1. Adapted from Pearson Education, Inc.
Bags
Chapter 1
Adapted from Pearson Education, Inc.

2. Adapted from Pearson Education, Inc.
Contents & Objectives
Contents
Objectives
The Bag
A Bag’s Behaviors
Specifying a Bag
An Interface
Using the ADT Bag
Using an ADT Is Like Using a Vending Machine
Describe the concept of abstract data type (ADT)
Describe ADT bag
Use ADT bag in a Java program
Adapted from Pearson Education, Inc.

3. Bag: Definition & Behavior
A finite collection of objects
In no particular order
May contain duplicate items
Determine how many objects in bag
Full?
Empty?
Add, remove objects
Count duplicates
Test for specific object
View all objects
Adapted from Pearson Education, Inc.

4. A CRC card for a class Bag
Adapted from Pearson Education, Inc.

5. Specifying a Bag Describe data Specify methods for bag’s behaviors
Name methods
Choose parameters
Decide return types
Write comments
UML notation for the class Bag
Adapted from Pearson Education, Inc.

6. Design Decisions
What should the method add do when it cannot add a new entry?
Nothing?
Leave bag unchanged, signal client of condition?
What should happen when an unusual condition occurs?
Assume invalid never happens?
Ignore invalid event?
Guess at client’s intention?
Return flag value?
Return boolean value – success/failure?
Throw exception?
Adapted from Pearson Education, Inc.

7. Adapted from Pearson Education, Inc.
Interface
What does that mean?
public interface BagInterface < T > { /** Gets the current number of entries in this the integer number of entries currently in the bag */ public int getCurrentSize (); /** Sees whether this bag is true if full, or false if not */ public boolean isFull (); /** Sees whether this bag is true if empty, or false if not */ public boolean isEmpty (); /** Counts the number of times a given entry appears in this anEntry the entry to be the number of times anEntry appears in the bag */ public int getFrequencyOf (T anEntry); /** Tests whether this bag contains a given anEntry the entry to true if the bag contains anEntry, or false otherwise */ public boolean contains (T anEntry);
Adapted from Pearson Education, Inc.

8. Adapted from Pearson Education, Inc.
Interface
/** Adds a new entry to this newEntry the object to be added as a new true if the addition is successful, or false if not */ public boolean add (T newEntry); /** Removes one unspecified entry from this bag, if either the removed entry, if the removal was successful, or null */ public T remove (); /** Removes one occurrence of a given entry from this bag, if anEntry the entry to be true if the removal was successful, or false if not */ public boolean remove (T anEntry); /** Removes all entries from this bag. */ public void clear (); /** Creates an array of all entries that are in this a newly allocated array of all the entries in the bag */ public T [] toArray (); } // end BagInterface
Adapted from Pearson Education, Inc.

9. Using ADT Bag Implementation done from specifications
User (client) needs know what ADT does, not how
Type of object in bag specified by program (client) using the ADT
Adapted from Pearson Education, Inc.

10. Using ADT Bag Implementation done from specifications
User (client) needs know what ADT does, not how
Type of object in bag specified by program (client) using the ADT
An ADT is like a Vending Machine:
Perform only available tasks
User must understand the tasks
Cannot access inside of mechanism
Usable without knowing inside implementation
New inside implementation unknown to users
Adapted from Pearson Education, Inc.

11. Example of Bag for online shopping
public class OnlineShopper { public static void main (String [] args) { Item [] items = { new Item ("Bird feeder", 2050), new Item ("Squirrel guard", 1547), new Item ("Bird bath", 4499), new Item ("Sunflower seeds", 1295) }; BagInterface < Item > shoppingCart = new Bag < Item > (); int totalCost = 0; // simulate getting item from shopper and add them to the shopping cart: for (int index = 0 ; index < items.length ; index++) { Item nextItem = items [index]; shoppingCart.add (nextItem); totalCost = totalCost + nextItem.getPrice (); } // end for // simulate checkout while (!shoppingCart.isEmpty ()) System.out.println (shoppingCart.remove ()); System.out.println ("Total cost: " + "\t$" + totalCost / "." + totalCost % 100); } // end main } // end OnlineShopper
Adapted from Pearson Education, Inc.

12. Using ADT Bag In what ways is a piggybank similar to a bag?
In what ways is it different?
Example of Bag for class of piggy banks Listing 1-3
Demonstration of class PiggyBank Listing 1-4
Adapted from Pearson Education, Inc.

13. Class Library The interface Set public boolean add(T newEntry)
public boolean remove(Object anEntry)
public void clear()
public boolean contains(Object anEntry)
public boolean isEmpty()
public int size()
public Object[] toArray()
Adapted from Pearson Education, Inc.

14. End
Chapter 1
Adapted from Pearson Education, Inc.