Carron Shankland. Content. String manipula3on in Java The use of files in Java The use of the command line arguments References:

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CSCU9T4 (Managing Informa3on):

Strings and Files in Java

Carron Shankland

Content

•  String manipula3on in Java

•  The use of ﬁles in Java

•  The use of the command line arguments

•  References:

–  Java For Everyone, 2nd Edi3on (2013), by Cay Horstmann

•  Chapter 02: sec3on 2.3 and 2.5 (strings)

•  Chapter 07: sec3ons 7.1, 7.2, 7.3 (ﬁles)

–  The lecture material comes from this book

–  M. T. Goodrich and R. Tamassia, Data Structures and Algorithms in Java, 5th edi3on

•  Character strings (such as those displayed in the board) are

important data types in any Java program. We will learn how to work with text, and how to perform useful tasks with them.

•  We will also learn how to write programs that manipulate text ﬁles,

a very useful skill for processing real world data.

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String Processing facili3es in Java

•  A common method to manipulate strings in Java is to use the the String class (you can also use StringBuffer)

•  The String class provides a lot of useful methods, including those for

–  crea3ng and manipula3ng strings

–  inspec3ng the characters in a string

–  spliXng up a string into tokens

•  We will take a quick tour of some of the most useful of these (aZer a quick recap of the features you know already)

–  substring, trim, split

–  toUpperCase, toLowerCase

–  equals, endsWith, startsWith

–  charAt, indexOf, lastindexOf

•  We will also look at the StringTokenizer class

TIP: Refer to Java docs! h\p://www.cs.s3r.ac.uk/doc/java/jdk1.6/

©University of S3rling

Strings

•

Many programs process text, not numbers

•

Text consists of characters: le\ers, numbers,

punctua3ons, spaces, etc.

•

What is a string?

–

An ordered collec3on of characters of arbitrary

length

–

Consider S3rling, You rock!, “67.435”, 3.8x104, or

5.2e6?

–

In our ‘world’ strings delineated by quota3ons: “ “

4 ©University of S3rling

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Common uses of strings

•  Input

–  from a user, or from a (data) ﬁle

–  the program must understand what the string represents

–  making sense of a string (determining its syntax) is called

parsing e.g. a Url •  Output

–  maybe on the screen, or to a ﬁle

•  Strings are oZen converted to/from other formats, e.g.

–  string/number conversions are very common, including

integers, ﬂoa3ng point numbers

–  can also have general string/object conversions

•  As programmers, we need to be able to process strings

Examples of string processing

•

A compiler takes the text of a program as input,

and its ﬁrst task is to parse the input

•

A word processor looks to see which are the

individual words of a line of text so that it can

spell check them.

•

A browser must parse a URL into pieces:

–  which protocol to use

–  which web server to contact

–  which ﬁle to ask for from that web server

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2.5

Strings

q 

The

String

Type:

§  Type Variable Literal

§  String name = Harry

q 

Once you have a

String

variable, you can use

methods such as:

int n = name.length(); // n will be assigned 5

q 

A

String

s length is the number of characters

inside:

§  An empty String (length 0) is shown as

§  The maximum length is quite large (an int)

String concatena3on

•  Java: + operator is used to concatenate strings. Put them together to

produce a longer string. Example:

–  String fName = “Harry”, String lName = “Morgan”

–  String name = fName + lName

–  Results in the string: “HarryNorman”

•  If you’d like the ﬁrst and last name separated by a space:

–  String name = fName + “ “ + lName –  Results in the string: “Harry Norman”

•  When the expression to the leZ or right of a ‘+’ operator is a string,

the other one is automa3cally forced to be a string, and both strings are concatenated. Example:

–  String jobTitle = “Agent”, int empID = 7

–  String bond = jobTitle + empID

–  Results in the string: “Agent7”

8 ©University of S3rling

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2.3 Input and Output

Reading Input

•

You might need to ask for input (aka prompt for input)

and then save what was entered.

–  We will be reading input from the keyboard

–  For now, don t worry about the details

•

This is a three step process in Java

1)  Import the Scanner class from its package java.util

import java.util.Scanner;

2)  Setup an object of the Scanner class

Scanner in = new Scanner(System.in);

3)  Use methods of the new Scanner object to get input

int bottles = in.nextInt(); double price = in.nextDouble();

Page 9

Syntax 2.3: Input Statement

•  The Scannerclass allows you to read keyboard input from the user

–  It is part of the Java API util package

Java classes are grouped into packages. Use the import statement to use

classes from packages.

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String

Input

q 

You can read a

String

from the console with:

System.out.print("Please enter your name: "); String name = in.next();

§  The next method reads one word at a time

§  It looks for white space delimiters

q 

You can read an entire line from the console with:

System.out.print("Please enter your address: "); String address = in.nextLine();

§  The nextLine method reads until the user hits Enter

q 

Converting a

String

variable to a number

System.out.print("Please enter your age: "); String input = in.nextLine();

int age = Integer.parseInt(input); // only digits!

String

Escape Sequences

q 

How would you print a double quote?

§  Preface the " with a \ inside the double quoted String

System.out.print("He said \"Hello\"");

q 

OK, then how do you print a backslash?

§  Preface the \ with another \!

System.out.print(" C:\\Temp\\Secret.txt );

q 

Special characters inside

String

s

§  Output a newline with a \n

System.out.print("*\n**\n***\n");

* ** ***

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String

s and Characters

q 

String

s are sequences of characters

§  Unicode characters to be exact

§  Characters have their own type: char

§  Characters have numeric values

•  See the ASCII code chart in Appendix B

•  For example, the letter H has a value of 72 if it were a number

q 

Use single quotes around a

char

char initial = B ;

q 

Use double quotes around a

String

String initials = BRL ;

Copying a

char

from a

String

q 

Each

char

inside a

String

has an index number:

q 

The first

char

is index zero

(0)

q 

The

charAt

method returns a

char

at a given

index inside a

String

:

String greeting = "Harry";

char start = greeting.charAt(0); char last = greeting.charAt(4);

0 1 2 3 4 5 6 7 8 9

c h a r s h e r e

0 1 2 3 4

H a r r y

(8)

Copying portion of a

String

q 

A substring is a portion of a

String

q 

The

substring

method returns a portion of a

String

at a given index for a number of

char

s,

starting at an index:

String greeting = "Hello!";

String sub = greeting.substring(0, 2);

String sub2 = greeting.substring(3, 5);

0 1 2 3 4 5 H e l l o ! 0 1 H e Page 15

Example: initials.java

1 import java.util.Scanner; 2 3 /**

4 This program prints a pair of initials.

5 */

6 public class Initials

7 {

8 public static void main(String[] args)

9 {

10 Scanner in = new Scanner(System.in);

11

12 // Get the names of the couple

13

14 System.out.print("Enter your first name: ");

15 String first = in.next();

16 System.out.print("Enter your significant other's first name: ");

17 String second = in.next();

18

19 // Compute and display the inscription

20

21 String initials = first.substring(0, 1)

22 + "&" + second.substring(0, 1);

23 System.out.println(initials);

24 }

25 }

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The StringTokenizer Class

•  The StringTokenizer class allows a string to be split into pieces know as ‘tokens’

•  A delimiter character is speciﬁed, and this is used to break down the original string into tokens. We start a new token every 3me a delimiter character is detected.

•  For example, with the string

"http://www.cs.stir.ac.uk/courses/CSC9V4/"

•  and the delimiter '/', the individual tokens in that string would be

"http:", "www.cs.stir.ac.uk", "courses", and "CSC9V4".

•  More usually, with the default space character delimiter, a line of text can be broken up into individual words. This is how a word processor works out where words begin and end.

StringTokenizer Example 1

StringTokenizer st; // Declare a reference st = new StringTokenizer("this is a test");

while (st.hasMoreTokens()) {

System.out.println(st.nextToken()); }

The above program produces: this

is a test

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StringTokenizer Example 2

String input="http://www.cs.stir.ac.uk/index.htm";!

String delims= "/";!

!

StringTokenizer st; !// Declare a reference!

st = new StringTokenizer(input,delims); !

!

while (st.hasMoreTokens()) !

System.out.println(st.nextToken());

The above program produces:

http:

www.cs.stir.ac.uk index.htm

The String.split method

•  In Java 1.5, a new method of tokenizing strings was introduced. An addi3onal

method called split was added to the String class.

•  The API guide for split is:

public String[] split(String regex)

where regex is a ‘regular expression’ or pa\ern used to determine how to break up the String.

•  For example we could just use the forward slash delimiter as before “/”,

alterna3vely we can use “\\s+” which means one or more white spaces, or “two\\s +” which looks for the word ‘two’ followed by one or more white spaces. (What are \\d , \\D , \\w , \\W ?)

•  Regular expressions are very powerful selectors

•  split returns an array of tokens; each token is just a String

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Example: String.split

String input="http://www.cs.stir.ac.uk/index.htm";!

String delims="/";!

!

String [] tokens = input.split(delims);! !

for (int t=0; t<tokens.length; t++)!

System.out.println(tokens[t]);

The above code produces:

http:

www.cs.stir.ac.uk index.htm

Table 9: String Operations (1)

Page 22

2.5 Strings 63

12_{// Get the names of the couple} 13

14 System.out.print("Enter your first name: ");

15 String first = in.next();

16 System.out.print("Enter your significant other's first name: ");

17 String second = in.next();

18

19_{// Compute and display the inscription} 20

21 String initials = first.substring(0, 1)

22 + "&" + second.substring(0, 1);

23 System.out.println(initials);

24 }

25 }

Program Run

Enter your first name: Rodolfo

Enter your significant other's first name: Sally

R&S

Table 9 String Operations

Statement Result Comment

string str = "Ja"; str = str + "va";

str is set to "Java" When applied to strings,

+ denotes concatenation.

System.out.println("Please" + " enter your name: ");

Prints

Please enter your name:

Use concatenation to break up strings that don’t fit into one line.

team = 49 + "ers" team is set to "49ers" Because "ers" is a string, 49 is converted

to a string.

String first = in.next(); String last = in.next(); (User input: Harry Morgan)

first contains "Harry"

last contains "Morgan"

The next method places the next word

into the string variable.

String greeting = "H & S"; int n = greeting.length();

n is set to 5 Each space counts as one character.

String str = "Sally"; char ch = str.charAt(1);

ch is set to 'a' This is a char value, not a String. Note

that the initial position is 0.

String str = "Sally";

String str2 = str.substring(1, 4);

str2 is set to "all" Extracts the substring starting at

position 1 and ending before position 4.

String str2 = str.substring(1);

str2 is set to "ally" If you omit the end position, all

characters from the position until the end of the string are included.

String str2 = str.substring(1, 2);

str2 is set to "a" Extracts a String of length

1; contrast with str.charAt(1).

String last = str.substring( str.length() - 1);

last is set to the string

containing the last

character in str

The last character has position

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Table 9: String Operations (2)

Formatted Output

q 

Outputting floating point values can look strange:

Price per liter: 1.21997

q 

To control the output appearance of numeric

variables, use formatted output tools such as:

System.out.printf( %.2f , price);

Price per liter: 1.22

System.out.printf( %10.2f , price); Price per liter: 1.22

§  The %10.2f is called a format specifier

10 spaces 2 spaces

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Format Types

q 

Formatting is handy to align columns of output

q 

You can also include text inside the quotes:

System.out.printf( Price per liter: %10.2f , price);

Page 25

Summary:

Strings

q  Strings are sequences of characters.

q  The length method yields the number of characters in a

String.

q  Use the + operator to concatenate Strings; that is, to put

them together to yield a longer String.

q  Use the next (one word) or nextLine (entire line)

methods of the Scanner class to read a String.

q  Whenever one of the arguments of the + operator is a

String, the other argument is converted to a String.

q  If a String contains the digits of a number, you use the

Integer.parseInt or Double.parseDouble method to obtain the number value.

q  String index numbers are counted starting with 0.

q  Use the substring method to extract a part of a String

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Files

7.1 Reading and Writing Text Files

q 

Text Files are very commonly used to store

information

§  Both numbers and words can be stored as text

§  They are the most portable types of data files

q 

The

Scanner

class can be used to read text files

§  We have used it to read from the keyboard

§  Reading from a file requires using the File class

q 

The

PrintWriter

class will be used to write text

files

§  Using familiar print, println and printf tools

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Text File Input

q 

Create an object of the

File

class

§  Pass it the name of the file to read in quotes

q 

Then create an object of the

Scanner

class

§  Pass the constructor the new File object

q 

Then use

Scanner

methods such as:

§  next() §  nextLine() §  hasNextLine() §  hasNext() §  nextDouble() §  nextInt()...

File inputFile = new File("input.txt");

while (in.hasNextLine()) {

String line = in.nextLine(); // Process line;

}

Scanner in = new Scanner(inputFile);

Text File Output

q 

Create an object of the

PrintWriter

class

§  Pass it the name of the file to write in quotes

•  If output.txt exists, it will be emptied

•  If output.txt does not exist, it will create an empty file PrintWriter is an enhanced version of PrintStream • System.out is a PrintStream object!

PrintWriter out = new PrintWriter("output.txt");

out.println("Hello, World!");

out.printf("Total: %8.2f\n", totalPrice);

System.out.println( Hello World! );

q 

Then use

PrintWriter

methods such as:

§  print()

§  println()

§  printf()

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Closing Files

q 

You must use the

close

method before file

reading and writing is complete

q  Closing a Scanner

String line = in.nextLine(); // Process line;

}

in.close();

out.println("Hello, World!");

out.printf("Total: %8.2f\n", totalPrice);

out.close();

q  Closing a PrintWriter

Your text may not be saved to the file until you use the

close method!

Exceptions Preview

q 

One additional issue that we need to tackle:

§

If the input file for a Scanner doesn’t exist, a

FileNotFoundException

occurs when the

Scanner object is constructed.

§

The PrintWriter constructor can generate this

exception if it cannot open the file for writing.

•  If the name is illegal or the user does not have the

authority to create a file in the given location

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Exceptions Preview

§

Add two words to any method that uses File I/O

•  Until you learn how to handle exceptions yourself

public static void main(String[] args) throws FileNotFoundException

import java.io.File;

import java.io.FileNotFoundException; import java.io.PrintWriter;

import java.util.Scanner; public class LineNumberer {

public void openFile() throws FileNotFoundException {

. . . }

}

And an important

import

or two..

q 

Exception classes are part of the

java.io

package

§  Place the import directives at the beginning of the source

file that will be using File I/O and exceptions

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Example: Total.java (1)

required! Some examples may use import java.io.*;

Note the throws clause

Example: Total.java (2)

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Common Error 7.1

q 

Backslashes in File Names

§  When using a String literal for a file name with path

information, you need to supply each backslash twice:

§  A single backslash inside a quoted string is the escape

character, which means the next character is interpreted

differently (for example, \n for a newline character)

§  When a user supplies a filename into a program, the

user should not type the backslash twice

File inputFile = new File("c:\\homework\\input.dat");

Common Error 7.2

q 

Constructing a

Scanner

with a

String

§  When you construct a PrintWriter with a String, it writes

to a file:

§  This does not work for a Scanner object

§  It does not open a file. Instead, it simply reads through

the String that you passed ( input.txt )

§  To read from a file, pass Scanner a File object:

§  or

PrintWriter out = new PrintWriter("output.txt");

Scanner in = new Scanner("input.txt"); // Error?

File myFile = new File("input.txt"); Scanner in = new Scanner(myFile);

Scanner in = new Scanner(new File ( input.txt ) );

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7.2 Text Input and Output

q 

In the following sections, you will learn how to

process text with complex contents, and you will

learn how to cope with challenges that often occur

with real data.

q 

Reading Words Example:

while (in.hasNext()) {

String input = in.next(); System.out.println(input); }

Mary had a little lamb

Mary had a little lamb input output

Processing Text Input

q 

There are times when you want to read input by:

§  Each Word

§  Each Line

§  One Number

§  One Character

q 

Java provides methods of the

Scanner

and

String

classes to handle each situation

§  It does take some practice to mix them though!

Processing input is required for almost all types of programs that interact with the user.

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Reading Words

q  In the examples so far, we have read text one line at a time

q  To read each word one at a time in a loop, use:

§  The Scanner object s hasNext()method to test if there

is another word

§  The Scanner object s next() method to read one word

§  Input: Output:

String input = in.next(); System.out.println(input); }

Mary had a little lamb

White Space

q 

The

Scanner

’s

next()

method has to decide

where a word starts and ends.

q 

It uses simple rules:

§  It consumes all white space before the first character

§  It then reads characters until the first white space

character is found or the end of the input is reached

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White Space

q 

What is whitespace?

§  Characters used to separate:

•  Words

•  Lines

her fleece was white as\tsnow

Common White Space

Space

\n NewLine

\r Carriage Return

\t Tab

\f Form Feed

The

useDelimiter

Method

q 

The

Scanner

class has a method to change the

default set of delimiters used to separate words.

§  The useDelimiter method takes a String that lists all

of the characters you want to use as delimiters:

Scanner in = new Scanner(. . .); in.useDelimiter("[^A-Za-z]+");

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The

useDelimiter

Method

§  You can also pass a String in regular expression format

inside the String parameter as in the example above.

§  [^A-Za-z]+ says that all characters that ^not either

A-Z uppercase letters A through Z or a-z lowercase a

through z are delimiters.

§  Search the Internet to learn more about regular

expressions.

Scanner in = new Scanner(. . .); in.useDelimiter("[^A-Za-z]+");

Summary: Input/Output

q 

Use the

Scanner

class for reading text files.

q 

When writing text files, use the

PrintWriter

class

and the

print/println/printf

methods.

q 

Close all files when you are done processing them.

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Summary: Processing Text Files

q 

The

by white space.

q 

nextDouble

(and

nextInt

…

) read double,

Integer, respectively.

§  Should be used with hasNextDouble and hasNextInt

respectively to avoid exceptions

§  They do not consume white space following a number

q 

Next lecture

§  How to read complete lines with mixed input (data record)

§  How to read one character at a time

§  Command line arguments

Reading Characters

q 

There are no

hasNextChar()

or

nextChar()

methods of the

Scanner

class

§  Instead, you can set the Scanner to use an empty

delimiter ("")

§  next returns a one character String

§  Use charAt(0) to extract the character from the String

at index 0 to

a

char

variable

Scanner in = new Scanner(. . .); in.useDelimiter("");

char ch = in.next().charAt(0); // Process each character

}

(25)

Classifying Characters

q 

The Character class provides several useful

methods to classify a character:

§  Pass them a char and they return a boolean

if ( Character.isDigit(ch) ) …

Reading Lines

q  Some text files are used as simple databases

§  Each line has a set of related pieces of information

§  This example is complicated by:

•  Some countries use two words

–  United States

§  It would be better to read the entire line and process it

using powerful String class methods

q  nextLine() reads one line and consumes the ending \n China 1330044605

India 1147995898

United States 303824646

String line = in.nextLine(); // Process each line

}

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Breaking Up Each Line

q 

Now we need to break up the line into two parts

§  Everything before the first digit is part of the country

§  Get the index of the first digit with Character.isdigit

int i = 0;

while (!Character.isDigit(line.charAt(i))) { i++; }

Breaking Up Each Line

§  Use String methods to extract the two parts

String countryName = line.substring(0, i); String population = line.substring(i);

// remove the trailing space in countryName

countryName = countryName.trim();

trim removes white space at the beginning and the end.

303824646 United States

(27)

Or Use

Scanner

Methods

q 

Instead of

String

methods, you can sometimes use

Scanner

methods to do the same tasks

§  Read the line into a String variable

•  Pass the String variable to a new Scanner object

§  Use Scanner hasNextInt to find the numbers

•  If not numbers, use next and concatenate words

United States 303824646

Scanner lineScanner = new Scanner(line); String countryName = lineScanner.next(); while (!lineScanner.hasNextInt())

{

countryName = countryName + " " + lineScanner.next(); }

Remember the

next method

consumes white space.

Converting Strings to Numbers

q 

Strings can contain

digits

, not

numbers

§  They must be converted to numeric types

§  Wrapper classes provide a parseInt method

String pop = 303824646 ;

int populationValue = Integer.parseInt(pop);

String priceString = 3.95 ;

int price = Double.parseInt(priceString);

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Converting Strings to Numbers

q 

Caution:

§  The argument must be a string containing only digits

without any additional characters. Not even spaces are

allowed! So… Use the trim method before parsing!

int populationValue = Integer.parseInt(pop.trim());

Safely Reading Numbers

q 

Scanner

nextInt

and

nextDouble

can get

confused

§  If the number is not properly formatted, an Input

Mismatch Exception occurs

§  Use the hasNextInt and hasNextDouble methods to test

your input first

q  They will return true if digits are present

§  If true, nextInt and nextDouble will return a value

§  If not true, they would throw an input mismatch exception if (in.hasNextInt())

{

int value = in.nextInt(); // safe }

(29)

Reading Other Number Types

q 

The

Scanner

class has methods to test and read

almost all of the primitive types

q 

What is missing?

§  Right, no char methods!

Data Type Test Method Read Method

byte hasNextByte nextByte short hasNextShort nextShort

int hasNextInt nextInt

long hasNextLong nextLong float hasNextFloat nextFloat double hasNextDouble nextDouble boolean hasNextBoolean nextBoolean

Mixing Number, Word and Line Input

q 

nextDouble

(and

nextInt

…

) do not consume

white space following a number

§  This can be an issue when calling nextLine after

reading a number

§  There is a newline at the end of each line

§  After reading 1330044605 with nextInt

• nextLine will read until the \n (an empty String) China

1330044605 India

while (in.hasNextInt()) {

String countryName = in.nextLine(); int population = in.nextInt();

in.nextLine(); // Consume the newline }

(30)

Formatting Output

q 

Advanced

System.out.printf

§  Can align strings and numbers

§  Can set the field width for each

§  Can left align (default is right)

q 

Two format specifiers example:

§  %-10s : Left justified String, width 10

§  %10.2f : Right justified, 2 decimal places, width 10

System.out.printf("%-10s%10.2f", items[i] + ":", prices[i]);

printf

Format Specifier

q 

A format specifier has the following structure:

§  The first character is a %

§  Next, there are optional flags that modify the format,

such as - to indicate left alignment. See Table 2 for the most common format flags

§  Next is the field width, the total number of characters in

the field (including the spaces used for padding), followed by an optional precision for floating-point numbers

q 

The format specifier ends with the format type,

such as f for floating-point values or s for strings.

See Table 3 for the most important formats

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printf

Format Flags

printf

Format Types

(32)

7.3 Command Line Arguments

q 

Text based programs can be parameterized by

using command line arguments

§  Filename and options are often typed after the program

name at a command prompt:

§  Java provides access to them as an array of Strings

parameter to the main method named args

§  The args.length variable holds the number of args

§  Options (switches) traditionally begin with a dash

-public static void main(String[] args) >java ProgramClass -v input.dat

args[0]: "-v"

args[1]: "input.dat"

Caesar Cipher Example

q 

Write a program that encrypts a file – scrambles it

so it is unreadable except to those who know the

encryption method

q 

Use a method familiar to the emperor

Julius

Caesar

(ignoring 2,000 years of progress in

encryption)

q 

Replacing A

→

D, B

→

E, C

→

F

… (shift of a fixed

length)

(33)

Caesar Cipher Example

q 

Write a command line program that uses character

replacement (Caesar cipher) to:

1)  Encrypt a file provided input and output file names

2)  Decrypt a file as an option

>java CaesarCipher input.txt encrypt.txt

>java CaesarCipher –d encrypt.txt output.txt

7.3 Command Line Arguments 331

Regular Expressions

Regular expressions describe character patterns. For example, numbers have a simple form. They contain one or more digits. The regular expression describing numbers is [0-9]+. The set

[0-9] denotes any digit between 0 and 9, and the + means “one or more”.

The search commands of professional programming editors understand regular expres-sions. Moreover, several utility programs use regular expressions to locate matching text. A commonly used program that uses regular expres sions is grep (which stands for “global regu-lar expression print”). You can run grep from a command line or from inside some compila-tion environments. Grep is part of the UNIX operating system, and versions are available for Windows. It needs a regular expression and one or more files to search. When grep runs, it displays a set of lines that match the regular expression.

Suppose you want to find all magic numbers (see Programming Tip 2.2) in a file.

grep [0-9]+ Homework.java

lists all lines in the file Homework.java that contain sequences of digits. That isn’t terribly useful;

lines with variable names x1 will be listed. OK, you want sequences of digits that do not

imme-diately follow letters:

grep [^A-Za-z][0-9]+ Homework.java

The set [^A-Za-z] denotes any characters that are not in the ranges A to Z and a to z. This works

much better, and it shows only lines that contain actual numbers.

The useDelimiter method of the Scanner class accepts a regular expression to describe

delim-iters—the blocks of text that separate words. As already mentioned, if you set the delimiter pattern to [^A-Za-z]+, a delimiter is a sequence of one or more characters that are not letters.

For more information on regular expressions, consult one of the many tutorials on the Internet by pointing your search engine to “regular expression tutorial”.

7.3

Command Line Arguments

Depending on the operating system and Java development environment used, there are different methods of starting a program—for example, by selecting “Run” in the compilation environment, by clicking on an icon, or by typing the name of the pro-gram at the prompt in a command shell window. The latter method is called “invok-ing the program from the command line”. When you use this method, you must of course type the name of the program, but you can also type in additional information

that the program can use. These additional strings are called command line

argu-ments. For example, if you start a pro gram with the command line

java ProgramClass -v input.dat

then the program receives two command line arguments: the strings "-v" and "input.

dat". It is entirely up to the program what to do with these strings. It is customary to

interpret strings starting with a hyphen (-) as program options.

Special Topic 7.4

VIDEO EXAMPLE 7.1 Computing a Document's Readability

In this Video Example, we develop a program that computes the Flesch Readability Index for a document.

Should you support command line arguments for your programs, or should you prompt users, per haps with a graphical user interface? For a casual and infrequent user, an interactive user interface is much better. The user interface guides the user along and makes it possible to navigate the application without much knowledge. But for a frequent user, a command line interface has a major advantage: it is easy to auto-mate. If you need to process hundreds of files every day, you could spend all your time typing file names into file chooser dialog boxes. However, by using batch files or shell scripts (a feature of your computer’s operating system), you can automatically call a program many times with different command line arguments.

Your program receives its command line arguments in the args parameter of the

main method:

public static void main(String[] args)

In our example, args is an array of length 2, containing the strings

args[0]: "-v"

args[1]: "input.dat"

Let us write a program that encrypts a file—that is,

scrambles it so that it is unreadable except to those who know the decryption method. Ignoring 2,000 years of progress in the field of encryption, we will use a method familiar to Julius Caesar, replacing A with a D, B with an E, and so on (see Figure 1).

The program takes the following command line arguments:

• An optional -d flag to indicate decryption instead of

encryption

• The input file name

• The output file name

For example,

java CaesarCipher input.txt encrypt.txt

encrypts the file input.txt and places the result into

encrypt.txt.

java CaesarCipher -d encrypt.txt output.txt

decrypts the file encrypt.txt and places the result into output.txt.

section_3/CaesarCipher.java

1 import java.io.File;

2 import java.io.FileNotFoundException;

3 import java.io.PrintWriter;

4 import java.util.Scanner;

Programs that start from the command line receive the command line arguments in the main method.

The emperor Julius Caesar used a simple scheme to encrypt messages.

Figure 1 Caesar Cipher

M e e t m e a t t h e

P h h w p h d w w k h

Plain text Encrypted text

CaesarCipher.java (1)

(34)

CaesarCipher.java (2)

If the switch is present, it is the first argument

CaesarCipher.java (3)

Process the input file one character at a time

Don’t forget the close the files!

(35)

Steps to Processing Te

xt Files

1) Understand the Processing Task

-- Process on the go or store data and then process? 2) Determine input and output files

3) Choose how you will get file names

4) Choose line, word or character based input processing -- If all data is on one line, normally use line input

5) With line-oriented input, extract required data

-- Examine the line and plan for whitespace, delimiters…

6) Use methods to factor out common tasks

Summary: Input/Output

q 

Use the

Scanner

class for reading text files.

q 

When writing text files, use the

PrintWriter

class

and the

print/println/printf

methods.

q 

Close all files when you are done processing them.

q 

Programs that start from the command line receive

command line arguments in the main method.

(36)

Summary: Processing Text Files

q 

The

by white space.

q 

The

Character

class has methods for classifying

characters.

q 

The

nextLine

method reads an entire line.

q 

If a string contains the digits of a number, you use

the

Integer.parseInt

or

Double.parseDouble

method to obtain the number value.

q 

Programs that start from the command line

receive the command line arguments in the

main

method.