SENG 531: Labs

TA: Brad Cossettebrad.cossette@gmail.com

cossette@cpsc.ucalgary.ca

http://pages.cpsc.ucalgary.ca/~cossette/

Office Hours:Monday, Wednesday 3-4pm ICT 524

How the Labs Work

Labs will coverCourse material in depth as neededAssignments, Presentations, Final ReviewGeneral help with course work

Labs are optional, but encouragedUpcoming lab schedule posted on TA site

Labs This Week:

Monday Assignments Overview Polymorphism in OO Polymorphic Overloading

Wednesday AST’s Single/Double Dispatch The Visitor Pattern

Assignments: Stuff to Remember

Groups allowed/encouraged

Everyone must submit individual write-upsThe write-ups are worth more then actual

code, so don’t blow them off! Short, Concise, & Quality = Happy T.A. Long, Wordy, & full of B.S. = Grumpy T.A.

Assignments: Assignment 1

Link: http://pages.cpsc.ucalgary.ca/~rwalker/teaching/seng531/W07/

Key Points: Determine all method invocations in the code AND Determine what concrete methods are potentially invoked there

Rob has suggested a starting form of your solution Not required, but unless you have a better idea . . .

Write-up – look at the questions posted on the assignment page

Assignments: Assignment 1

Decaff no member, local, or anonymous types

local = int x1, x2; no arrays no for statements no exceptions, and so, no throw, throws, try/catch, etc. no call chains allowed, e.g., a.b().c() (which means that a.b() would

have to be assigned to a local variable) no explicit constructor calls allowed, i.e., no this() or super() no class selector

Foo.class no generics no "on-demand" import statements

import java.io.*

Assignments: Assignment 1

Coding style Should mostly follow the Java Coding Standard http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html I also expect JavaDoc formatted comments

Some recommended exceptions (IMHO): Use all lowercase for variable names, _ to separate words Use iii, jjj for loop iterators, not i,j. Use super when invoking any parent methods/variables Always use generics when possible Never use reflection unless absolutely necessary

Polymorphism and OO

Polymorphism: Definition

In more precise terms, polymorphism (object-oriented programming theory) is the ability of objects belonging to different types to respond to method calls of methods of the same name, each one according to an appropriate type-specific behaviour. The programmer (and the program) does not have to know the exact type of the object in advance, so this behavior can be implemented at run time (this is called late binding or dynamic binding).

Simpler VersionLets you treat derived class members just like their parent class's members.

Source: http://en.wikipedia.org/wiki/Polymorphism_in_object-oriented_programming

Polymorphism: So What?

AbstractGrungeBand

FooFighters

Nirvana

<<interface>>IRockBand

Polymorphism: Aggregation

AbstractGrungeBand

playLoudMusic()::void

FooFighters

singerOverdose()::voidpumpOutAlbum()::void

Nirvana

singerOverdose()::void

<<interface>>IRockBand

playMusic()::void This is how you’re probably used to thinking about Polymorphism in an OOcontext.

The FooFighters class has all the functionality of the Nirvana class, perhaps even does a few things differently, and provides some new stuff.

Polymorphism: Sub-Typing

AbstractGrungeBand

playLoudMusic()::void

FooFighters

singerOverdose()::voidpumpOutAlbum()::void

Nirvana

singerOverdose()::void

<<interface>>IRockBand

playMusic()::void

Look at this example:

public class RockConcert {

public void playAGig( AbstractGrungeBand band ) {

band.playMusic(); band.playLoudMusic(); }

}

We can pass either Nirvana or FooFighters objects as arguments to playMusic().

How hard would it to be to add new bands, or new rock music genres to the system?

Polymorphic Overloading

PolymorphismAn object can be treated as having the type

of any of its parents

OverloadingMore then one method with the same name,

but accepting different types of arguments, exists

Are there situations where we want function overloading, ANDcalling a method using an object’s real sub-

type, not its “current” sub-type?

Polymorphic Overloading

Single Dispatch: Example

Suppose we have the following classes:

public class A { public String text = "This is A";}

public class A1 extends A { public String text_1 = "This is really A1";}

public class A2 extends A { public String text_2 = "This is really A2";}

Source:http://pages.cpsc.ucalgary.ca/~rwalker/teaching/seng531/W07/priv/OO%20Review.pdf

Single Dispatch: Example

And this is our Main class:

public class Main {public void doIt( A object ) {

System.out.println( object.text );}public void doIt( A1 object ) {

System.out.println( object.text_1 );}public void doIt( A2 object ) {

System.out.println( object.text_2 );}public void foo( A object ) {

doIt( object );}

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.doIt( new A() );test.doIt( new A1() );test.doIt( new A2() );

}

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.doIt( new A() );test.doIt( new A1() );test.doIt( new A2() );

}

This is AThis is really A1This is really A2

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.foo( new A() );test.foo( new A1() );test.foo( new A2() );

}

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.foo( new A() );test.foo( new A1() );test.foo( new A2() );

}

This is AThis is AThis is A

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.doIt( new A() );test.doIt( ( A )( new A1() ) );test.doIt( ( A )( new A2() ) );

}

This is AThis is AThis is A

Single Dispatch: Example

What happens when we execute:

public static void main( String args[] ) {Main test = new Main();test.doIt( new A() );test.doIt( ( A )( new A1() ) );test.doIt( ( A )( new A2() ) );

}

The crummy way to do this:

Single Dispatch: Fix

public class CrummyMain {

public void foo( A object ) {if ( object instanceof A1 )

doIt( ( A1 )object );else if ( object instanceof A2 )

doIt( ( A2 )object );else

doIt( object );}

The elegant way to do this:

Single Dispatch: Fix

The Visitor Design Pattern

Extra slides . . .

public class PolymorphicOverloading {

public static void main( String args[] ) {

try { throw new FileNotFoundException( "Blah!" );}

catch( Exception e ) {

System.out.println( "We caught an exception" );

}

catch( FileNotFoundException fnfe ) {

System.out.println( "We caught a File Not Found Exception" );

}

}

}

Polymorphic Overloading

What’s the output from this code?

Caveat: this isn’t really polymorphic overloading, but it’s a related example that gets the gist

public class PolymorphicOverloading {

public static void main( String args[] ) {

try { throw new FileNotFoundException( "Blah!" );}

catch( Exception e ) {

System.out.println( "We caught an exception" );

}

catch( FileNotFoundException fnfe ) {

System.out.println( "We caught a File Not Found Exception" );

}

}

}

Polymorphic Overloading

What’s the output from this code?

public class PolymorphicOverloading2 {

public static void main( String args[] ) {

try { throw new FileNotFoundException( "Blah!" ); }

catch( FileNotFoundException fnfe ) {

System.out.println( "We caught a File Not Found Exception" );

}

catch( Exception e ) {

System.out.println( "We caught an exception" );

}

}

}

Polymorphic Overloading

This is how a SANE person would have written that example: