Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Fall 2013 Chapter 9 Strings and Text I/O

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Objectives To use the String class to process fixed strings (§9.2). To use the Character class to process a single character (§9.3). To use the StringBuilder/StringBuffer class to process flexible strings (§9.4). To distinguish among the String, StringBuilder, and StringBuffer classes (§ ). To learn how to pass arguments to the main method from the command line (§9.5). To discover file properties and to delete and rename files using the File class (§9.6). To write data to a file using the PrintWriter class (§9.7.1). To read data from a file using the Scanner class (§9.7.2). (GUI) To open files using a dialog box (§9.8). 3

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The String Class Constructing a String: – String message = "Welcome to Java“; – String message = new String("Welcome to Java“); – 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Constructing Strings String newString = new String(stringLiteral); String message = new String("Welcome to Java"); Since strings are used frequently, Java provides a shorthand initializer for creating a string: String message = "Welcome to Java"; 5

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code String s = "Java"; s = "HTML"; 7 animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code String s = "Java"; s = "HTML"; 8 animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Examples display s1 == s2 is false s1 == s3 is true You are comparing the pointer values, not the actual values referenced by the variables 10 A new object is created if you use the new operator. If you use the string initializer, no new object is created if the interned object is already created.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code 11 animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code 12

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code 13

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Comparisons 14

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Comparisons Equals – Returns true if this string is equal to string s1. 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 } 15

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Comparisons, cont. compareTo(Object object) – Returns an integer greater than 0, equal to 0, or less than 0 to indicate whether this string is greater than, equal to, or less than s1. 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 16

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Length, Characters, and Combining Strings 17

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Finding String Length Finding string length using the length() method: Returns the number of characters in this string. message = "Welcome"; message.length() (returns 7 ) 18

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) – Returns the character at the specified index from this string. Index starts from 0 19

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Concatenation Concat(String) Returns a new string that concatenate this string with string s1. 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); 20

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Lecture 2 21

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Extracting Substrings 22

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Converting, Replacing, and Splitting Strings 24

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Splitting a String String[] tokens = "Java#HTML#Perl".split("#", 0); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " "); 26 Java HTML Perl displays

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Lecture 3 27

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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.F, “Regular Expressions,” for further studies. "Java".matches("Java"); "Java".equals("Java"); matches and equals in the above statements both evaluate to true, but matches is more powerful "Java is fun".matches("Java.*"); "Java is cool".matches("Java.*"); Both evaluate to true, because “Java.*” is a regular expression

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Regular Expressions Describe a set of strings based on common characteristics Used to search, edit, or manipulate text and data You must learn a specific syntax to create regular expressions Metacharacters have special meaning – ([{\^-$|]})?*+. There are two ways to force a metacharacter to be treated as an ordinary character: – precede the metacharacter with a backslash, or – enclose it within \Q (which starts the quote) and \E (which ends it). When using this technique, the \Q and \E can be placed at any location within the expression, provided that the \Q comes first. 29

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Regular Expressions Character Classes Regular ExpressionDescription [abc]a, b, or c (simple class) [^abc] Any character except a, b, or c (negation) [a-zA-Z] a through z, or A through Z, inclusive (range) [a-d[m-p]] a through d, or m through p: [a- dm-p] (union) [a-z&&[def]]d, e, or f (intersection) [a-z&&[^bc]] a through z, except for b and c: [ad-z] (subtraction) [a-z&&[^m-p]] a through z, and not m through p: [a-lq-z] (subtraction) 30

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Regular Expressions Predefined Character Classes. Any character (may or may not match line terminators) \dA digit: [0-9] \DA non-digit: [^0-9] \sA whitespace character: [ \t\n\x0B\f\r] \SA non-whitespace character: [^\s] \wA word character: [a-zA-Z_0-9] \WA non-word character: [^\w] 31

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Regular Expressions Quantifiers Meaning Greedy Reluctant Possessive X? X?? X?+ X, once or not at all X* X*? X*+ X, zero or more times X+ X+? X++ X, one or more times X{n} X{n}? X{n}+ X, exactly n times X{n,} X{n,}? X{n,}+ X, at least n times X{n,m} X{n,m}? X{n,m}+ X, at least n but not more than m times 32

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 33

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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]); 34

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Finding a Character or a Substring in a String 35

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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’. 37

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Finding Palindromes Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward. 38 CheckPalindrome

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Character Class 39

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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 40

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Lecture 4 41

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Counting Each Letter in a String This example gives a program that counts the number of occurrences of each letter in a string. Assume the letters are not case- sensitive. 42 CountEachLetter

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 43

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved StringBuilder Constructors 44

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Modifying Strings in the Builder 45

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 46

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The toString, capacity, length, setLength, and charAt Methods 47

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Checking Palindromes Ignoring Non-alphanumeric 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. 48 PalindromeIgnoreNonAlphanumeric

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Palindromes String s1 = input.nextLine(); //change s to s1 String s = s1.toLowerCase(); //create new String s that has s1 in lower case // Display result System.out.println("Ignoring non-alphanumeric characters, \nis " + s1 + " a palindrome? " + isPalindrome(s)); //change + s + to + s1 + to display original string mon dad noon moon Do geese see God Never odd or even Are we not drawn onward, we few, drawn onward to new era? 49

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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: 50

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Command-Line Parameters class TestMain { public static void main(String[] args) {... } java TestMain arg0 arg1 arg2... argn 51

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 52

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Set Command Line Input in Eclipse Run menu Run Configuration… Arguments tab Program arguments box Variables… button (bottom right of box) Scroll down to string_prompt OK button ${string_prompt} should appear in box OK button Close button When you run the program a window will appear titled Variable input 53

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Set Command Line Input in JCreator Configure menu Options… Java Tools in left menu box Run Application in Select Tool Type: drop-down menu at top Select Select Edit… button at right Parameters tab on top Check box Prompt for main method arguments OK button Apply button OK button When you run the program a window will appear titled Set main method arugments 54

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Calculator Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers. 55 Calculator java Calculator java Calculator java Calculator 2 / 3 java Calculator 2 “*” 3

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 56 Companion Website

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Matching Strings "Java".matches("Java"); "Java".equals("Java"); 57 "Java is fun".matches("Java.*") "Java is cool".matches("Java.*") "Java is powerful".matches("Java.*") Companion Website

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Regular Expression Syntax 58 Companion Website

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Replacing and Splitting Strings 59 Companion Website

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; 60 String s = "Java Java Java".replaceFirst("v\\w", "wi") ; String[] s = "Java1HTML2Perl".split("\\d"); Companion Website

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Lecture 4 61

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 62

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Obtaining file properties and manipulating file 63

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Explore File Properties 64 TestFileClass 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. Figure 16.1 shows a sample run of the program on Windows, and Figure 16.2 a sample run on Unix.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 65

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Writing Data Using PrintWriter 66 WriteData

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Reading Data Using Scanner 67 ReadData

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. 68 ReplaceText

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved (GUI) File Dialogs 69 ReadFileUsingJFileChooser

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Homework 9.7: Phone keypads Write a method that returns a number, given an uppercase letter, as follows: public static int getNumber(char uppercaseLetter) Write a test program that prompts the user to enter a phone number as a string. The input number may contain letters. The program translates a letter ( upper or lowercase) to a digit and leaves all other characters intact. Here is a sample run of the program: Enter a string: Flowers Enter a string: 1800flowers A, B, C = 2 D, E, F = 3 G, H, I = 4 J, K, L = 5 M, N, O = 6 P, Q, R, S = 7 T, U, V = 8 W, X, Y, Z = 9 70 Input a string Look at each character in string using charAt(i) If character is a letter, getNumber (character) and print Else print character. public static int getNumber (char uc) { if (uc == ‘a’ | uc = =‘b’ | uc == ‘c’) return 2; if (uc == ‘d’ | uc = =‘e’ | uc == ‘f’) return 3; … Maybe you might want to use switch statement?

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Homework 9.15: Finding the number of uppercase letters in a string Write a program that passes a string to the main method and displays the number of uppercase letters in a string. 71 // Check command-line arguments if (args.length != 1) { System.out.println( "Usage: java Exercise9_15 string"); System.exit(0); } Use the following methods: charAt(i) isUpperCase(charAt(i) or check to see if charAt(i) >=‘A’ and <= ‘Z’ Start with total = 0 Check each character, if it is an upper case letter add 1 to total.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Homework 9.19: Writing/ Reading data Write a program to create a file named Exercise9_ 19.txt if it does not exist. Write 100 integers created randomly into the file using text I/ O. Integers are separated by spaces in the file. Read the data back from the file and display the sorted data 72 1.Create File type object 2.Check to see if it exists a.If not, create a PrintWriter object for the File and write 100 random integers to the file. (int) (Math.random() * 100). Be sure to close the file after you have finished the 100 integers. 3.Create a Scanner to read from the file 4.Create an array to store the 100 integers when you read the file 5.Use a for loop to read the numbers from the file into the array 6.Sort the values in the array in ascending order 7.Display the array values on the monitor.