt(Position<T> p);}

List v4 public class NodeList<T> implements List<T> private static class Node<T> implements Position<T> { Node<T> next;

Node<T> prev;T data;List<T> color; // avoid “fake positions”

private Node<T> front; private Node<T> back; private int elements; public int length() { return this.elements; }

public Position<T> insertFront(T t) { ... }

public Position<T> insertBack(T t) { ...

public void remoteAt(Position<T> p) { ... }}

Public Position interface, but nested (private static) Node Implementation

List v4 List l = new List<String>();

Position a = l.insertFront(“Mike”);Position b = l.insertBack(“Peter”);Position c = l.insertFront(“Kelly”);

l.removeAt(a);

Position<T> insertFront(T t);Position<T> insertBack(T t);

// TODO: void is temporaryvoid removeAt(Position<T> p);

Kelly next prev

Mike next prev

Peter next prev

List v4 public String toString () { String s ="["; Node<T> n = this.front; while (n != null) { s += n.data.toString(); if (n.next != null) { s +=""; } n = n.next; } s +="]"; return s; }

$ java NodeList[Mike][Mike, Peter][Kelly, Mike, Peter][Kelly, Peter]

public static void main(String[] args) { List l = new NodeList(); Position a = l.insertFront("Mike"); System.out.println(l); Position b = l.insertBack("Peter"); System.out.println(l); Position c = l.insertFront("Kelly"); System.out.println(l);

l.remove(a); System.out.println(l);}

How to test the code? public class NodeList<T> implements List<T> {

private static final class Node<T> implements Position<T> {Node<T> next;

Node<T> prev;T data;List<T> owner;public T get() { return this.data; }public void put(T t) { this.data = t; }

}...public Position<T> front() throws EmptyException { … }public Position<T> back() throws EmptyException { … }public Position<T> insertFront(T t) { … }public Position<T> insertBack(T t) { … }public void removeFront() throws EmptyException { … }public void removeBack() throws EmptyException { … }public Position<T> insertBefore(Position<T> p, T t)

throws PositionException { … }public Position<T> insertAfter(Position<T> p, T t)

throws PositionException { … }public void remove(Position<T> p) throws PositionException { … }public String toString() { … }

How to test the code? public class NodeList<T> implements List<T> {

private static final class Node<T> implements Position<T> {Node<T> next;

Node<T> prev;T data;List<T> owner;public T get() { return this.data; }public void put(T t) { this.data = t; }

}...public Position<T> front() throws EmptyException { … }public Position<T> back() throws EmptyException { … }public Position<T> insertFront(T t) { … }public Position<T> insertBack(T t) { … }public void removeFront() throws EmptyException { … }public void removeBack() throws EmptyException { … }public Position<T> insertBefore(Position<T> p, T t)

throws PositionException { … }public Position<T> insertAfter(Position<T> p, T t)

throws PositionException { … }public void remove(Position<T> p) throws PositionException { … }public String toString() { … }

The output for every method should be tested, including any exceptions for invalid inputs

Use toString() method to test the state of private member

variables

TestList.java (5) @Test(expected=InvalidPositionException.class) public void testRemoveAtFakePosition () {

List <String> faker = new NodeList<String>();Position <String> fake = faker.insertFront("Hehehe");list.insertBack("Paul");list.insertBack ("Mary");assertEquals("[Paul, Mary]", list.toString ()); assertEquals(2, list.length());list.removeAt(fake);

Testing code is significantly longer than the implementation

Part 3: Make Lists Useful Again J

l.removeAt(a);

Kelly next prev

Mike next prev

Peter next prev

l.removeAt(a);

Kelly next prev

Mike next prev

Peter next prev

This interface protects the integrity of the List, but is “weird” in that we can have references to objects but cant get their value or do anything else with them except remove them

List v5 public interface Position<T> {

T get();void put(T t);

public interface List<T> {private static class Node<T> implements Position<T> {

Node<T> next; Node<T> prev; T data; List<T> color; public T get() { return this.data; }

public void put(T t) { this.data = t; }}

List v5 public interface Position<T> {

T get();void put(T t);

public interface List<T> {private static class Node<T> implements Position<T> {

Node<T> next; Node<T> prev; T data; List<T> color; public T get() { return this.data; }

public void put(T t) { this.data = t; }}

Hooray, now we can get/set the value from a Position

Why wouldn’t you want to do it this way?

What if you wanted a UniqueList<T> that only stored unique items? This would have to be checked in the UniqueList<T>

implementation

List v5 Test @Testpublic void testPositionGet() { Position<String> p1 = list.insertBack("Peter"); Position<String> p2 = list.insertBack("Paul"); assertEquals("[Peter Paul]", list.toString()); assertEquals("Peter", p1.get()); assertEquals("Paul", p2.get());}

@Testpublic void testPositionPut() { Position<String> p1 = list.insertBack("Peter"); list.insertBack("Paul"); assertEquals("[Peter Paul]", list.toString()); p1.put("Mary"); assertEquals("Mary", p1.get()); assertEquals("[Mary Paul]", list.toString());}

What else are we missing?

List v6 public interface List<T> { ... Position<T> front() throws EmptyListException;

Position<T> back() throws EmptyListException;

Position<T> next(Position<T> p) throws InvalidPositionException;

Position<T> previous(Position<T> p) throws InvalidPositionException;

boolean hasNext(Position<T> p) throws InvalidPositionException;

boolean hasPrevious(Position<T> p) throws InvalidPositionException;

boolean valid(Position<T> p); …}

Why do we put next() and prev() into the list and not Position?

For more complex data structures, like trees or graphs, next() and prev() will be more complicated

List v6 Iterating // Very C++ likePosition<String> current = list.front();Position<String> last = list.back();while (current != last) { // do whatever we need to do at the current position current = list.next(current);}

// More Java-likePosition<String> current = list.front();for (;;) { // do whatever we need to do at the current position if (list.hasNext(current)) { current = list.next(current); } else {

break; }

// Very Java-likePosition<String> current = list.front();while (list.valid(current)) { // do whatever we need to do at the current position current = list.next(current);}

Iterator Interface public interface Iterator<T> { boolean valid(); void next(); // next element, not necessarily “next” T get(); // get ok, but put may break invariants}

public interface List<T> { ... Iterator<T> forwardIterator(); Iterator<T> backwardIterator(); ...}

Iterator<String> i = list.forwardIterator();while (i.valid()) { String e = i.get(); // do whatever with the element e i.next();}

Iterator Interface private static class NodeListIterator<T> implements Iterator<T> { private Node<T> current; private boolean forward;

NodeListIterator(Node<T> start, boolean forward) { this.current = start; this.forward = forward;

public boolean valid() { return this.current != null; }

public void next() { this.current = this.forward ? this.current.next

: this.current.prev; }

public T get() { return this.current.get();

Ternary operator: If (this.forward) {

this.current = this.current.next; } else {

this.current = this.current.prev; }

Iterator Interface public Iterator<T> forwardIterator() { return new NodeListIterator<T>(this.front, true);}

public Iterator<T> backwardIterator() { return new NodeListIterator<T>(this.back, false);}

@Testpublic void testForwardIterator() { list.insertBack("Peter"); list.insertBack("Paul"); list.insertBack("Mary"); String[] expected = {"Peter", "Paul", "Mary"}; int current = 0; Iterator<String> i = list.forwardIterator(); while (i.valid()) { String e = i.get(); assertEquals(expected[current], e); i.next(); current += 1;

}assertEquals(3, current);

Iterator Interface public Iterator<T> forwardIterator() { return new NodeListIterator<T>(this.front, true);}

public Iterator<T> backwardIterator() { return new NodeListIterator<T>(this.back, false);}

@Testpublic void testBackwardIterator() { list.insertBack("Peter"); list.insertBack("Paul"); list.insertBack("Mary"); String[] expected = {"Mary", "Paul", "Peter"}; int current = 0; Iterator<String> i = list.backwardIterator(); while (i.valid()) { String e = i.get(); assertEquals(expected[current], e); i.next(); current += 1;

} assertEquals(3, current);}

Java Iterators http://docs.oracle.com/javase/6/docs/api/java/util/Iterator.html

Will become even more important for more complex data structures to simplify certain operations like printing every element in sorted order

Java Iterable https://docs.oracle.com/javase/7/docs/api/java/lang/Iterable.html

List v7 public interface List<T> extends Iterable<T> { ...

private static class NodeListIterator<T> implements Iterator<T> {

Iterator<String> i = list.iterator();while (i.hasNext()) { String e = i.next(); // do something with element e}

for (String e: list) { // do something with element e}

Living in a null world

Mike next prev

Peter next prev

Living in a null world

Mike next prev

Peter next prev

public Position <T> insertBack(T t) { ...if (this.back != null) {

this.back.next = n; }if (this.front == null) {

this.front = n; }...

public Position <T> insertFront(T t ) {...if (this.front != null) {

this.front.prev =n; }if (this.back==null) {

this.back = n; }...

public void removeAt(Position<T> p) {...if (n.next != null) { n.next.prev = n.prev; }if (n.prev != null) { n.prev.next = n.next; }...

f next prev

Doubly Linked List with Sentinels

b next prev

f next prev

1 next prev

b next prev

f next prev

1 next prev

2 next prev

b next prev

For the cost of a tiny bit of extra memory, the code gets significantly simpler!

Part 4: Midterm Review!

Next Steps 1.  Reflect on the magic and power of stacks, queues, deques! 2.  Work on Assignment 4: Due Sunday 3 Oct 2 @ 10:00 pm

3.  Start to review for Midterm on Monday Oct 10

1.  Your notes from class 2.  Lecture Notes on Piazza 3.  Slides on course webpage 4.  Online & printed textbooks 5.  Sample Midterm!!!

Welcome to CS 600.226 http://www.cs.jhu.edu/~cs226/

Questions?

cs 600.226: data structuresphf/2016/fall/cs226/lectures/13.morelists.pdf · cs 600.226: data...

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