Teach.NET Workshop Series Track 4: AP Computer Science with.NET and J#

“AP Computer Science with.NET and J#” Lecture 13: Interfaces and Interface-based Programming Microsoft.NET Workshops for Teachers

13-3 MicrosoftAP Computer Science with.NET and J# Workshop Track LectureTopic 7Algorithms and Algorithm Analysis 8Debugging and Exception Handling 9Applying Object-oriented Design Principles in Your Classes 10Using Inheritance to Design a Set of Classes 11MBS CS: the Marine Biology Simulation Case Study 12Extending the MBS CS 13Interfaces and Interface-based Programming 14Extending the MBS CS, Part 2 15Stacks, Queues and Invariants 16Linked Data Structures and the Linked-list 17Trees and Recursion 18Collection Classes and Iteration 19Additional Resources and Ideas

13-4 MicrosoftAP Computer Science with.NET and J# Lecture — Objectives “Good object-oriented programming is really about good object-oriented design. And good OOD is all about making things easier for ourselves, as well as for our class users…” Topics: –Design by contract –Interfaces [ Joe Hummel, Lake Forest College ]

13-5 MicrosoftAP Computer Science with.NET and J# Part 1 Design by Contract

13-6 MicrosoftAP Computer Science with.NET and J# Motivation We program by relying on "contracts" –i.e. agreements between objects –"Object X agrees to do Y, so I'll call X when I need to do Y" java.io.FileReader file = new java.io.FileReader("…"); java.io.BufferedReader reader = new java.io.BufferedReader(file); s = reader.readLine(); java.io.FileReader file = new java.io.FileReader("…"); java.io.BufferedReader reader = new java.io.BufferedReader(file); s = reader.readLine(); public class BufferedReader {. public String readLine() {. } public class BufferedReader {. public String readLine() {. }

13-7 MicrosoftAP Computer Science with.NET and J# Design by Contract Design by contract formalizes this process –design first –implement second Advantages? –encourages "think before you code“ for better designs –classes that implement the same contract are interchangeable — i.e. “plug-compatible” — yielding a more flexible, easier to evolve system –enforces a consistent design across a set of classes (much like inheritance), making it easier to program against these classes — can you say polymorphism !

13-8 MicrosoftAP Computer Science with.NET and J# Part 2 Interfaces

13-9 MicrosoftAP Computer Science with.NET and J# Interfaces Interfaces are a formal way of specifying contracts An interface = a set of method signatures –representing the contract between two or more parties –pure design, no data / code public interface SomeInterfaceName { public void Method1(); public int Method2(int x, String y);. } public interface SomeInterfaceName { public void Method1(); public int Method2(int x, String y);. }

13-10 MicrosoftAP Computer Science with.NET and J# Conceptual view Here's how to visualize what's going on: "server" (object performing service according to interface contract) "client" (object requesting service) Interface –"client" uses interface to communicate with server –"server" implements interface to fulfill contract

13-11 MicrosoftAP Computer Science with.NET and J# Example Sorting and binary search These algorithms are a part of the Java Class Library –great examples of the power of polymorphism — one sort and one binary search for all objects in Java! –use an interface to communicate with objects… sort( ) object binarySearch( ) object

13-12 MicrosoftAP Computer Science with.NET and J# Working with Interfaces Working with interfaces involves 3 steps: 1.Define the interface 2.Implement the interface 3.Program against the interface Let’s work through the sorting example…

13-13 MicrosoftAP Computer Science with.NET and J# (1) Comparable Interface Java’s Comparable interface formalizes what it means to compare 2 objects Defines a compare method that returns an integer: –< 0 means this object < parameter obj –= 0 means this object = parameter obj –> 0 means this object > parameter obj public interface Comparable { public int compareTo(Object obj); } public interface Comparable { public int compareTo(Object obj); } if (s1.compareTo(s2) < 0)...

13-14 MicrosoftAP Computer Science with.NET and J# (2) Sortable Objects Implement Comparable To fulfill contract, objects must implement Comparable: public class BankCustomer implements Comparable { private String firstName, lastName;. public int compareTo(Object obj) { // make sure we are comparing apples to apples… BankCustomer other = (BankCustomer) obj; int diff; diff = this.lastName.compareTo(other.lastName); if (diff == 0) // last names are the same, so look at first names: diff = this.firstName.compareTo(other.firstName); return diff; } public class BankCustomer implements Comparable { private String firstName, lastName;. public int compareTo(Object obj) { // make sure we are comparing apples to apples… BankCustomer other = (BankCustomer) obj; int diff; diff = this.lastName.compareTo(other.lastName); if (diff == 0) // last names are the same, so look at first names: diff = this.firstName.compareTo(other.firstName); return diff; } student Bank Customer

13-15 MicrosoftAP Computer Science with.NET and J# (3) Writing a Generic Sort() Now we can sort any list of Comparable objects! –For simplicity, here’s Selection sort: public static void sort(ArrayList list) { for (int i = 0; i < list.size(); i++) { for (int j = i+1; j < list.size(); j++) { Comparable obj1; Object obj2; obj1 = (Comparable) list.get(i); obj2 = list.get(j); if (obj1.compareTo(obj2) > 0) // swap! { list.set(i, obj2); list.set(j, obj1); } } //inner } //outer } public static void sort(ArrayList list) { for (int i = 0; i < list.size(); i++) { for (int j = i+1; j < list.size(); j++) { Comparable obj1; Object obj2; obj1 = (Comparable) list.get(i); obj2 = list.get(j); if (obj1.compareTo(obj2) > 0) // swap! { list.set(i, obj2); list.set(j, obj1); } } //inner } //outer }

13-16 MicrosoftAP Computer Science with.NET and J# Demo! Sorting bank customers…

13-17 MicrosoftAP Computer Science with.NET and J# Java’s Predefined Sort and Search Methods As long as our objects implement Comparable, we can take advantage of Java’s sort and binarySearch methods: ArrayList bank = new ArrayList(); bank.add(... );. java.util.Collections.sort(bank); int i; BankCustomer c; // we need a temporary object holding search criteria to pass to binary search: c = new BankCustomer(firstname, lastname, 0, null, null); i = java.util.Collections.binarySearch(bank, c); if (i >= 0) System.out.println("Found in arraylist at index " + i); else System.out.println("Not found!"); ArrayList bank = new ArrayList(); bank.add(... );. java.util.Collections.sort(bank); int i; BankCustomer c; // we need a temporary object holding search criteria to pass to binary search: c = new BankCustomer(firstname, lastname, 0, null, null); i = java.util.Collections.binarySearch(bank, c); if (i >= 0) System.out.println("Found in arraylist at index " + i); else System.out.println("Not found!");

13-18 MicrosoftAP Computer Science with.NET and J# Demo! Java’s sort and search methods…

13-19 MicrosoftAP Computer Science with.NET and J# Hey, what about Linear Search? Note that binary search only works if objects are sorted –Binary search is fast (O(lgn)), but sorting is slow (O(nlgn)) Suppose we only need to do 1 search, and data isn’t sorted? –Then a linear search (O(n)) makes more sense ArrayList bank;. int i; BankCustomer c; // we need a temporary object holding search criteria to pass to linear search: c = new BankCustomer(firstname, lastname, 0, null, null); i = bank.indexOf(c); // performs a linear search if (i >= 0) System.out.println("Found in arraylist at index " + i); else System.out.println("Not found!"); ArrayList bank;. int i; BankCustomer c; // we need a temporary object holding search criteria to pass to linear search: c = new BankCustomer(firstname, lastname, 0, null, null); i = bank.indexOf(c); // performs a linear search if (i >= 0) System.out.println("Found in arraylist at index " + i); else System.out.println("Not found!"); will *not* be found…

13-20 MicrosoftAP Computer Science with.NET and J# Overriding Equals Linear search is based on finding an object that is equal –And equality is defined by equals method, not compareTo... –… so override default version inherited from Object class public class BankCustomer implements Comparable { private String firstName, lastName;. public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false; BankCustomer other = (BankCustomer)obj; return this.lastName.equals(other.lastName) && this.firstName.equals(other.firstName); } public class BankCustomer implements Comparable { private String firstName, lastName;. public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false; BankCustomer other = (BankCustomer)obj; return this.lastName.equals(other.lastName) && this.firstName.equals(other.firstName); }

13-21 MicrosoftAP Computer Science with.NET and J# Demo! Overriding equals…

13-22 MicrosoftAP Computer Science with.NET and J# Summary Interfaces are another powerful tool for OO design Interfaces are used throughout Java –effective use requires a solid understanding of OOD –e.g. linear search is built-in, but you need to override equals() –e.g. sorting & binary search are built-in, but you need to implement Comparable and compareTo() Use interfaces when: –You are designing a set of classes –Classes adhere to the “acts-like” rule — a BankCustomer acts-like a sortable and searchable object

13-23 MicrosoftAP Computer Science with.NET and J# Resources Web site for slides, demos, associated lab exercises: – – – Supplemental Reading: –M. Weisfeld, “ The Object-Oriented Thought Process ” (language-neutral) –D. West, “ Object Thinking ” (language-neutral) –Sierra & Bates, “ Head First Java ” Your students may really like this one, a very hip approach to learning Java

13-24 MicrosoftAP Computer Science with.NET and J# That’s it! Next up: LectureTopic.. 14Extending the MBS CS, Part