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6.005 Elements of Software Construction

Fall 2008

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(name, retur

whi ch

Classes

Rob Miller

Fall 2008

© Robert Miller 2008 

Review: How To Write a Method/ *

* Ret urns t he content s of t he web page i denti f i ed

1. Write the method signature n type, arguments 

*/publ i c stat i c Str i ng f etch(Str i ng url Str i ng) {

* by url Str i ng, must be a val i d URL.* e. g. f etch( "htt p: / / www. mi t . edu")

* r eturns t he MI T home page as a st r i ng of HTML.

. . .

}

2. Write a specification (a comment that defines what it returns, any side‐effects, and assumptions about the arguments. 

3. Write the method body so that it conforms to your specification.  (Revise the signature or specification if  you discover you can’t implement it!) 

© Robert Miller 2007 

Today’s Topicsobject-oriented programming in Java

¾exceptions¾classes

© Robert Miller 2007 

Getting Data from the Webi mport  j ava. net . URL;

/ *

imports the class URL from the  java.net package 

* Ret urns t he cont ent s of t he web page i dent i f i ed

* by url St ri ng. e.g. f etch( "htt p: / / www. mi t . edu")

* r et urns t he MI T home page as a st r i ng of HTML. */

publ i c stat i c Str i ng f etch(Str i ng url Str i ng) {URL url = new URL(url St ri ng) ; . . .

}

constructs a new URL object 

© Robert Miller 2007 

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1. Write the method signature

(name, return type, arguments

2. Write a specification (a comment 

that defines what it returns, any side‐

effects, and assumptions about the 

arguments.

3. Write the method body so that it conforms to your 

specification.  (Revise the signature or specification if  

you discover you can’t implement it!)

imports the class URL from 

the  java.net package

constructs a new URL object

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Static Fields and Methods¾Fields and methods declared static are associated with the class itself,

rather than an individual object• A static field has only one value for the whole program (rather than

one value per object)

– All objects of the class share that single copy of the static field

• A static method has no this object

• Static methods and fields are referenced using the class name (e.g.

 Web.fetch()) rather than an object variable

• Some classes are purely containers for static code (e.g. Hailstone,Web, java.lang.Math), and no objects of the class are ever constructed

    methods¾static final is commonly used for constants, e.g.:

publ i c st at i c f i nal PI = 3. 14159;

© Robert Miller 2007 

Using the Cache

publ i c Page( URL ur l ) t hrows I OExcept i on {

t hi s. ur l = ur l ;

Page p = getPageFromCache( url ) ;

i f  (p != nul l ) {

t hi s. content = p. content ;

} el se {t hi s. content = Web. f etch(ur l ) ;

putPageI nCache( t hi s) ;

}}

Implementing the Cachepr i vate stat i c f i nal Page[] cache = new Page[ 100]; pr i vate stat i c i nt cachePoi nter = 0;

/ / i ndex of next page to r epl ace i n the cache

pr i vate stat i c Page getPageFromCache(URL ur l ) {

f or ( Page p : cache) {

i f  (p != nul l && p. get URL( ) . equal s( url ) ) return p;

}

ret urn nul l ;

why might p be null? what happens if  we don’t check? 

/ / page not f ound

}pri vate stati c voi d put PageI nCache( Page page) {

cache[ cachePoi nter ] = page; ++cachePoi nter ; i f  ( cachePoi nter >= cache. l engt h) cachePoi nter = 0;

}

© Robert Miller 2007 

SummaryExceptions

¾Exceptions are abnormal returns from a method

¾Exceptions can be caught or declared

¾Members (fields, constructors, methods)¾Access control (public, protected, private) ¾Static members¾Overloading

© Robert Miller 2007 © Robert Miller 2007 

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why might p be null?

what happens if  we don’t check?