Java程序辅导

More Input File Processing
Using the Scanner Class
• Review Token by Token Processing
• Line by Line Processing
• String Scanner
Exception Handling
• throws clause
• try catch block
Scanner Review –
Token Based Processing
• We can use the Scanner class to read from (and 
tokenize) the standard input stream, System.in:
– Scanner console = new Scanner(System.in);
– Prompt user to enter data
• We can use the Scanner class to read (and 
tokenize) a file:
– Scanner input = new Scanner(new File(“data.txt”));
• Same Scanner methods used in both cases to 
look for and read in tokens
import java.util.*;
import java.io.*;
public class DoubleAverage {
public static void main(String[] args) throws FileNotFoundException { 
userInterface();
}
public static void userInterface() throws FileNotFoundException { 
Scanner console = new Scanner(System.in);
Scanner fileScanner= getInputScanner(console);
System.out.println("Average: " + processFile(fileScanner));
fileScanner.close();
}
…
…
}
Calculate the Average of all doubles in a file
//getInputScanner method option 1 – exception thrown if file not found
public static Scanner getInputScanner(Scanner console) throws FileNotFoundException { 
System.out.print("input file name? ");
String name = console.next();
Scanner fileScanner = new Scanner(new File (name));
return fileScanner;
}
// getInputScanner method – option 2 – exit without exception if file not found
public static Scanner getInputScanner(Scanner console) throws FileNotFoundException {
System.out.print("input file name? ");
String name = console.next(); 
Scanner fileScanner = null; 
File f = new File(name);
if (f.exists()) {
fileScanner = new Scanner(f);
}
else {
System.out.println("File: " + name + " not found"); 
System.exit(1);
}
return fileScanner;
}
Calculate the double average (cont)
getInputScanner – reprompt
public static Scanner getInputScanner(Scanner
console) throws FileNotFoundException { 
Scanner fileScanner = null;
while (fileScanner == null) { 
System.out.print("input file name? ");
File f = new File(console.next());
if (f.exists()) {
fileScanner = new Scanner(f);
}
else {
System.out.println("File not found . Please
try again.");
}
}
return fileScanner;
}
public static Scanner getInputScanner(Scanner
console) throws FileNotFoundException { 
System.out.print("Enter a file name to
process: "); 
File file = new File(console.next());
while (!file.exists()) {
System.out.print("File doesn't exist. “  
+ "  Enter a file name to process: ");
file = new File(console.next());
}
Scanner fileScanner = new Scanner(file);
return fileScanner;
}
5
Calculate the double average (cont)
public static double processFile(Scanner input) { 
double sum = 0;
int count = 0;
while (input.hasNext()) {
if (input.hasNextDouble()) { 
sum += input.nextDouble(); 
count++;
} else {
input.next();
}
}
return sum / count;
}
Another Token-Based File Processing Example
Let’s write a program HoursWorked1 that processes 
a file hours1.txt which contains employees and hours 
worked each week.  We want to calculate the total 
hours worked by each employee.
Desired Output:
Erica 7.5 8.5 10.25 8 8.5
Erin 10.5 11.5 12 11 10.75
Simone 8 8 8
Ryan 6.5 8 9.25 8
Kendall 2.5 3
Solution
Solution (cont.)
Solution (cont.)
Modified Input File
101 Erica 7.5 8.5 10.25 8 8.5
783 Erin 10.5 11.5 12 11 10.75
114 Simone 8 8 8
238 Ryan 6.5 8 9.25 8
156 Kendall 2.5 3
Employee Id
What if our input file is now hours2.txt and it 
also contains an employee id as the first token?
Flawed Solution
Flawed Output:
101 Erica 7.5 8.5 10.25 8 8.5
783 Erin 10.5 11.5 12 11 10.75
114 Simone 8 8 8
238 Ryan 6.5 8 9.25 8
156 Kendall 2.5 3
Reason for Flawed Solution
The inner while loop is grabbing the next person's ID.
101 Erica 7.5 8.5 10.25 8 8.5
783 Erin 10.5 11.5 12 11 10.75
114 Simone 8 8 8
238 Ryan 6.5 8 9.25 8
156 Kendall 2.5 3
101 Erica 7.5 8.5 10.25 8 8.5\n783 Erin 10.5 11.5 12 11 10.75\n…
The Scanner looks at the file as one continuous stream of 
input,  line breaks are ignored
We want to process the tokens, but we also care about 
the line breaks (they mark the end of a person's data).
Better Solution
101 Erica 7.5 8.5 10.25 8 8.5
783 Erin 10.5 11.5 12 11 10.75
114 Simone 8 8 8
238 Ryan 6.5 8 9.25 8
156 Kendall 2.5 3
Read this 
line first 
and process 
it token by 
token
A hybrid approach:
1. First, break the overall input into 
lines.
2. Then break each line into tokens
Line-Based Processing
• Processing input line by line
– Use nextLine() and hasNextLine()
methods
– Takes all of the text up to the new line character
(\n)
• Preserves white space and line breaks of 
the text being processed:
- Processing a poem or formatted text
- count lines in file, words in line
Counting Lines
// Counts the number of lines in the input.txt file
Scanner input = new Scanner(new File("input.txt"));
int count = 0;
while(input.hasNextLine()) {
String line = input.nextLine();
count++;
}
System.out.println(“File has " + count + " lines");
• What if we want to count the words on a line?
Scanners on Strings
17
Copyright 2008 by Pearson Education
 A Scanner can tokenize the contents of a String:
Scanner name = new Scanner(String);
 Example:
String text = "15 3.2 hello 9 27.5";
Scanner scan = new Scanner(text); 
int num = scan.nextInt();
System.out.println(num);
double num2 = scan.nextDouble(); 
System.out.println(num2);
String word = scan.next(); 
System.out.println(word);
// 15
// 3.2
// hello
Tokenizing Lines of a File
Input file input.txt: Output to console:
The 
the
quick brown 
lazy dog.
fox jumps over Line 
Line
has 
has
6
3
words 
words
Scanner to read the 
file
Scanner to tokenize the line
Close Scanner for the file
Close the Scanner for the line
Return to Hours Worked Example
So how would we process hours2.txt ?
101 Erica 7.5 8.5 10.25 8 8.5
783 Erin 10.5 11.5 12 11 10.75
114 Simone 8 8 8
238 Ryan 6.5 8 9.25 8
156 Kendall 2.5 3
Desired Output:
Corrected Solution
Close 
Scanner 
for the 
line
Line-by-Line File Processing Template
public static void userInterface () throws FileNotFoundException {
Scanner console = new Scanner(System.in);
Scanner input = getInputScanner(console);
processFile (input);
input.close();
}
public static void processFile(Scanner fileScanner) {
while (fileScanner.hasNextLine()) {
String line = fileScanner.nextLine();
processLine(line);
}       
}
public static void processLine(String line) {
Scanner lineScanner = new Scanner(line); 
while (lineScanner.hasNext()) {
String token = lineScanner.next();
// process the token
}
lineScanner.close();
}
Line-by-Line File Processing Template
public static void userInterface () throws FileNotFoundException {
Scanner console = new Scanner(System.in);
Scanner input = getInputScanner(console);
processFile (input);
input.close();
}
public static void processFile(Scanner fileScanner) {
while (fileScanner.hasNextLine()) {
String line = fileScanner.nextLine();
Scanner lineScanner = new Scanner(line); 
while (lineScanner.hasNext()) {
String token = lineScanner.next();
// process the token
}
lineScanner.close();
}       
}
In-class Exercise
• Go to the moodle page and work on the LineProcessing.java assignment.
• Your LineProcessing class must contain the following methods:
• public static void main (String[] args)
– call the userInterface() method
• public static void userInterface()
– create a Scanner for console input
– pass this Scanner to the getInputScanner method
– pass the Scanner returned from the getInputScanner method to the 
processFile method
• public static Scanner getInputScanner(Scanner console)
– prompt the user for the file name of a file to process
(use the console Scanner parameter)
– create a Scanner to read from this file and return it
– if the file does not exist, print a file does not exist message and reprompt
• public static void processFile(Scanner input)
– Read each line of the input file (using the input Scanner parameter).
– For each line:
» Create a Scanner that gets its input from the line.
» Calculate and print the number of tokens and the length of the 
longest token in the line (use the output format shown below).
•
If the file contains the following text: Your method should produce the 
following output: 
Beware the Jabberwock, my son, 
the jaws that bite, the claws that catch, 
Beware the JubJub bird and shun 
the frumious bandersnatch.
Line 1 has 5 tokens (longest = 11) 
Line 2 has 8 tokens (longest = 6) 
Line 3 has 6 tokens (longest = 6) 
Line 4 has 3 tokens (longest = 13) 
Exception Handling
• Review
– A FileNotFoundException is a “checked exception”
– The compiler “checks” to make sure we are aware
that it could happen.
– We can choose to not worry about the
FileNotFoundException:
public static void main(String [] args) throws FileNotFoundException { 
Scanner input = new Scanner(new File(“data.txt”));
}
Exception Handling (cont.)
• We can use a “try catch” block to handle the
FileNotFoundException (a better idea☺):
• See Appendix C in Textbook
public static void main(String[] args) { 
try {
Scanner input = new Scanner(new File(“data.txt”));
//do our processing
} catch (FileNotFoundException e) { 
System.out.println(“File: data.txt does not exist!”);
}
}
Exception Handling (cont.)
NOTE: If method X throws a checked
exception, then any method that calls X must 
handle the exception with either
a “throws clause” 
or
a “try catch block.”
import java.util.*;
import java.io.*;
public class DoubleAverage {
public static void main(String [] args) throws FileNotFoundException { 
userInterface();
}
public static void userInterface() throws FileNotFoundException { 
Scanner console = new Scanner(System.in);
Scanner fileScanner= getInputScanner(console);
System.out.println("Average: " + processFile(fileScanner));
fileScanner.close();
}
…
…
}
Recall: Calculate the Average of all doubles in a file
Recall: Calculate the double average
(getInputScanner)
//get inputScanner method – reprompt option
public static Scanner getInputScanner(Scanner console) throws FileNotFoundException { 
Scanner fileScanner = null;
while (fileScanner == null) { 
System.out.print("input file name? ");
String name = console.next();
File f = new File(name);
if (f.exists()) {
fileScanner = new Scanner(f);
}
else {
System.out.println("File: " + name + " not found . Please try again.");
}
}
return fileScanner;
}
Redo: Calculate the double average
(getInputScanner – try/catch)
//get inputScanner method – try/catch
public static Scanner getInputScanner(Scanner console) { //remove throws
Scanner fileScanner = null;
while (fileScanner == null) { 
System.out.print("input file name? ");
String name = console.next();
try {                                                                      //add try catch
fileScanner = new Scanner(new File(name));
} catch (FileNotFoundException e) {
System.out.println("File: " + name + " not found . Please try again.");
}
}
return fileScanner;
}
import java.util.*;
import java.io.*;
public class DoubleAverage {
public static void main(String [] args) { //remove throws
userInterface();
}
public static void userInterface() {  //remove throws
Scanner console = new Scanner(System.in);
Scanner fileScanner= getInputScanner(console);
System.out.println("Average: " + processFile(fileScanner));
fileScanner.close();
}
…
…
}
Redo: Calculate the Average – try/catch
For example, a file with the following content: Should result in the 
following output:
Students are often asked to write term papers containing a certain number of words. Counting words 
in a long paper is a tedious task, but the computer can help.  Write a program that counts the number
of words, lines, and total characters (not including whitespace) in a paper, assuming that 
consecutive words are separated either by spaces or end-of-line characters. Your program should be 
named WordCount.java. 
Total lines = 5 
Total words = 66 
Total chars = 346
In-class Group Exercise
• Go to GitHub
• Clone the csc116-xxx-Lab14-yy  repository, where yy is your team 
number, xxx is your section number
– Under clone or download, copy the url
– In exercises folder on local machine:   git clone url
• Go to the moodle page and work on the WordCount.java assignment. 
• Write a WordCount class that counts the number of words, lines, 
and total characters (not including whitespace) in a paper, assuming 
that consecutive words are separated by whitespace.
• Use try/catch (do not use the throws clause).
• Push to github,  do not submit to Moodle.