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6.005 Elements of Software Construction

Fall 2008

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7/28/2019 MIT6_005f08_lec03


Subclassing and Interfaces

Rob Miller

Fall 2008

© Robert Miller 2008

Getting Weather InformationLet’s build on Page to get weather conditions

¾Yahoo Weather feed: http://weather.yahooapis.com/forecastrss?p=02139

¾Returns an XML page that looks like this:

. . . <. . . t i t l e>Condi ti ons f or Cambri d e MA at 3: 54 m

, EDT</ t i t l e>. . . <yweat her : condi t i on t ext="Cl oudy"

code="26" t emp="82"date="Sat, 06 Sep 2008 3: 54 pmEDT" / >. . .

Some string manipulation can extract the current weather condition:

Page p = new Page( new URL( "ht t p:/ / weat her. . . =02139") ; St ri n weat her = Match. between( . etContent( ) ,

"<yweat her : condi t i on" , "/ >") ;

St ri ng condi t i on = Match. between(yweat her,"t ext =\ "" , " \ " " ) ;


Today’s TopicsMore Java

¾subclassing¾ interfaces

¾collections


Simple Pattern Matchingpubl i c cl ass Mat ch {

/ * Fi nds the f i rst match to pat tern i n st ri ng, and

* returns t he rest of t he str i ng. e. g. ,

* af t er( "Your Name: Ben", "Name: ") => "Ben“

* Returns nu i at te rn never occ urs i n st r i n . * /

publ i c stat i c Str i ng after( Str i ng str i ng, Str i ng pattern)

/ * Fi nds the f i rst match to pat tern i n st ri ng, and

* returns the part of t he str i ng before i t . e. g. ,* bef ore("hel l o/t here", "/ ") => "hel l o“

* Returns nul l i f patt ern never occurs i n str i ng. */

publ i c stat i c St ri ng bef ore(St ri ng content, St ri ng patt ern)

/ * F inds the f i rst match to l ef t i n str i ng, and

* the f i rst match to ri ght af t er t hat, and

* r etur ns t he subst r i ng between t he t wo mat ches.

* e. g. bet ween(" a <b>bol d</ b> word", "<b>", "</ b>") => "bol d“

* Returns nul l i f l ef t. . . r i ght never occurs i n str i ng. */

publ i c stat i c St ri ng between(St ri ng cont ent , Stri ng l ef t, Str i ng ri ght) © Robert Miller 2007

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c s t at c t ass o t

Switch Statement

switch tests a value against a set of cases¾equivalent to a sequence of if-else clauses¾ the value can be either an enum or integer type (int, char, etc.)

u l i c st at i c i nt e rees ComassPoi nt oi nt {

i nt resul t ;

swi t ch (poi nt) {

case NORTH: r esul t = 0; break;

case EAST: resul t = 90; break;

case SOUTH: resul t = 180; break;

case WEST: resul t = 270; break;

def aul t : t hrow new Runt i meExcepti on("i nval i d compass

The cases of a switch must be terminated by break, or they will fall through to the next case!

oi nt" ;}

return resul t ;

}default is like the else clause of a switch – it ’s good practice to always include one. Often it just throws an exception.


Summary

Subclassing¾ Inheritance of fields and methods¾Declared type vs. actual type

¾DowncastingInterfaces

¾An interface captures the essence of a class: its method specifications

Packages

¾Packages define separate namespaces for classesamespaces are a use u organ z ng pr ncp e orl arge sys ems

Collections

¾List<Type> is a list of Type objects¾Map<Key,Value> is a set of <Key,Value> pairs ¾Generic types like List and Map take a type parameter <Type>


7

The cases of a switch must be

terminated by break, or they

will fall through to the next

case!

default is like the else clause of a switch – it ’s good practice

to always include one. Often it just throws an exception.

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