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

Teach .NET Workshop SeriesTrack 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

13-3MicrosoftAP Computer Science with .NET and J#

Workshop Track

Lecture Topic

7 Algorithms and Algorithm Analysis

8 Debugging and Exception Handling

9 Applying Object-oriented Design Principles in Your Classes

10 Using Inheritance to Design a Set of Classes

11 MBS CS: the Marine Biology Simulation Case Study

12 Extending the MBS CS

13 Interfaces and Interface-based Programming

14 Extending the MBS CS, Part 2

15 Stacks, Queues and Invariants

16 Linked Data Structures and the Linked-list

17 Trees and Recursion

18 Collection Classes and Iteration

19 Additional Resources and Ideas


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 ][ Joe Hummel, Lake Forest College ]


Part 1

• Design by Contract


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() { . . . }


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!


Part 2

• Interfaces


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); . . .}


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


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( ) objectobjectobject

binarySearch( ) objectobjectobject


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…


(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) ...if (s1.compareTo(s2) < 0) ...


(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 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; }

studentstudentBankCustomer


(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}


Demo!

Sorting bank customers…


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( ... );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( ... );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!");


Demo!

Java’s sort and search methods…


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 searchif (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 searchif (i >= 0) System.out.println("Found in arraylist at index " + i);else System.out.println("Not found!");

will *not* be found…


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 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 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); }


Demo!

Overriding equals…


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


Resources

• Web site for slides, demos, associated lab exercises:

– http://blogs.msdn.com/daryllmc/default.aspx

– http://www.lakeforest.edu/~hummel/workshops-HS.htm

– https://www.mainfunction.com/home/training/

• 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

http://blogs.msdn.com/daryllmc/default.aspx

http://blogs.msdn.com/daryllmc/default.aspx

http://www.lakeforest.edu/~hummel/workshops-HS.htm

https://www.mainfunction.com/home/training/


That’s it!

• Next up:

Lecture Topic

. .

14 Extending the MBS CS, Part 2

. .

. .

. .

Date post:	14-Dec-2015
Category:	Documents
Upload:	beatrice-thomas
View:	215 times
Download:	0 times

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

Documents