Java程序辅导

Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
Lab	13					Name:________________________		Checked:______	
Objectives:	
In	this	lab	we	will	learn	how	to	access	text	files	using	Scanner	to	get	their	contents.	We	
will	also	use	Scanner	in	other	novel	ways,	including	reading	data	directly	from	a	webpage!	
Before	you	begin,	create	a	folder	for	your	Lab	13	files.	It	is	important	to	save	all	the	data	
and	program	files	for	this	lab	in	the	same	folder.	
Part	I:		
Preparation:	Submit	4	files:	YouVeGotMoreShoes.java,	ShoeCollection.java,	Shoe.java	(Lab	12,	part	B)	and	Lab13a.java	(see	below)	through	Blackboard	by	8:00am	the	morning	of	Lab.		
a.	Reading	values	from	a	file	and	storing	them	in	an	array	In	part	(d)	of	Lab	11	we	explored	how	to	input	values	from	the	user	and	store	them	in	an	array.	Review	your	program	Lab11d.java	–	recall	that	it	uses	Scanner	to	input	integer	values	from	the	user.	Save	a	copy	of	this	program	in	your	Lab	13	folder,	renaming	it	as	
Lab13a.java	Suppose	you	have	a	long	list	of	values	stored	in	a	file,	and	you	want	to	store	them	in	an	array	of	100	elements.		For	example,	the	file:	www.csc.villanova.edu/~map/1051/s16/examples/oneHundredInts.txt		You	would	not	want	to	type	the	numbers	in	order	to	enter	them	in	your	program	interactively!	
• Download	this	file	and	save	in	your	Lab	13	folder.	Open	it	in	jGrasp	to	inspect	it.		
• Make	sure	the	file	was	not	corrupted	during	download	(this	happens	sometimes)–	it	should	contain	100	numbers	and	nothing	else	–check	before	proceeding.		
• In	Java,	you	can	direct	the	Scanner	to	input	from	a	file,	instead	of	from	the	keyboard.	Do	this	as	follows:	
o In	the	instantiation	of	the	Scanner	object,	replace	System.in	by	the	file	from	which	you	are	“reading”,	i.e.:	
Scanner scan = new Scanner(new File("oneHundredInts.txt")); 
o Import	classes	from	java.io: 
import java.io.*; 
o Add	“throws IOException”	to	the	heading	of	your	main	method:	 
   public static void main(String[] args) throws IOException 
 
• Since	you	want	to	input	100	numbers,	you	need	to	enlarge	the	array	to	hold	100	integers.		Troubleshooting:	Remember	to	double-check	that	the	file	oneHundredInts.txt	is	saved	in	the	same	folder	as	your	program	and	contains	the	correct	data	(a	list	of	100	numbers).	
Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
Part	II:		
b.	Processing	a	text	file,	line	by	line	www.csc.villanova.edu/~map/1051/s16/examples/FileInput.java	
//*************************************************************** 
//  FileInput.java       Author: MAP 
//  Demonstrates the use of Scanner to read text file input. 
//*************************************************************** 
import java.util.Scanner; 
import java.io.*; 
 
public class FileInput 
{ 
   //------------------------------------------------------------ 
   //  Reads text from a file and prints it in uppercase. 
   //------------------------------------------------------------ 
   public static void main (String[] args) throws IOException 
   { 
      String line; 
      Scanner fileScan; 
 
      File myFile = new File("sample.txt"); 
      fileScan = new Scanner (myFile); 
 
      // Read and process each line of the file 
      while (fileScan.hasNext()) 
      { 
         line = fileScan.nextLine(); 
         System.out.println (line.toUpperCase()); 
      } 
      
   } 
} 
1) Download and compile this program; create a small text file named sample.txt and 
save it in the same folder as the program (you can create the file directly in JGrasp – under 
File-> New, choose “Plain text” instead of “Java” for the file type). Run FileInput – what 
does it do? 
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2) Modify it to use the parameter args[0] of main() as the file name. Do this as follows: 
• Replace the use of 
File myFile = new File("sample.txt"); 
with  
File myFile = new File(args[0]);  
• In jGrasp, select “Run Arguments” from the Build menu, and provide the file name as 
an argument (parameter) to main by typing sample.txt in the box that appears 
above your program. In this way, you can run your program with different files, 
without modifying the code. Try it with the dataset file for next project:  
• www.csc.villanova.edu/~map/1051/s16/examples/titanic.txt 	
Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
c.	Scanning	from	a	String	Just	as	we	can	use	a	Scanner	to	input	from	a	file	or	from	System.in,	we	can	also	use	a	Scanner	to	“input”	from	a	String!		1)	Try	this	code:	Lab13c.java		
//******************************************************************** 
//  Lab13c.java        MA Papalaskari 
//  Simple example: scanning from a String 
//******************************************************************** 
 
import java.util.Scanner; 
 
public class Lab13c 
{ 
   public static void main(String[] args) 
   { 
      Scanner scan = new Scanner(System.in); 
       
      System.out.print("Please type 3 words:  "); 
      String line = scan.nextLine(); 
            
      Scanner scanLine = new Scanner(line); 
       
      String word1 = scanLine.next(); 
      String word2 = scanLine.next(); 
      String word3 = scanLine.next(); 
       
      System.out.println("Word 1: " + word1); 
      System.out.println("Word 2: " + word2); 
      System.out.println("Word 3: " + word3); 
 
   } 
} 
Run :	Lab13c.java	– what does it do? 
 
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____________________________________________________________________	
 
d.	Scanning	from	a	String	and	doing	something	useful		Next,	we	will	create	Lab13d.java	by	modifying	Lab13c.java	so	that	it	does	something	more	interesting	with	the	input.	Our	new	program	will	treat	the	input	as	a	command	for	a	simple	numeric	computation.		For	example,	the	input	might	be:	
55 * 83	We	want	the	program	to	compute	and	print	the	product	4565.	First,	run	Lab13c.java	with	this	input	and	observe	how	it	picks	out	the	“55”,	“*”,	“83”	as	word1,	word2,	and	
word3,	respectively.	Note	that	the	code	uses	scanLine.next()	which	produces	String	tokens	and	that	was	fine	because	word1,	word2,	and	word3	are	Strings.	But	now	you	want	to	use	the	values	55	and	83	as	numbers,	so	the	variables	have	to	be	of	type	double	(we	
scanning	from	a	String	
Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
could	use	int,	but	double	will	allow	you	to	handle	a	wider	range	of	values),	and	you	need	to	obtain	their	values	using	scanLine.nextDouble()	instead	of	scanLine.next().		Can	you	use	these	ideas	to	create	a	simple	calculator?	Change	the	prompt	from	“Please	enter	3	words”	to	“Calculate:	”		Note	that	you	can	test	the	value	of	word2.charAt(0)	to	see	if	it	is	equal	to	‘+’,	‘*’,	etc,	and,	accordingly,	compute	the	result.	(If	you	want	to	be	able	to	handle	more	than	2	operators,	it	is	best	to	use	a	switch	statement.)		Sample	runs:	
----jGRASP exec: java Lab13d 
Calculate:   
9.3 + 44.7 
= 54.0 
 
----jGRASP exec: java Lab13d 
Calculate:   
55 * 83 
= 4565.0 
 
 
e.	Input	data	into	an	array	The	technique	described	in	Parts	C	is	also	useful	for	processing	data	organized	in	columns	and	inputting	that	into	an	array.	Go	back	to	the	code	for	PartC	(NOT	Part	D)	and	modify	the	code	so	that	it	inputs	8	words	into	an	array	of	8	Strings.	(Be	sure	to	replace	the	variables	word1,	word2	etc	by	an	array	word[0],	word[1],	etc.	and	use	a	for-loop	to	get	the	input.	The	words	should	then	be	printed	backwards.		
Tab	delimited	data:		Sometimes	the	input	tokens	can	contain	spaces.	For	example,	the	“words”	could	be	country	names:				India United States France China Germany Greece South Korea Brazil	These	are	still	just	8	countries!	In	such	situations,	a	tab	can	be	used	as	a	delimiter,	so	the	String	would	be	stored	as:		
"India\tUnited States\tFrance\tChina\tGermany\tGreece\tSouth Korea\tBrazil" In	order	for	your	Scanner	to	use	a	delimiter	other	than	whitespace,	you	need	to	specify	this	before	doing	any	input:	
scanLine.useDelimiter("\t"); 		Sample	run:	
----jGRASP exec: java Lab13d 
Enter 8 country names, all in one line, separated by tabs: 
India   France    Japan   India    Greece   United States   South Korea     Mali 
Mali 
South Korea 
United States 
Greece 
India 
Japan 
France 
India 
 
Note:	these	are	tab	characters	
Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
f.	Processing	data	from	text	files,	organized	in	columns	(Combine	Parts	b	&	e)	The	 technique	 described	 in	 (e)	 is	 useful	 for	 processing	 text	 files	 containing	 data	organized	in	columns.	We	now	modify	FileInput.java from	(b)	,	above,	so	that	after	it	inputs	each	line,	it	uses	the	technique	of	Lab13e.java,	above	(i.e.,	a	second	Scanner)	to	“scan”	8	words	from	each	line	in	the	file	and	store	these	words	in	an	array,	then	print	the	contents	of	the	array	backwards.	Try	this	with	the	following	file:		http://www.csc.villanova.edu/~map/1051/s16/examples/eightwords.txt	
 
Sample	output:	
Line: India   France  Japan   United Arab Emirates    Greece  United States   South Korea Mali 
Mali 
South Korea 
United States 
Greece 
United Arab Emirates 
Japan 
France 
India 
 
Line:  
apple     orange  asian pear      fig     persimmon       grape   raspbery        pineapple 
pineapple 
raspbery 
grape 
persimmon 
fig 
asian pear 
orange 
apple 
 
Line: black     white   gray    light gray      dark gray       red     blue    green 
green 
blue 
red 
dark gray 
light gray 
gray 
white 
black	
g.	(Optional)	Input	directly	from	a	website	Would	you	like	your	program	to	access	a	website	directly?	Here	is	how.	You	need	to	1)		Add	another	import	directive	at	the	beginning	or	your	program:	
 
import java.net.URL; 	2)	Set	up	your	Scanner	to	read	from	the	url	instead	of	a	file.	Here	is	an	example:		 	  
String myurl =   
  "http://www.csc.villanova.edu/~map/1051/s16/examples/oneHundredInts.txt"; 
     
InputStream inStream = new URL(myurl).openStream();     
Scanner webScan = new Scanner (inStream); 
 3)	Now	you	can	use webScan as	any	other	Scanner object,	to	input	from	a	webpage	as	if	it	were	any	other	text	file.	Try	it	with	your	program	lab13a.java.	This	technique	will	work	with	most	webpages,	as	long	as	they	can	be	read	as	text	(including	html	files).	
Villanova	University							CSC	1051											www.csc.villanova.edu/~map/1051											Dr.	Papalaskari	
Lab	13	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?