CC316: Object Oriented Programming

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1 CC316: Object Oriented Programming Lecture 5: Strings and Text I/O - Ch 9. Dr. Manal Helal, Fall

2 Motivations Often you encounter the problems that involve string processing and file input and output. Suppose you need to write a program to replace all occurrences of a word with a new word in a file. How do you solve this problem? This chapter introduces strings and text files, which will enable you to solve this problem. 2

3 Objectives To use the String class to process fixed strings ( 9.2). To construct strings ( 9.2.1). To understand that strings are immutable and to create an interned string ( 9.2.2). To compare strings ( 9.2.3). To get string length and characters, and combine strings ( 9.2.4). To obtain substrings ( 9.2.5). To convert, replace, and split strings ( 9.2.6). To match, replace, and split strings by patterns ( 9.2.7). To search for a character or substring in a string ( 9.2.8). To convert between a string and an array ( 9.2.9). To convert characters and numbers into a string ( ). To obtain a formatted string ( ). To check whether a string is a palindrome ( 9.3). To convert hexadecimal numbers to decimal numbers ( 9.4). To use the Character class to process a single character ( 9.5). To use the StringBuilder and StringBuffer classes to process flexible strings ( 9.6). To distinguish among the String, StringBuilder, and StringBuffer classes ( ). To learn how to pass arguments to the main method from the command line ( 9.7). To discover file/directory properties, to delete and rename files/directories, and to create directories using the File class ( 14.10). To write data to a file using the PrintWriter class ( ). To read data from a file using the Scanner class ( ). To understand how data is read using a Scanner ( ). To develop a program that replaces text in a file ( ). To open files using a file dialog box ( 14.12). To read data from the Web ( 14.13). 3

4 The String Class Constructing a String: o String message = "Welcome to Java ; o String message = new String("Welcome to Java ); o String s = new String(); Obtaining String length and Retrieving Individual Characters in a string String Concatenation (concat) Substrings (substring(index), substring(start, end)) Comparisons (equals, compareto) String Conversions Finding a Character or a Substring in a String Conversions between Strings and Arrays Converting Characters and Numeric Values to Strings 4

5 Constructing Strings String newstring = new String(stringLiteral); String message = new String("Welcome to Java"); Since strings are used frequently, Java provides a shorthand initialiser for creating a string: String message = "Welcome to Java"; 5

6 Strings Are Immutable A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML"; 6

7 Trace Code String s = "Java"; s = "HTML"; 7

8 Trace Code String s = "Java"; s = "HTML"; 8

9 Interned Strings Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. For example, the following statements: 9

10 Examples display s1 == s is false s1 == s3 is true A new object is created if you use the new operator. If you use the string initialiser, no new object is created if the interned object is already created. 10

11 Trace Code 11

12 Trace Code 12

13 Trace Code 13

14 String Comparisons 14

15 String Comparisons equals: String s1 = new String("Welcome ); String s2 = "welcome"; if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference }

16 String Comparisons, cont. compareto(object object) String s1 = new String("Welcome ); String s2 = "welcome"; if (s1.compareto(s2) > 0) { // s1 is greater than s2 } else if (s1.compareto(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2

17 String Length, Characters, and Combining Strings 17

18 Finding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7)

19 Retrieving Individual Characters in a String Do not use message[0] Use message.charat(index) Index starts from 0

20 String Concatenation String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 same as (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);

21 Extracting Substrings 21

22 Extracting Substrings You can extract a single character from a string using the charat method. You can also extract a substring from a string using the substring method in the String class. String s1 = "Welcome to Java"; String s2 = s1.substring(0, 11) + "HTML";

23 Converting, Replacing, and Splitting Strings 23

24 Examples "Welcome".toLowerCase() returns a new string, welcome. "Welcome".toUpperCase() returns a new string, WELCOME. " Welcome ".trim() returns a new string, Welcome. "Welcome".replace('e', 'A') returns a new string, WAlcomA. "Welcome".replaceFirst("e", "AB") returns a new string, WABlcome. "Welcome".replace("e", "AB") returns a new string, WABlcomAB. "Welcome".replace("el", "AB") returns a new string, WABcome.

25 Splitting a String String[] tokens = "Java#HTML#Perl".split("#", 0); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " "); displays Java HTML Perl 25

26 Matching, Replacing and Splitting by Patterns You can match, replace, or split a string by specifying a pattern. This is an extremely useful and powerful feature, commonly known as regular expression. Regular expression is complex to beginning students. For this reason, two simple patterns are used in this section. Please refer to Supplement III.H, Regular Expressions, for further studies. "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("java.*"); "Java is cool".matches("java.*");

27 Matching, Replacing and Splitting by Patterns The replaceall, replacefirst, and split methods can be used with a regular expression. For example, the following statement returns a new string that replaces $, +, or # in "a+b$#c" by the string NNN. String s = "a+b$#c".replaceall("[$+#]", "NNN"); System.out.println(s); Here the regular expression [$+#] specifies a pattern that matches $, +, or #. So, the output is annnbnnnnnnc.

28 Matching, Replacing and Splitting by Patterns The following statement splits the string into an array of strings delimited by some punctuation marks. String[] tokens = "Java,C?C#,C++".split("[.,:;?]"); for (int i = 0; i < tokens.length; i++) System.out.println(tokens[i]);

29 Finding a Character or a Substring in a String 29

30 Finding a Character or a Substring in a String "Welcome to Java".indexOf('W') returns 0. "Welcome to Java".indexOf('x') returns -1. "Welcome to Java".indexOf('o', 5) returns 9. "Welcome to Java".indexOf("come") returns 3. "Welcome to Java".indexOf("Java", 5) returns 11. "Welcome to Java".indexOf("java", 5) returns -1. "Welcome to Java".lastIndexOf('a') returns 14.

31 Convert Character and Numbers to Strings The String class provides several static valueof methods for converting a character, an array of characters, and numeric values to strings. These methods have the same name valueof with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a string, use String.valueOf(5.44). The return value is string consists of characters 5,., 4, and 4.

32 Problem: Finding Palindromes Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward. CheckPalindrome

33 The Character Class 33

34 Examples Character charobject = new Character('b'); charobject.compareto(new Character('a')) returns 1 charobject.compareto(new Character('b')) returns 0 charobject.compareto(new Character('c')) returns -1 charobject.compareto(new Character('d') returns 2 charobject.equals(new Character('b')) returns true charobject.equals(new Character('d')) returns false

35 Problem: Counting Each Letter in a String This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive. CountEachLetter

36 StringBuilder and StringBuffer The StringBuilder/StringBuffer class is an alternative to the String class. In general, a StringBuilder/ StringBuffer can be used wherever a string is used. StringBuilder/StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer, whereas the value of a String object is fixed once the string is created. String StringBuffer StringBuilder Storage Area Constant String Pool Heap Heap Modifiable No (immutable) Yes( mutable ) Yes( mutable ) Thread Safe Yes Yes No Performance Fast Very slow Fast

37 StringBuilder Constructors 37

38 Modifying Strings in the Builder 38

39 Examples stringbuilder.append("java"); stringbuilder.insert(11, "HTML and "); stringbuilder.delete(8, 11) changes the builder to Welcome Java. stringbuilder.deletecharat(8) changes the builder to Welcome o Java. stringbuilder.reverse() changes the builder to avaj ot emoclew. stringbuilder.replace(11, 15, "HTML") changes the builder to Welcome to HTML. stringbuilder.setcharat(0, 'w') sets the builder to welcome to Java.

40 The tostring, capacity, length, setlength, and charat Methods 40

41 Problem: Checking Palindromes Ignoring Nonalphanumeric Characters This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive. PalindromeIgnoreNonAlphanumeric

42 Main Method Is Just a Regular Method You can call a regular method by passing actual parameters. Can you pass arguments to main? Of course, yes. For example, the main method in class B is invoked by a method in A, as shown below:

43 Command-Line Parameters class TestMain { public static void main(string[] args) {... } } java TestMain arg0 arg1 arg2... argn

44 Processing Command-Line Parameters In the main method, get the arguments from args[0], args[1],..., args[n], which corresponds to arg0, arg1,..., argn in the command line.

45 Problem: Calculator Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers. java Calculator "2 + 3" Calculator java Calculator "2-3" java Calculator "2 / 3" java Calculator "2 * 3"

46 Regular Expressions A regular expression (abbreviated regex) is a string that describes a pattern for matching a set of strings. Regular expression is a powerful tool for string manipulations. You can use regular expressions for matching, replacing, and splitting strings.

47 Matching Strings "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("java.*") "Java is cool".matches("java.*") "Java is powerful".matches("java.*")

48 48

49 Replacing and Splitting Strings 49

50 Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; String s = "Java Java Java".replaceFirst("v\\w", "wi") ; String[] s = "Java1HTML2Perl".split("\\d");

51 The File Class The File class is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine-independent fashion. The filename is a string. The File class is a wrapper class for the file name and its directory path.

52 52 OOP in in Java Java :: W. W. Milner 2005 ::

53 Problem: Explore File Properties Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties. The following figures show a sample run of the program on Windows and on Unix. 53 TestFileClass OOP in in Java Java :: W. W. Milner 2005 ::

54 File Class Example java.io.file file = new java.io.file("image/us.gif"); System.out.println("Does it exist? " + file.exists()); System.out.println("The file has " + file.length() + " bytes"); System.out.println("Can it be read? " + file.canread()); System.out.println("Can it be written? " + file.canwrite()); System.out.println("Is it a directory? " + file.isdirectory()); System.out.println("Is it a file? " + file.isfile()); System.out.println("Is it absolute? " + file.isabsolute()); System.out.println("Is it hidden? " + file.ishidden()); System.out.println("Absolute path is " + file.getabsolutepath()); System.out.println("Last modified on " + new java.util.date(file.lastmodified())); OOP in in Java Java :: W. W. Milner 2005 ::

55 Text I/O A File object encapsulates the properties of a file or a path, but does not contain the methods for reading/writing data from/to a file. In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/writing data from/to a file. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes.

56 Writing Data Using PrintWriter 56 WriteData OOP in in Java Java :: W. W. Milner 2005 ::

57 Write Data java.io.file file = new java.io.file("scores.txt"); if (file.exists()) { System.out.println("File already exists"); System.exit(0); } // Create a file java.io.printwriter output = new java.io.printwriter(file); // Write formatted output to the file output.print("john T Smith "); output.println(90); output.print("eric K Jones "); output.println(85); // Close the file output.close(); OOP in in Java Java :: W. W. Milner 2005 ::

58 Reading Data Using Scanner 58 ReadData OOP in in Java Java :: W. W. Milner 2005 ::

59 Read Data // Create a File instance java.io.file file = new java.io.file("scores.txt"); // Create a Scanner for the file Scanner input = new Scanner(file); // Read data from a file while (input.hasnext()) { String firstname = input.next(); String mi = input.next(); String lastname = input.next(); int score = input.nextint(); System.out.println( firstname + " " + mi + " " + lastname + " " + score); } // Close the file input.close(); OOP in in Java Java :: W. W. Milner 2005 ::

60 Problem: Replacing Text Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows: java ReplaceText sourcefile targetfile oldstring newstring For example, invoking java ReplaceText FormatString.java t.txt StringBuilder StringBuffer replaces all the occurrences of StringBuilder by StringBuffer in FormatString.java and saves the new file in t.txt. ReplaceText

61 Reading Data from the Web Just like you can read data from a file on your computer, you can read data from a file on the Web. 61 OOP in in Java Java :: W. W. Milner 2005 ::

62 Reading Data from the Web URL url = new URL(" After a URL object is created, you can use the openstream() method defined in the URL class to open an input stream and use this stream to create a Scanner object as follows: Scanner input = new Scanner(url.openStream());

63 Case Study: Web Crawler This case study develops a program that travels the Web by following hyperlinks.

64 Case Study: Web Crawler The program follows the URLs to traverse the Web. To avoid that each URL is traversed only once, the program maintains two lists of URLs. One list stores the URLs pending for traversing and the other stores the URLs that have already been traversed. The algorithm for this program can be described as follows:

65 Case Study: Web Crawler Add the starting URL to a list named listofpendingurls; while listofpendingurls is not empty { Remove a URL from listofpendingurls; if this URL is not in listoftraversedurls { Add it to listoftraversedurls; Display this URL; Exit the while loop when the size of S is equal to 100. Read the page from this URL and for each URL contained in the page { Add it to listofpendingurls if it is not is listoftraversedurls; } } }

$showopendialog(null) == JFileChooser.APPROVE_OPTION) { java.io.file file = filechooser.$

66 (GUI) File Dialogs ReadFileUsingJFileChooser JFileChooser filechooser = new JFileChooser(); if (filechooser.showopendialog(null) == JFileChooser.APPROVE_OPTION) { java.io.file file = filechooser.getselectedfile(); } 66 OOP in in Java Java :: W. W. Milner 2005 ::

67 Assignment ass5 **9.5 (Occurrences of each digit in a string) Write a method that counts the occurrences of each digit in a string using the following header: public static int[] count(string s) The method counts how many times a digit appears in the string. The return value is an array of ten elements, each of which holds the count for a digit. For example, after executing int[] counts = count("12203ab3"), counts[0] is 1, counts[1] is 1, counts[2] is 2, and counts[3] is 2. Write a test program that prompts the user to enter a string and displays the number of occurrences of each digit in the string.

Chapter 12 Strings and Characters. Dr. Hikmat Jaber

Chapter 12 Strings and Characters Dr. Hikmat Jaber 1 The String Class Constructing a String: String message = "Welcome to Java ; String message = new String("Welcome to Java ); String s = new String();