Bài giảng Chapter 9: Characters and Strings

Chapter 9Characters and Strings©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Chapter 9 ObjectivesAfter you have read and studied this chapter, you should be able toDeclare and manipulate data of the char data type.Write string processing programs, using String and StringBuffer objects.Differentiate the String and StringBuffer classes and use the correct class in solving a given task.Tell the difference between equality and equivalence testings for String objects.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.1 CharactersIn Java, single characters are represented using the data type char.Character constants are written as symbols enclosed in single quotes:char ch1 = ‘X’;©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.1 CharactersASCII stands for American Standard Code for Information Interchange. ASCII is one of the document coding schemes widely used today. This coding scheme allows different computers to share information easily.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.1 CharactersASCII works well for English-language documents because all characters and punctuation marks are included in the ASCII codes.ASCII does not represent the full character sets of other languages. ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.1 CharactersThe 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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.2 StringsA 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 access individual characters of a string by calling the charAt method of the String object.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.2 StringsEach character in a string has an index we use to access the character. Java uses zero-based indexing; the first character’s index is 0, the second is 1, and so on. To refer to the first character of the word name, we sayname.charAt(0).©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 9.1An indexed expression is used to refer to individual characters in a string.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.2 StringsSince String is a class, we can create an instance of a class by using the new method.The statements we have used so far, such asString name1 = “Kona”;works as a shorthand for String name1 = new String(“Kona”);But this shorthand works for the String class only.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.2 StringsString comparison may be done in several ways. The methods equals and equalsIgnoreCase compare string values; one is case-sensitive and one is not.The method compareTo returns a value:Zero (0) if the strings are equal. A negative integer if the first string is less than the second.A positive integer if the first string is greater than the second.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionPattern matching is a common function in many applications. In Java 2 SDK 1.4, two new classes, Pattern and Matcher, are added. The String class also includes several new methods that support pattern matching.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe matches method from the String class is similar to the equals method. However, unlike equals, the argument to matches can be a pattern.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionSuppose that all new 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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe valid code pattern for computer science majors living on-campus:©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe pattern is called a regular expression that allows us to denote a large set of “words” (any sequence of symbols) succinctly.Brackets [] represent choices, so [abc] means a, b, or c.For example, the definition for a valid Java identifier may be stated as[a-zA-Z][a-zA-Z0-9_$]*©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionExpressionDescription[013]A single digit 0, 1, or 3.[0-9][0-9]Any two-digit number from 00 to 99.A[0-4]b[05]A string that consists of four characters. The first character is A. The second character is a number between 0 and 4, inclusive. The third character is b. The last character is either 0 or 5.[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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionExpressionDescriptionX{N}Repeat X exactly N times, where X is a regular expression for a single character.X{N,}Repeat X at least N times.X{N,M}Repeat X at least N but no more than M times.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe period symbol (.) is used to match any character except a line terminator (\n or \r).String document;document = ...; //assign text to ‘document’if (document.matches(“.*zen of objects.*”){ System.out.println(“Found”); }else{ System.out.println(“Not found”);}©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionBrackets ([ ]) are used for expressing a range of choices for a given character. To express a range of choices for multiple characters, use parentheses and the vertical bar.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionExpressionDescription[wb](ad|eed)Matches wad, weed, bad, and beed.(pro|anti)-OOPMatches pro-OOP and anti-OOP(AZ|CA|CO)[0-9]{4}Matches AZxxxx,CAxxxx, and COxxxx, where x is a single digit.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe replaceAll method is new to the Version 1.4 String class. This method allows us to replace all occurrences of a substring that matches a given regular expression with a given replacement string.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionFor example, to replace all vowels in a string with the @ symbol:String originalText, modifiedText;originalText = ...; //assign string to ‘originalText’modifiedText = originalText.replaceAll(“[aeiou]”,”@”);Note that this method does not change the original text; it simply returns a modified text as a separate string.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionTo match a whole word, use the \b symbol to designate the word boundary.str.replaceAll(\\btemp\\b, “temporary”);Two backslashes are necessary because we must write the expression in a String representation. Two backslashes prevents the system from interpreting the regular expression backslash as a control character.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.3 Pattern Matching and Regular ExpressionThe backslash is also used to search for a command character. For example:To search for the plus symbol (+) in text, we use the backslash as \+.To express it as a string, we write “\\+”.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesThe matches and replaceAll methods of the String class are shorthand for using the Pattern and Matcher classes from the java.util.regex package.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesIf str and regex are String objects, then bothstr.matches(regex);andPattern.matches(regex, str);are equivalent to Pattern pattern = Pattern.compile(regex);Matcher matcher = p.matcher(str);matcher.matches();©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesCreating Pattern and Matcher objects gives us more options and efficiency.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 or Pattern class is executed.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher Classes/*Chapter 9 Sample Program: Checks whether the input string is a valid identifier. This version uses the Matcher and Pattern classes.File: Ch9MatchJavaIdentifier2.java*/import javax.swing.*;import java.util.regex.*;class Ch9MatchJavaIdentifier2 { private static final String STOP = STOP"; private static final String VALID ="Valid Java identifier"; private static final String INVALID ="Not a valid Java identifier";©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher Classesprivate static final String VALID_IDENTIFIER_PATTERN = "[a-zA-Z][a-zA-Z0-9_$]*";public static void main (String[] args) { String str, reply; Matcher matcher; Pattern pattern =Pattern.compile(VALID_IDENTIFIER_PATTERN); while (true) { str = JOptionPane.showInputDialog(null, "Identifier:"); if (str.equals(STOP)) break; ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher Classes matcher = pattern.matcher(str); if (matcher.matches()) { reply = VALID; } else { reply = INVALID; } JOptionPane.showMessageDialog(null, str + ":\n" + reply); } }}©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesThe find method is another powerful method of the Matcher class.The method searches for the next sequence in a string that matches the pattern, and returns true if the pattern is found.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesWhen a matcher finds a matching sequence of characters, we can query the location of the sequence by using the start and end methods.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.4 The Pattern and Matcher ClassesThe 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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.5 Comparing StringsComparing String objects is similar to comparing other objects.The equality test (==) is true if the contents of the variables are the same.For a reference data type, the equality test is true if both variables refer to the same object, because they both contain the same address.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.5 Comparing StringsThe equals method is true if the String objects to which the two variables refer contain the same string value.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 9.2AThe difference between the equality test and the equals method.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 9.2B and C©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.5 Comparing StringsAs long as a new String object is created using the new operator, the rule for comparing objects applies to comparing strings.String str = new String (“Java”);If the new operator is not used, string data are treated as if they are of the primitive data type.String str = “Java”;©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 9.3The difference between using and not using the new operator for String.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.6 StringBufferWhen a String object is created, it cannot be changed.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.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.6 StringBufferFor example:StringBuffer word = new StringBuffer(“Java”);word.setCharAt(0, ‘D’);word.setCharAt(1, ‘i’ );changes the string from “Java” to “Diva.”©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.6 StringBufferThe following example reads a sentence and replaces all vowels in the sentence with the character X. /*Chapter 9 Sample Program: Replace every vowel in a given sentence with 'X‘using StringBuffer.File: Ch9ReplaceVowelsWithX.java*/import javax.swing.*;class Ch9ReplaceVowelsWithX { public static void main (String[] args) { ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.6 StringBufferStringBuffer tempStringBuffer;String inSentence;int numberOfCharacters;char letter;inSentence = JOptionPane.showInputDialog(null, "Enter a sentence:"); tempStringBuffer= new StringBuffer(inSentence);numberOfCharacters = tempStringBuffer.length();for (int index = 0; index .insert(, );where must be greater than or equal to 0 and less than or equal to the length of , and the is an object or a value of the primitive data type.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.9.6 StringBufferFor example, executingStringBuffer str = new StringBuffer(“Java is great”);str.insert(8, “really”);changes the stringJava is greatto Java is really great.©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.