1. Intro to OOP with Java, C. Thomas Wu Characters and Strings
Chapter 9
Characters and Strings
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2. Intro to OOP with Java, C. Thomas Wu
Objectives
Declare and manipulate data of the char data type.
Write string processing program using String, StringBuilder, and StringBuffer objects.
Differentiate the three string classes and use the correct class for a given task.
Specify regular expressions for searching a pattern in a string.
Use the Pattern and Matcher classes.
Compare the String objects correctly.
We will cover the basic string processing in this lesson, manipulating char and String data.
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3. Intro to OOP with Java, C. Thomas Wu
Characters
In Java, single characters are represented using the data type char.
Character constants are written as symbols enclosed in single quotes.
Characters are stored in a computer memory using some form of encoding.
ASCII, which stands for American Standard Code for Information Interchange, is one of the document coding schemes widely used today.
Java uses Unicode, which includes ASCII, for representing char constants.
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4. Intro to OOP with Java, C. Thomas Wu
ASCII Encoding
For example, character 'O' is 79 (row value 70 + col value 9 = 79). O 9 70
Characters can be stored in a computer memory using the ASCII encoding. The ASCII codes range from 0 to 127. The character 'A' is represented as 65, for example. The ASCII values from 0 to 32 are called nonprintable control characters. For example, ASCII code 04 eot stands for End of Transmission. We can use this character to signal the end of transmission of data when sending data over a communication line.
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5. Intro to OOP with Java, C. Thomas Wu
Unicode Encoding
The Unicode Worldwide Character Standard (Unicode) supports the interchange, processing, and display of the written texts of diverse languages.
Java uses the Unicode standard for representing char constants.
char ch1 = 'X';
System.out.println(ch1);
System.out.println( (int) ch1);
ASCII works well for English-language documents because all characters and punctuation marks are included in the ASCII codes. But ASCII codes cannot be used to represent character sets of other languages. To overcome this limitation, unicode encoding was proposed. Unicode uses two bytes to represent characters and adopts the same encoding for the first 127 values as the ASCII codes.
Encoding value of a character can be accessed by converting it to an int as the sample code illustrates. X 88
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6. Intro to OOP with Java, C. Thomas Wu
Character Processing
Declaration and initialization
char ch1, ch2 = ‘X’;
Type conversion between int and char.
System.out.print("ASCII code of character X is " + (int) 'X' );
System.out.print("Character with ASCII code 88 is " + (char)88 );
This comparison returns true because ASCII value of 'A' is 65 while that of 'c' is 99.
‘A’ < ‘c’
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7. Intro to OOP with Java, C. Thomas Wu
Strings
A string is a sequence of characters that is treated as a single value.
Instances of the String class are used to represent strings in Java.
We can access individual characters of a string by calling the charAt method of the String object.
We introduced the String class in Chapter 2. We will study additional String methods.
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8. Accessing Individual Elements
Intro to OOP with Java, C. Thomas Wu
Accessing Individual Elements
Individual characters in a String accessed with the charAt method.
String name = "Sumatra"; 1 2 3 4 5 6 S u m a t r
name
This variable refers to the whole string.
name.charAt( 3 )
The method returns the character at position # 3.
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9. Example: Counting Vowels
Intro to OOP with Java, C. Thomas Wu
Example: Counting Vowels
char letter;
String name = JOptionPane.showInputDialog(null,"Your name:");
int numberOfCharacters = name.length();
int vowelCount = 0;
for (int i = 0; i < numberOfCharacters; i++) {
letter = name.charAt(i);
if ( letter == 'a' || letter == 'A' ||
letter == 'e' || letter == 'E' ||
letter == 'i' || letter == 'I' ||
letter == 'o' || letter == 'O' ||
letter == 'u' || letter == 'U' ) {
vowelCount++; }
System.out.print(name + ", your name has " + vowelCount + " vowels");
Here’s the code to count the number of vowels in the input string.
This sample code counts the number of vowels in a given input.
Using the toUpperCase method, that converts all alphabetic characters to uppercase, we can rewrite the code as
char letter;
String name = inputBox.getString("What is your name?");
int numberOfCharacters = name.length();
int vowelCount = 0;
String nameUpper = name.toUpperCase();
for (int i = 0; i < numberOfCharacters; i++) {
letter = nameUpper.charAt(i);
if ( letter == 'A' ||
letter == 'E' ||
letter == 'I' ||
letter == 'O' ||
letter == 'U' ) {
vowelCount++; }
System.out.print(name + ", your name has " + vowelCount + " vowels");
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10. Example: Counting ‘Java’
Intro to OOP with Java, C. Thomas Wu
Example: Counting ‘Java’
int javaCount = 0;
boolean repeat = true;
String word;
while ( repeat ) {
word = JOptionPane.showInputDialog(null,"Next word:");
if ( word.equals("STOP") ) {
repeat = false;
} else if ( word.equalsIgnoreCase("Java") ) {
javaCount++; }
Continue reading words and count how many times the word Java occurs in the input, ignoring the case.
Notice how the comparison is done. We are not using the == operator.
This sample code counts the number of times the word 'Java' is entered.
Notice that we wrote
word.equals( “STOP”)
not
word == “STOP”
We described the differences in Lesson 5-2. In this situation, we need to use 'equals' because we are testing whether two String objects have the same sequence of characters.
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11. Other Useful String Operators
Intro to OOP with Java, C. Thomas Wu
Other Useful String Operators
Method
Meaning
compareTo
Compares the two strings. str1.compareTo( str2 )
substring
Extracts the a substring from a string. str1.substring( 1, 4 )
trim
Removes the leading and trailing spaces. str1.trim( )
valueOf
Converts a given primitive data value to a string. String.valueOf( )
startsWith
Returns true if a string starts with a specified prefix string. str1.startsWith( str2 )
endsWith
Returns true if a string ends with a specified suffix string. str1.endsWith( str2 )
Here are some examples:
String str1 = “Java”, str2 = “ Wow “;
str1.compareTo( “Hello” ); //returns positive integer
//because str1 >= “Hello”
str1.substring( 1, 4 ); //returns “ava”
str2.trim( ) //returns “Wow”, str2 stays same
str1.startsWith( “Ja” ); //returns true
str1.endsWith( “avi” ); //returns false
See the String class documentation for details.
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12. Intro to OOP with Java, C. Thomas Wu
Pattern Example
Suppose students are assigned a three-digit code:
The first digit represents the major (5 indicates computer science);
The second digit represents either in-state (1), out-of-state (2), or foreign (3);
The third digit indicates campus housing:
On-campus dorms are numbered 1-7.
Students living off-campus are represented by the digit 8.
The 3-digit pattern to represent computer science majors living on-campus is
We can use a pattern to represent a range of valid codes succintly. Without using the sample 3-digit pattern for computer science majors living on-campus, we must spell out 21 separate codes.
5[123][1-7]
first
character
is 5
second
character
is 1, 2, or 3
third
character
is any digit
between 1 and 7
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13. Intro to OOP with Java, C. Thomas Wu
Regular Expressions
The pattern is called a regular expression.
Rules
The brackets [ ] represent choices
The asterisk symbol * means zero or more occurrences.
The plus symbol + means one or more occurrences.
The hat symbol ^ means negation.
The hyphen – means ranges.
The parentheses ( ) and the vertical bar | mean a range of choices for multiple characters.
Regular expression allows us to express a large set of “words” (any sequence of symbols) succinctly. We use specially designated symbols such as the asterisk to formulate regular expressions.
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14. Regular Expression Examples
Intro to OOP with Java, C. Thomas Wu
Regular Expression Examples
Expression
Description
[013]
A single digit 0, 1, or 3.
[0-9][0-9]
Any two-digit number from 00 to 99.
[0-9&&[^4567]]
A single digit that is 0, 1, 2, 3, 8, or 9.
[a-z0-9]
A single character that is either a lowercase letter or a digit.
[a-zA-z][a-zA-Z0-9_$]*
A valid Java identifier consisting of alphanumeric characters, underscores, and dollar signs, with the first character being an alphabet.
[wb](ad|eed)
Matches wad, weed, bad, and beed.
(AZ|CA|CO)[0-9][0-9]
Matches AZxx,CAxx, and COxx, where x is a single digit.
Here are some examples of regular expressions.
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15. Intro to OOP with Java, C. Thomas Wu
The replaceAll Method
The replaceAll method replaces all occurrences of a substring that matches a given regular expression with a given replacement string.
Replace all vowels with the
String originalText, modifiedText;
originalText = ...; //assign string
modifiedText =
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16. The Pattern and Matcher Classes
Intro to OOP with Java, C. Thomas Wu
The Pattern and Matcher Classes
The matches and replaceAll methods of the String class are shorthand for using the Pattern and Matcher classes from the java.util.regex package.
If str and regex are String objects, then
str.matches(regex); // matches all str
is equivalent to
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(str);
matcher.matches(); // boolean
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17. Intro to OOP with Java, C. Thomas Wu
The compile Method
The compile method of the Pattern class converts the stated regular expression to an internal format to carry out the pattern-matching operation.
This conversion is carried out every time the matches method of the String class is executed, so it is more efficient to use the compile method when we search for the same pattern multiple times.
See the sample programs Ch9MatchJavaIdentifier2 and Ch9PMCountJava
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18. Intro to OOP with Java, C. Thomas Wu
The find Method
The find method is another powerful method of the Matcher class.
It searches for the next sequence in a string that matches the pattern, and returns true if the pattern is found.
When a matcher finds a matching sequence of characters, we can query the location of the sequence by using the start and end methods.
Ex: Find all substrings of the form a?z where ? is b or c …or y.
Pattern p = Pattern.compile(“a[b-y]z”);
Matcher m = p.matcher(str); // str is the target string
while(m.find() ) {
out.printf(“[%d,%d) ->%s”, m.start(), m.end(), m.group ) }
See Ch9PMCountJava2 for further exmple
The start method returns the position in the string where the first character of the pattern is found.
The end method returns the value 1 more than the position in the string where the last character of the pattern is found.
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19. The String Class is Immutable
Intro to OOP with Java, C. Thomas Wu
The String Class is Immutable
In Java a String object is immutable
This means once a String object is created, it cannot be changed, such as replacing a character with another character or removing a character
The String methods we have used so far do not change the original string. They created a new string from the original. For example, substring creates a new string from a given string.
The String class is defined in this manner for efficiency reason.
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20. Effect of Immutability
Intro to OOP with Java, C. Thomas Wu
Effect of Immutability
We can do this
because String
objects are
immutable.
By making String objects immutable, we can treat it much like a primitive data type for efficient processing. If we use the new operator to create a String object, a separate object is created as the top diagram illustrates. If we use a simple assignment as in the bottom diagram, then all literal constants refer to the same object. We can do this because String objects are immutable.
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21. The StringBuffer Class
Intro to OOP with Java, C. Thomas Wu
The StringBuffer Class
In many string processing applications, we would like to change the contents of a string. In other words, we want it to be mutable.
Manipulating the content of a string, such as replacing a character, appending a string with another string, deleting a portion of a string, and so on, may be accomplished by using the StringBuffer class.
If the situation calls for directing changing the contents of a string, then we use the StringBuffer class.
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22. Intro to OOP with Java, C. Thomas Wu
StringBuffer Example
word
: StringBuffer
Java
Before
word
: StringBuffer
Diva
After
Changing a string Java to Diva
This sample code illustrates how the original string is changed. Notice that the String class does not include the setCharAt method. The method is only defined in the mutable StringBuffer class.
StringBuffer word = new StringBuffer("Java");
word.setCharAt(0, 'D');
word.setCharAt(1, 'i');
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23. Intro to OOP with Java, C. Thomas Wu
Sample Processing
Replace all vowels in the sentence with ‘X’.
char letter;
String inSentence = JOptionPane.showInputDialog(null, "Sentence:");
StringBuffer tempStringBuffer = new StringBuffer(inSentence);
int numberOfCharacters = tempStringBuffer.length();
for (int index = 0; index < numberOfCharacters; index++) {
letter = tempStringBuffer.charAt(index);
if ( letter == 'a' || letter == 'A' || letter == 'e' || letter == 'E' ||
letter == 'i' || letter == 'I' || letter == 'o' || letter == 'O' ||
letter == 'u' || letter == 'U' ) {
tempStringBuffer.setCharAt(index,'X'); }
JOptionPane.showMessageDialog(null, tempStringBuffer );
Notice how the input routine is done. We are reading in a String object and converting it to a StringBuffer object, because we cannot simply assign a String object to a StringBuffer variable.
For example, the following code is invalid:
StringBuffer strBuffer = inputBox.getString( );
We are required to create a StringBuffer object from a String object as in
String str = "Hello";
StringBuffer strBuf = new StringBuffer( str );
You cannot input StringBuffer objects. You have to input String objects and convert them to StringBuffer objects.
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24. The append and insert Methods
Intro to OOP with Java, C. Thomas Wu
The append and insert Methods
We use the append method to append a String or StringBuffer object to the end of a StringBuffer object.
The method can also take an argument of the primitive data type.
Any primitive data type argument is converted to a string before it is appended to a StringBuffer object.
We can insert a string at a specified position by using the insert method.
The append and insert are the two very useful methods of the StringBuffer class for string manipulation.
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25. The StringBuilder Class
Intro to OOP with Java, C. Thomas Wu
The StringBuilder Class
This class is new to Java 5.0 (SDK 1.5)
The class is added to the newest version of Java to improve the performance of the StringBuffer class.
StringBuffer and StringBuilder support exactly the same set of methods, so they are interchangeable.
There are advanced cases where we must use StringBuffer, but all sample applications in the book, StringBuilder can be used.
Since the performance is not our main concern and that the StringBuffer class is usable for all versions of Java, we will use StringBuffer only in this book.
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26. Intro to OOP with Java, C. Thomas Wu
Problem Statement
Problem statement:
Write an application that will build a word concordance of a document. The output from the application is an alphabetical list of all words in the given document and the number of times they occur in the document. The documents are a text file (contents of the file are an ASCII characters) and the output of the program is saved as an ASCII file also.
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27. Intro to OOP with Java, C. Thomas Wu
Overall Plan
Tasks expressed in pseudocode:
while ( the user wants to process
another file ) {
Task 1: read the file;
Task 2: build the word list;
Task 3: save the word list to a file; }
For this application, we are given two helper classes. The FileManager class handles the file input and output. The WordList class handles the maintenance of word lists. Our responsibility is to extract the words from a given document and use the helper classes correctly.
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28. Intro to OOP with Java, C. Thomas Wu
Design Document
Class
Purpose
Ch9WordConcordanceMain
The instantiable main class of the program that implements the top-level program control.
Ch9WordConcordance
The key class of the program. An instance of this class managers other objects to build the word list.
FileManager
A helper class for opening a file and saving the result to a file. Details of this class can be found in Chapter 12.
WordList
Another helper class for maintaining a word list. Details of this class can be found in Chapter 10.
Pattern/Matcher
Classes for pattern matching operations.
There will be a total of six key classes in this application. We will be designing two classes: Ch9WordConcordanceMain and Ch9WordConcordance.
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29. Intro to OOP with Java, C. Thomas Wu
Class Relationships
FileManger
WordList
Ch9Word
ConcordanceMain
(main class)
Ch9Word
Concordance
Pattern
Matcher
class we implement
helper class given to us
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30. Intro to OOP with Java, C. Thomas Wu
Development Steps
We will develop this program in four steps:
Start with a program skeleton. Define the main class with data members. Begin with a rudimentary Ch9WordConcordance class.
Add code to open a file and save the result. Extend the existing classes as necessary.
Complete the implemention of the Ch9WordConcordance class.
Finalize the code by removing temporary statements and tying up loose ends.
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31. Intro to OOP with Java, C. Thomas Wu
Step 1 Design
Define the skeleton main class
Define the skeleton Ch9WordConcordance class that has only an empty zero-argument constructor
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32. Intro to OOP with Java, C. Thomas Wu
Step 1 Code
Directory: Chapter9/Step1
Source Files: Ch9WordConcordanceMain.java
Ch9WordConcordance.java
Program source file is too big to list here. From now on, we ask
you to view the source files using your Java IDE.
Please use your Java IDE to view the source files and run the program.
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33. Intro to OOP with Java, C. Thomas Wu
Step 1 Test
The purpose of Step 1 testing is to verify that the constructor is executed correctly and the repetition control in the start method works as expected.
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34. Intro to OOP with Java, C. Thomas Wu
Step 2 Design
Design and implement the code to open and save a file
The actual tasks are done by the FileManager class, so our objective in this step is to find out the correct usage of the FileManager helper class.
The FileManager class has two key methods: openFile and saveFile.
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35. Intro to OOP with Java, C. Thomas Wu
Step 2 Code
Directory: Chapter9/Step2
Source Files: Ch9WordConcordanceMain.java
Ch9WordConcordance.java
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36. Intro to OOP with Java, C. Thomas Wu
Step 2 Test
The Step2 directory contains several sample input files. We will open them and verify the file contents are read correctly by checking the temporary echo print output to System.out.
To verify the output routine, we save to the output (the temporary output created by the build method of Ch9WordConcordance) and verify its content.
Since the output is a textfile, we can use any word processor or text editor to view its contents.
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37. Intro to OOP with Java, C. Thomas Wu
Step 3 Design
Complete the build method of Ch9WordConcordance class.
We will use the second helper class WordList here, so we need to find out the details of this helper class.
The key method of the WordList class is the add method that inserts a given word into a word list.
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38. Intro to OOP with Java, C. Thomas Wu
Step 3 Code
Directory: Chapter9/Step3
Source Files: Ch9WordConcordanceMain.java
Ch9WordConcordance.java
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39. Intro to OOP with Java, C. Thomas Wu
Step 3 Test
We run the program against varying types of input textfiles.
We can use a long document such as the term paper for the last term’s economy class (don’t forget to save it as a textfile before testing).
We should also use some specially created files for testing purposes. One file may contain one word repeated 7 times, for example. Another file may contain no words at all.
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40. Intro to OOP with Java, C. Thomas Wu
Step 4: Finalize
Possible Extensions
One is an integrated user interface where the end user can view both the input document files and the output word list files.
Another is the generation of different types of list. In the sample development, we count the number of occurences of each word. Instead, we can generate a list of positions where each word appears in the document.
For the second extension, the WordList class itself needs to be modified. Details on how to implement this extension can be found in Chapter 10 of the textbook.
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