Workshop for CS-AP Teachers

Workshop for CS-AP Teachers Chapter 6 Data Structures Georgia Institute of Technology

Learning Objectives • Understand at the conceptual level • The need to group objects • Limitations of Arrays • Collections • Lists and Linked Lists • Sets and Maps • Stacks and Queues • Trees Georgia Institute of Technology

Grouping Objects • We often group objects • A list of items to buy at a grocery store • Your friends names and phone numbers • Your homework for each class • A record of all of your ancestors • A sorted list of people in a class Georgia Institute of Technology

Array Limitations • You can use arrays to store multiple objects • You need to know many items there will be • You specify the size when you create an array Item[] shoppingList = new Item[10]; • What happens if the array runs out of space? • If you try to add an element passed the last valid index you get • java.lang.ArrayIndexOutOfBoundsException • You could create a bigger array • You would have to copy all the elements from the old array to the new array • What if you don’t need all the space in an array? Georgia Institute of Technology

Collection Classes • Java has collection classes to handle grouping objects • The classes don’t require you to know how many objects you will need to store • The collections will grow and shrink as needed • There are different types of collections depending on what you need • Keep the order of the objects - List • Make sure there are no duplicates – Set • Associate one object with another - Map Georgia Institute of Technology

Collection Exercise • Look up the Collection Interface • How do you add objects to a collection? • Is there a way to add two collections together? • Is there a way to get an intersection of two collections? • Is there a way to remove an object from a collection? • How do you empty a collection? • Can you get an array from a collection? Georgia Institute of Technology

Collections hold object references • When you add an object to a collection • You add a reference to the object • Not a copy of the object • Many collections can hold references to the same object • Variables may also reference the same object Ham: Item Eggs: Item Lettuce: Item Cheerios: Item Georgia Institute of Technology

List and Set Interfaces and Classes <<interface>> Collection <<interface>> List <<interface>> Set <<interface>> SortedSet HashSet ArrayList Vector LinkedList TreeSet Georgia Institute of Technology

List • We often keep ordered lists of things • “To do” list • People in a line • Parts • A list has an order • First thing, second thing, third thing, etc. • Lists may have duplicate items • You can get, add, or remove an item anywhere in a list Georgia Institute of Technology

Java Lists • The first index is 0 • The last valid index is list.size() – 1 • ArrayList is a class that implements the List interface • Using an array and allows null values in the list • Vector is an older class that also uses an array • It is like ArrayList but it is synchronized • Linked list is a class that implement the List interface • Using a linked structure, not an array Georgia Institute of Technology

Linked List - java.util.LinkedList • A linked list has nodes that contain data and a reference to the next node • A doubly linked list has references to previous nodes as well head Sue Mary Tasha null null Sue Mary Tasha null head tail Georgia Institute of Technology

Ideas for Teaching Linked Lists • Give random students a paper that tells them who the next and previous student is • Give one student the name of the first person in the list • Walk through • adding a new student to the front of the list • getting the 5th person in the list • removing the 3rd person in the list • removing the 1st person in the list Georgia Institute of Technology

Arrays versus Linked List • A book is like an array • The pages are ordered sequentially • It is easy to find a particular page • A magazine article is like a linked list • Has groups of pages and • a reference to the next group of pages • A treasure hunt is like a linked list • You start with one clue that takes you to the location of the next clue Georgia Institute of Technology

ArrayList versus LinkedList • If you need to access items randomly • Use an ArrayList • Quick to access a random location • Can be slower to add to and remove from • If it needs to create a new array and copy old items • If you are doing lots of adding/removing from a list • Use a LinkedList • Quick to add to or remove from • Slow to do random access Georgia Institute of Technology

Using Iterator • One way to access all elements of a List is to use a for loop and increment the index from 0 to < list.size() • Use the index to get items from the list item = (Item) itemList.get(index); • Another approach is to use an iterator Iterator iterator = itemList.iterator(); while (iterator.hasNext()) item = (Item) iterator.next(); Georgia Institute of Technology

Iterator Exercise • Is it better to use an iterator or an index to get all of the elements • of an ArrayList? • of a LinkedList? • What about if you want to access every other element • of an ArrayList? • of a LinkedList? • Which should you use if you don’t know the implementing class? Georgia Institute of Technology

ListIterator • Inherits from Iterator • Adds • The ability to traverse a list in either direction • The ability to modify the list during iteration • Add a new element before the current next element • public void add(Object obj); • Change the last accessed element • public void set(Object obj); Georgia Institute of Technology

ListNode AP Class • Has value and next fields • Can get and set the fields • Has a constructor that take the value and next node • Uses the keyword null to indicate the end of the linked list ListNode Object value ListNode next public Object getValue() public ListNode getNext() public void setValue(Object value) public void setNext(ListNode node) Georgia Institute of Technology

Loop through a linked list with ListNode • Start with a reference to the head of the list • Each time through the loop move the reference to the next node • Stop the loop when the reference is null • Continue while the reference is not null ListNode node = null; for (node = head; node != null; node = node.getNext()) Georgia Institute of Technology

Testing the Loop • Does this work when head is null? • Does it work when there is one node in the list? • Does it work when there is more than one node in the list? head null head Sue null head Sue Mary Tasha null Georgia Institute of Technology

Add to the front of a linked list • Set the new nodes next to the node referenced by head • Change head to point to the new node head Sue Mary Tasha null Fred null head Fred Sue Mary Tasha null Georgia Institute of Technology

Stacks • A stack holds objects with the last object put in the stack being the first one returned • Last-in-first-out structure (LIFO) • Like a stack of cafeteria plates • Or a Pez container • Stacks are used to hold the list of operations that you might want to undo • When you click “Undo” the last thing you did is undone Georgia Institute of Technology

Teaching Stacks • Have each student put a book on a stack of books • Then ask a student to take off a book from the stack • Where did people put the new books? • Where did people take books from? Georgia Institute of Technology

Stack AP Interface public interface Stack { /** Method that returns true if the stack is empty else false */ public boolean isEmpty(); /** Method that adds the passed object to the stack */ public void push(Object obj); /** Method that returns the top of the stack and removes the object from the stack */ public Object pop(); /** Method that returns the top of the stack but doesn't remove the object */ public Object peekTop(); } Georgia Institute of Technology

Implementing the Stack Interface • You could use an array • But you don’t know how many things will be in the stack • You could use a list • ArrayList or LinkedList • With a stack you add things to one end • The top • And remove things from one end • The top Georgia Institute of Technology

Implementing Stack with ArrayList • How would you check if the stack is empty if you use an ArrayList to hold the items? • It is empty if the list.size() is zero • How would you push an object on the stack? • You could add it to the 0 index but then you would have to move all other items in the array • So add it to the end of the array list.add() • How would you pop an object from the top of the stack • The last item entered in an ArrayList is at the number of objects in the array - 1 Object object = list.remove(list.size() – 1) • How would you peek at the top object? • Object object = list.get(list.size() – 1) Georgia Institute of Technology

Queues • A queue holds objects with the first object put in the queue the first one returned • First-in-first-out structure (FIFO) • Like the ticket line at the movies • Or a car wash with cars moving through • Use queues to track events and objects • A queue of requests for printing • Handle the first one before the next one • A queue of people in line to buy tickets for a movie • People at the front of the queue buy tickets first Georgia Institute of Technology

Teaching Queues • Have some students form a line as if in line to buy tickets for a movie • Who should be waited on first? Who would be waited on next? • When new people come where do they enter the line? Georgia Institute of Technology

Queue AP Interface public interface Queue { /** returns true if the queue is empty else false */ public boolean isEmpty(); /** adds the object to the end of the queue */ public void enqueue(Object obj); /** removes the first object in the queue and returns it */ public Object dequeue(); /** returns the first object in the queue without removing it */ public Object peekFront(); } Georgia Institute of Technology

Implementing the Queue Interface • You could use an array • But you don’t know how many objects it will need to hold • You could use an ArrayList • And add new objects to the end of the array • But when you remove an object from the front (0 index) all the remaining objects would have to move down one • You could use LinkedList Georgia Institute of Technology

Implementing Queue with LinkedList • How would you check if the queue is empty if you use a LinkedList to hold the items? • It is empty if the list.size() is zero • How would you add an object to the end of the queue? • list.addLast(object); • How would you remove and return the first object in the queue? • list.remove(0); • How would you peek at the first object? • list.get(0); Georgia Institute of Technology

Set • A set does not preserve order • The order things are retrieved from a set is not necessarily the same order they were placed in a set • Sets do not allow duplicate elements • elementA.equals(elementB) • If you try to add an element that is equal to another element of the set it won’t add it • And will return false Georgia Institute of Technology

Set Classes • HashSet • Uses equals and hashCode to compare objects and to check for duplicates • TreeSet • Objects must implement Comparable and are sorted based on the results of compareTo <<interface>> Set <<interface>> SortedSet HashSet TreeSet Georgia Institute of Technology

Maps • Maps hold key and value pairs • Use a key to put a value into the map • Use a key to get a value from a map • There can’t be duplicate keys • There can be duplicate values • A value can be associated with different keys • Used to look up associated data • Like look up a customer record from a phone number • Or like safety deposit boxes Georgia Institute of Technology

Map Interface • Get the number of keys in the map public int size(); • Put a value in the map for the given key • Returns the old object stored for this key public Object put(Object key, Object value); • Get a value from the map for the given key public Object get(Object key); • Check if the key is used in the map public boolean containsKey(Object key); • Get a set of the keys used in the map public Set keySet(); Georgia Institute of Technology

Map Interfaces and Classes <<interface>> Map HashMap Hashtable <<interface>> SortedMap TreeMap Georgia Institute of Technology

Map Classes • HashMap • Stores keys and values without regards to order entered • Allows null values and a null key • Hashtable • Older class like HashMap • Synchronized • TreeMap • Holds keys in sorted order Georgia Institute of Technology

Hashing • HashMap and Hashtable use hashing on the key to find the location where the value is stored • Using the hashCode() method inherited from Object • This method is overridden for String • You should override this method in your classes • Maps the key to an index in an array Georgia Institute of Technology

Hashing Procedure • When you put a value in a HashMap for a key • First the hashCode method is called on the key object • This returns an int value which is mapped from 0 to the array length – 1 • Often by using remainder (%) • There may be a value at that index from a different key • This is called a collision Georgia Institute of Technology

Handling Collisions • The array is often an array of lists • A bucket that holds more than one hash node • A good hashCode() method should result in few collisions and small lists • When more than one key has the same index • The hash node is added to the list • When you look for a value based on a key • If it maps to an index with a list • It looks for the key using equals Georgia Institute of Technology

hashCode() Method • The goal is to get a good spread of int results • Use some combination of fields • Like the hashCode for some String fields added to some prime number times some other field • Different keys can result in the same hashCode() result • The same key object must give the same hashCode() result Georgia Institute of Technology

Trees • Linked lists have nodes that hold a value and a reference to the “next” node • What if you need to track more than one “next” node? • Like you want to record your ancestors • You can use a tree • Each tree node has a value (a person) • And a reference to the person’s mother • And a reference to the person’s father Georgia Institute of Technology

Example Ancestor Tree root Barbara Ericson Janet Hund Charles Ericson Edna Wenzel Opal Peters Francis Hund Edward Ericson Georgia Institute of Technology

Binary Tree • Each tree node has at most one parent node • Each tree node can have at most 2 children • The top node in the tree is called the root • Tree nodes without any children nodes are called leaves root Right child Left child leaves Georgia Institute of Technology

Tree Node AP Class • Has fields: value, left, and right • Can get and set all fields • Has a constructor that takes a value, left tree node and right tree node TreeNode private Object value private TreeNode left private TreeNode right public Object getValue() public TreeNode getLeft() public TreeNode getRight() public void setValue(Object o) public void setLeft(TreeNode n) public void setRight(TreeNode n) Georgia Institute of Technology

Trees are Recursive • Each tree node is the root of a sub-tree of the original tree • This allows the use of recursion • A method invokes itself • On a subset of the original problem • Like a subtree • There has to be an end condition • That stops the recursion • No more subtrees Georgia Institute of Technology

Get the Number of Nodes in a Tree • If the root is null • The number of nodes is 0 • If the root isn’t null • Add one to the count • Add to the count the number of nodes in the left subtree • Add to the count the number of nodes in the right subtree Georgia Institute of Technology

Get the Number of Nodes Method • Some books use a class (static) method to count the number of nodes • And pass in the current node public static int getNumNodes(TreeNode node) { if (node == null) return 0; else return 1 + getNumNodes(node.getLeft()) + getNumNodes(node.getRight()); } Georgia Institute of Technology

What is wrong with this? • Static methods are used when there is no current object • Or for general methods • In this case there is a current tree node • And the method does operate on it • It is explicitly passed to the method • So this should be an object method • And the current object should be implicitly passed Georgia Institute of Technology

Modified Get Number of Nodes public int getNumNodes() { int count = 0; // increment count count = count + 1; // add to the count the number of nodes in the left subtree if (left != null) count = count + left.getNumNodes(); // add the to count the number of nodes in the right subtree if (right != null) count = count + right.getNumNodes(); return count; } Georgia Institute of Technology

Workshop for CS-AP Teachers