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

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

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. A string is a sequence of characters. Strings are treated as an array of characters, but in Java string is an object. The String class has 11 constructors and moreThe String class has 11 constructors and more than 40 methods than 40 methods for manipulating strings.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The String Class F Constructing a String: –String message = "Welcome to Java“; –String message = new String("Welcome to Java“); –String s = new String(); F Obtaining String length and Retrieving Individual Characters in a string F String Concatenation (concat) F Substrings (substring(index), substring(start, end)) F Comparisons (equals, compareTo) F String Conversions F Finding a Character or a Substring in a String F Conversions between Strings and Arrays F Converting Characters and Numeric Values to Strings

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"; Here are some more examples of how strings can be used: System.out.println("abc"); String cde = "cde"; System.out.println("abc" + cde); String c = "abc".substring(2,3); String d = cde.substring(1, 2);

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? The answer is no. String s = "Java"; /*creates a String object with content “Java” and assigns its reference to s. */ s = "HTML"; // create a new String object with content “HTML” and assigns to s.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code String s = "Java"; s = "HTML"; animation After executing String s= “Java”; Content cannot be changed After executing s = “HTML” String are immutable; once created, their contents cannot be changed.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Interned Strings Take a look a the String deeper. 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: String s1 = “Welcome to Java”; String s2 = new String (“Welcome to Java”); String s3 = “Welcome to Java”;

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 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 animation

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

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

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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Comparisons F Statements display true and false. String s1 = new String("Welcome“); String s2 = "welcome"; String s3 =“Welcome to C++”; system.out.println(s1.equals(s2)); //true system.out.println(s1.equals(s3)); //false if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { /* Check to see if only s1 and s2 are equal and refer to the same object. s1 and s2 have the same reference */ }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Comparisons, cont. F compareTo(Object object) String s1 = new String("Welcome“); String s2 = "welcome"; if (s1.compareTo(s2) > 0) { //compareTo method can also be used to compare two strings // s1 is greater than s2 } else if (s1.compareTo(s2) == 0) { //== compare // s1 and s2 have the same contents } else // s1 is less than s2

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Length, Characters, and Combining Strings The String class provides the methods for obtaining length, retrieving individual characters, and concate strings as shown below: You can get the length of a string by invoking its length() method. Example: message.length() //returns the length of the string message.

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)  Index starts from 0

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Concatenation We can use the concate method to concatenate two strings. The below example, cancate strings s1, s2, into s3. String s3 = s1.concat(s2); Also plus (+) sing works to concate two or more strings. String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 same as (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);

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";

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Converting, Replacing, and Splitting Strings The String class provides the methods for converting, replacing, and splitting string, as show below

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.

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] + " "); Java HTML Perl displays The split method can be sued to extract tokens from a string with specified a character that identified within the string beginning or end.

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"); "Java is fun".matches("Java.*"); "Java is cool".matches("Java.*");

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.

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]);

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

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 14.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Convert Character and Numbers to Strings String are not arrays, but a string can be converted into an array and vice versa. To convert a string to an array of characters, use the toCharArray method. Convert string “Java” to an array; char[] chars = “Java”.toCharArray(); So, char[0] is ‘J’, char[1] is ‘a’, char[2] ‘v’, char[3] ‘a’. You can also getChars (int srcBegin, int srcEnd)

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Character Class Java provides a wrapper class for every primitive type such as Character, Boolean, Byte, Short, Integer, Long, Float,, and more.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Case Study: Checking Palindromes 30 Video- Checking PalindromeVideo- Checking Palindromes This program presents whether a string is a palindrompalindrom A palindrome is a word or sentence that reads the same forward as it does backward. The words a and I are perhaps the simplest and least interesting palindromes; the word racecar and the name Hannah are more interesting and illustrative. Neither spaces nor punctuation are usually taken into consideration when constructing sentences that are palindromes -- one of the most famous palindromes is "A man, a plan, a canal, Panama" -- but when the spaces are properly positioned as well, so much the better. An example would be the also famous palindrome "Able was I ere I saw Elba," purportedly spoken by Napoleon, referring to his first sighting of Elba, the island where the British exiled him.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Command-Line Argument Case Study: Calculator program 31 The program takes an expression in one argument (operad1 operator operand2) from the command line and displays the expression and the result of the arithmetic operation. Video – Calculator program

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Examples You can create a character object from a char value Character character = new Character (‘a’); 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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Counting Each Letter in a String CountEachLetter Run import java.util.Scanner; public class CountEachLetter { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter a string System.out.print("Enter a string: "); String s = input.nextLine(); // Invoke the countLetters method to count each letter int[] counts = countLetters(s.toLowerCase()); // Display results for (int i = 0; i < counts.length; i++) { if (counts[i] != 0) System.out.println((char)('a' + i) + " appears " + counts[i] + ((counts[i] == 1) ? " time" : " times")); } } /** Count each letter in the string */ public static int[] countLetters(String s) { int[] counts = new int[26]; for (int i = 0; i < s.length(); i++) { if (Character.isLetter(s.charAt(i))) counts[s.charAt(i) - 'a']++; } return counts; } } This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive. Enter a string: Sarah North a appears 2 times h appears 2 times n appears 1 time o appears 1 time r appears 2 times s appears 1 time t appears 1 time

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. F In general, a StringBuilder/StringBuffer can be used wherever a string is used. F 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.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved String Builder and String Buffer F The most important difference between String and StringBuffer/StringBuilder in java is that –String object is immutable whereas –StringBuffer/StringBuilder objects are mutable. By immutable, we mean that the value stored in the String object cannot be changed. 35

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Internally a new String object is created to do the changes. So suppose you declare a String object: String myString = “Hello”; Next, you want to append “Guest” to the same String. What you can do is the following: myString = myString + ” Guest”; When you print the contents of myString the output will be “Hello Guest”. Although we made use of the same object(myString), internally a new object was created in the process.

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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Using append F since StringBuffer/StringBuilder objects are mutable, we can make changes to the value stored in the object. F What this effectively means is that string operations such as append would be more efficient if performed using StringBuffer/StringBuilder objects than String objects. 38

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.

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

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. PalindromeIgnoreNonAlphanumeric Run

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:

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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Description: This method is used to get the primitive data type of a certain String. parseXxx( ) is a static method and can have one argument or two. Syntax: All the variant of this method are given below: static int parseInt(String s) static int parseInt(String s, int radix) Parameters: Here is the detail of parameters: String s : This is a string representation of decimal. int radix : This would be used to convert String s into integer. Java - parseInt() Method

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Return Value : parseInt(String s): This returns an integer (decimal only). parseInt(int i): This returns an integer, given a string representation of decimal, binary, octal, or hexadecimal (radix equals 10, 2, 8, or 16 respectively) numbers as input. Example: public class Test{ public static void main(String args[]){ int x = Integer.parseInt("9"); double c = Double.parseDouble("5"); int b = Integer.parseInt("444",16); System.out.println(x); System.out.println(c); System.out.println(b); } Result

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. 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"); "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 Companion Website

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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Problem: Explore File Properties TestFileClass Run 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.

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

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

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Decimal to Binary Program F Program end of the chapter – 9.10 F Decimal to binary – write a method that parse a decimal number into a binary number as a string. The method header is as follows: public static String decimalToBinary(int value) F Write a test program that prompts the user to enter a decimal integer value and displays the corresponding binary value. 56 Video – Exercise 9-10 – Decimal to binary

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