Java程序辅导

	CS	Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
Lab	14					Name:________________________		Checked:______	
Objectives:	Practice	handling	exceptions	and	writing	text	files.		
a)	Revisit	Lab	9	and	Project	7	experiment	
• Run	ResponseTimeExperiment.java	and	experiment	with	“bad	inputs”	(e.g.,	not	a	number).	What	is	the	exception	when	your	input	cannot	be	parsed	as	a	number?			
Exception:	______________________________________________________________________________________		 	
b)	try/catch	NumberFormatException		Create	a	try	block	around	the	code	that	parses	a	response	and,	using	it,	computes	the	outcome.	Use	a	catch	clause	for	NumberFormatException	that	prints	"FAIL:	You	were	supposed	to	type	in	a	number".	
 
c)	Write	the	results	of	the	experiment	to	a	file	Let’s	now	save	the	results	of	the	experiment	to	a	file.		For	each	subject,	the	program	should	create	a	file	using	the	person’s	name	as	the	file	name,	with	“.txt”	appended	to	it,	and	then	output	the	results	of	that	run	(namely,	whether	they	got	the	question	right	and	the	number	of	milliseconds)	to	the	file,	separated	by	tabs.	For	example,	with	the	following	interaction:	
----jGRASP exec: java ResponseTimeExperiment 
Please enter your name: Megan 
Hello Megan. Please answer as fast as you can. 
 
Hit  when ready for the question. 
 
96 + 86 = 145 
Incorrect. 
That took 3025 milliseconds 
Thank you Megan, goodbye. 
 
 ----jGRASP: operation complete. the	program	should	create	a	file	named	Megan.txt	with	contents:		(“false”	since	the	answer	was	incorrect,	with	3025	milliseconds)		 If,	on	the	other	hand,	the	run	did	not	result	in	any	useful	data	(i.e.,	you	caught	a	NumberFromatException),	the	file	should	contain:			
Hints:	
• Create	a	PrintWriter	object	named	outfile,	using	the	name	that	the	person	entered	as	the	filename.	
• Use	the	println()	on	outfile	to	output	the	data	collected,	i.e.,	the	outcome	and	milliseconds,	separated	by	a	tab.	
• Don’t	forget	to	close()	the	file	at	the	end	(after	all	output	to	the	file	is	done)	
• Be	sure	to	import	java.io.*;		
• Remember,	PrintWriter can	throw	IOException.	We	will	NOT	handle	that	yet,	so	you	need	to	list	throws IOException	in	the	main()	method	heading.	
false  3025 
Bad data 
 
	CS	Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
	
d)	Repeat	using	your	own	code	from	Project	7	
• Modify	your	code	from	Project	7	to	save	the	results	of	the	experiment	to	a	file	and,	if	necessary,	to	catch	NumberFormatException.	For	each	subject,	the	program	should	create	a	file	using	the	person’s	name	as	the	file	name,	with	“.txt”	appended	to	it,	as	before.	The	file	should	contain	one	line	corresponding	to	each	question,	with	the	results	and	the	number	of	milliseconds,	unless	that	question	did	not	result	in	usable	data	(i.e.,	the	program	had	to	handle	a	NumberFormatException),	in	which	case	the	corresponding	line	in	the	file	should	contain	“bad	data”.			
• Run	the	experiment	using	yourself	and	your	partner	as	subjects	and	verify	that	the	corresponding	files	were	created	correctly.	
	
	
e)	What	if	the	file	cannot	be	created?	
• Can	you	figure	out	how	to	crash	your	programs	(ie,	cause	
FileNotFoundException)?	This	can	happen	if	the	name	entered	by	the	user	cannot	be	used	by	your	operating	system	to	create	a	file	with	that	name.	Play	around	with	the	program	using	different	inputs	for	your	name	(Hint:	try	various	symbols	and	non-alpha	characters),	until	you	cause	this	exception	and	make	a	note	of	the	input	here:			
Bad	file	name	that	crashed	the	program:			_________________________________________________		
Message:			_____________________________________________________________________________________		
	Now	let’s	try/catch	it:	
• First,	separate	the	PrintWriter	declaration	from	the	instantiation:	
PrintWriter outfile;      // declaration 
outFile = new PrintWriter( . . . ); // instantiation 	
• Next,	place	the	instantiation	for	the	PrintWriter	in	a	try	block,	and	catch	
IOException	(that	will	catch	FileNotFoundException	and	others	as	well).	In	the	
catch,	handle	the	exception	as	follows:	
o outFile = new PrintWriter(System.out); 
o print	a	warning	on	standard	output,	such	as:		
 
WARNING: Unable to create file (*name of file*).  
Data output to System.out. 	To	summarize:	The	experiment	proceeds	as	before.	If	no	IOException	occurs,	the	file	will	be	created;	if	that	is	impossible	(i.e.,	an	exception	occurs),	the	program	will	recover	and	proceed	after	a	warning,	and	the	contents	that	would	have	normally	gone	to	the	file	will	instead	get	printed	at	the	end	of	the	run,	after	all	the	other	output.		
	
	
	
	CS	Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
e)	What	if	your	subject	is	prone	to	obscenity?	Sometimes	programs	need	to	respond	to	rude	or	offensive	user	behavior.	Here	we	will	handle	this	situation	by	throwing	an	exception,	so	that	the	program	aborts	when	the	user	enters	a	forbidden	word	as	their	name.	Here	is	how	to	do	this:		
• Create	a	class	BadWordException.java that	extends	Exception:		
//******************************************************************** 
//  BadWordException.java       Author: M A Papalaskari 
//  Represents an exceptional condition for using bad words.  
//******************************************************************** 
 
public class BadWordException extends Exception 
{ 
   //----------------------------------------------------------------- 
   //  Sets up the exception object with a particular message. 
   //----------------------------------------------------------------- 
   BadWordException(String message) 
   { 
      super(message); 
   } 
} 
 
• Create	a	file	“excusemyfrench.txt”	of	banned	words.	Try	it	first	with	just	a	few	words	(such	lists	are	available	free	on	the	web,	you	can	download	one	later).			
• In	the	ResponseTimeExperiment	program	(or	use	your	own	program	from	Project	7),	check	that	the	name	entered	is	not	a	banned	word	before	creating	a	file	with	the	name	of	the	person.	Below	is	some	code	to	do	this:	run	through	the	banned	words	file,	comparing	the	name	with	each	word.	If	you	get	a	match,	punish	them	by	throwing	an	exception!	
 
            Scanner badWords = new Scanner(new File("excusemyfrench.txt")); 
            while (badWords.hasNext()) 
               if (name.equals(badWords.next())) 
                  throw (new BadWordException("Watch your tongue")); 	
Notes:		1)	By	default,	your	BadWordException	is	a	checked	exception	and	thus	needs	to	be	listed	with	a	throws	clause	in	the	heading	of	your	main	method.			2)	We	are	again	faced	with	the	possibility	of	an	IOException,	this	time	for	the	input	file	excusemyfrench.txt.		Handle	it	by	placing	the	above	code	inside	a	try	block	and	use	a	catch	clause	such	as:	
 
       catch (IOException e) 
        { 
          System.out.println("Warning: excusemyfrench.txt missing. "); 
    System.out.println("Not checking input for obscenity"); 
  } 
	CS	Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
Lab	14	Comments						 Name:________________________				Comments	on	this	lab,	please:		What	was	the	most	valuable	thing	you	learned	in	this	lab?								What	did	you	like	best	about	this	lab?								Was	there	any	particular	problem?									Do	you	have	any	suggestions	for	improving	this	lab	as	an	effective	learning	experience?