Programming Fundamentals I (COSC- 1336), Lecture 6 (prepared after Chapter 9 of Liang’s 2011 textbook) Stefan Andrei 11/24/20151 COSC-1336, Lecture 6

Overview of Previous Lecture To define methods, invoke methods, and pass arguments to a method (§ ). To develop reusable code that is modular, easy- to-read, easy-to-debug, and easy-to-maintain. (§5.6). To use method overloading and understand ambiguous overloading (§5.7). To design and implement overloaded methods (§5.8). 11/24/20152 COSC-1336, Lecture 6

Overview of Previous Lecture (cont) To determine the scope of variables (§5.9). To know how to use the methods in the Math class (§§ ). To learn the concept of method abstraction (§5.12). To design and implement methods using stepwise refinement (§5.12). 11/24/20153 COSC-1336, Lecture 6

Motivation of the current lecture 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. 11/24/20154 COSC-1336, Lecture 6

Overview of This Lecture 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). 11/24/20155 COSC-1336, Lecture 6

Overview of This Lecture (cont.) 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). 11/24/20156 COSC-1336, Lecture 6

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 11/24/20157 COSC-1336, Lecture 6

Constructing Strings String newString = new String(stringLiteral); String message = new String("Welcome to Java"); Since String s are used frequently, Java provides a shorthand initializer for creating a String : String message = "Welcome to Java"; 11/24/20158 COSC-1336, Lecture 6

Strings Are Immutable A String object is immutable: its contents cannot be changed. Does the following code change the contents of the String s ? String s = "Java"; s = "HTML"; 11/24/20159 COSC-1336, Lecture 6

Trace Code String s = "Java"; s = "HTML"; animation 11/24/ COSC-1336, Lecture 6

Trace Code String s = "Java"; s = "HTML"; animation 11/24/ COSC-1336, Lecture 6

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: 11/24/ COSC-1336, Lecture 6

Examples displays 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. 11/24/ COSC-1336, Lecture 6

Trace Code animation 11/24/ COSC-1336, Lecture 6

Trace Code 11/24/ COSC-1336, Lecture 6

Trace Code 11/24/ COSC-1336, Lecture 6

String Comparisons 11/24/ COSC-1336, Lecture 6

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 } 11/24/ COSC-1336, Lecture 6

The String Class – no need for new Because strings are so common, we do not have to use the new operator to create a String object: title = "Java programming language"; This is special syntax that works only for strings. Each string literal (enclosed in double quotes) represents a String object. 11/24/ COSC-1336, Lecture 6

Creating String objects Using the new operator: String s1 = new String("tree");  Will create a new String object whose value is "tree" and assign its address to s1 ; Without using the new operator: String s1 = "tree";  The Java compiler will check whether there exists already in memory a String object whose value is "tree", not created using the new operator.  If yes, then s1 will have the same reference as that String ;  If no, the Java compiler will create a new String object whose value is "tree", its address will be referred by s1. 11/24/2015 COSC-1336, Lecture 6 20

Example String s1 = new String("tree"), s2 = "tree", s3 = "tree"; if (s1.equals(s2)) System.out.println("Same name for s1 and s2"); if (s1 == s2) System.out.println("Same address for s1 and s2"); if (s2.equals(s3)) System.out.println("Same name for s2 and s3"); if (s2 == s3) System.out.println("Same address for s2 and s3"); This snippet of Java code will display: Same name for s1 and s2 Same name for s2 and s3 Same address for s2 and s3 11/24/2015 COSC-1336, Lecture 6 21

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 11/24/ COSC-1336, Lecture 6

String Length, Characters, and Combining Strings 11/24/ COSC-1336, Lecture 6

Finding a String’s Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7 ) 11/24/ COSC-1336, Lecture 6

Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) Index starts from 0 11/24/ COSC-1336, Lecture 6

String Concatenation String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 is equivalent to: (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5). 11/24/ COSC-1336, Lecture 6

Extracting Substrings 11/24/ COSC-1336, Lecture 6

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"; 11/24/ COSC-1336, Lecture 6

Converting, Replacing, and Splitting Strings 11/24/ COSC-1336, Lecture 6

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 11/24/ COSC-1336, Lecture 6

Splitting a String String[] tokens = "Java#HTML#Perl".split("#"); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " "); Java HTML Perl  displays 11/24/ COSC-1336, Lecture 6

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. "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("Java.*"); "Java is cool".matches("Java.*"); 11/24/ COSC-1336, Lecture 6

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. 11/24/ COSC-1336, Lecture 6

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]); 11/24/ COSC-1336, Lecture 6

Finding a Character or a Substring in a String 11/24/ COSC-1336, Lecture 6

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 /24/ COSC-1336, Lecture 6

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'. 11/24/ COSC-1336, Lecture 6

The Character Class 11/24/ COSC-1336, Lecture 6

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: charObject.compareTo(new Character('d')) returns: -2 charObject.equals(new Character('b')) returns: true charObject.equals(new Character('d')) returns: false 11/24/ COSC-1336, Lecture 6

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. 11/24/ COSC-1336, Lecture 6

CountEachLetter.java import java.util.Scanner; public class CountEachLetter { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a string: "); String s = input.nextLine(); int[] counts = countLetters(s.toLowerCase()); 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")); } } //... 11/24/2015 COSC-1336, Lecture 6 41

CountEachLetter.java (cont.) /** 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; } } // from CountEachLetter class 11/24/2015 COSC-1336, Lecture 6 42

Running CountEachLetter.java 11/24/2015 COSC-1336, Lecture 6 43

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. 11/24/ COSC-1336, Lecture 6

StringBuilder Constructors 11/24/ COSC-1336, Lecture 6

Modifying Strings in the Builder 11/24/ COSC-1336, Lecture 6

Examples stringBuilder.append("Welcome "); stringBuilder.insert(8, "to Java and"); stringBuilder.delete(16, 19); changes stringBuilder to:  Welcome to Java stringBuilder.deleteCharAt(8); changes stringBuilder to:  Welcome o Java stringBuilder.reverse() changes stringBuilder to:  avaJ o emocleW 11/24/ COSC-1336, Lecture 6

Examples (cont.) stringBuilder.reverse(); stringBuilder.insert(8, "t"); stringBuilder.replace(11, 15, "HTML"); changes stringBuilder to  Welcome to HTML stringBuilder.setCharAt(0, 'w') sets stringBuilder to  welcome to HTML 11/24/2015 COSC-1336, Lecture 6 48

The toString(), capacity(), length(), setLength(), and charAt() Methods 11/24/ COSC-1336, Lecture 6

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: 11/24/ COSC-1336, Lecture 6

Command-Line Parameters class TestMain { public static void main(String[] args) { System.out.println(args[0]); System.out.println(args[1]); System.out.println(args[2]); } By calling the Java bytecode like this: java TestMain arg0 arg1 arg2 args[0] becomes arg0, args[1] becomes arg1, and args[2] becomes arg2. 11/24/ COSC-1336, Lecture 6

Running TestMain.java 11/24/2015 COSC-1336, Lecture 6 52

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. 11/24/ COSC-1336, Lecture 6

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. 11/24/ COSC-1336, Lecture 6

Obtaining file properties and manipulating files 11/24/ COSC-1336, Lecture 6

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. 11/24/2015 COSC-1336, Lecture 6 56

public class TestFileClass { public static void main(String[] args) { 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())); } 11/24/2015 COSC-1336, Lecture 6 57

Left: run on Windows; right: run on Unix 11/24/2015 COSC-1336, Lecture 6 58

Text Input/Output (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. 11/24/ COSC-1336, Lecture 6

Writing Data Using PrintWriter 11/24/ COSC-1336, Lecture 6

WriteData.java public class WriteData { public static void main(String[] args) throws Exception { 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); output.print("John T Smith "); output.println(90); output.print("Eric K Jones "); output.println(85); output.close(); } 11/24/2015 COSC-1336, Lecture 6 61

Running and testing WriteData.java 11/24/2015 COSC-1336, Lecture 6 62

Reading Data Using Scanner 11/24/ COSC-1336, Lecture 6

import java.util.Scanner; public class ReadData { public static void main(String[] args) throws Exception { java.io.File file = new java.io.File("scores.txt"); Scanner input = new Scanner(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); } input.close(); } 11/24/2015 COSC-1336, Lecture 6 64

Running and testing ReadData.java 11/24/2015 COSC-1336, Lecture 6 65

ReadFileUsingJFileChooser.java import java.util.Scanner; import javax.swing.JFileChooser; public class ReadFileUsingJFileChooser { public static void main(String[] args) throws Exception { JFileChooser fileChooser = new JFileChooser(); if (fileChooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) { // Get the selected file java.io.File file = fileChooser.getSelectedFile(); // Create a Scanner for the file Scanner input = new Scanner(file); 11/24/2015 COSC-1336, Lecture 6 66

ReadFileUsingJFileChooser.java (cont.) // Read text from the file while (input.hasNext()) { System.out.println(input.nextLine()); } // Close the file input.close(); } else { System.out.println("No file selected"); } 11/24/2015 COSC-1336, Lecture 6 67

Running and testing ReadFileUsingJFileChooser.java 11/24/2015 COSC-1336, Lecture 6 68

Summary 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). 11/24/ COSC-1336, Lecture 6

Summary (cont.) 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). 11/24/ COSC-1336, Lecture 6

Reading suggestions From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, ]  Chapter 9 (Strings and Text I/O) 11/24/ COSC-1336, Lecture 6

Coming up next From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, ]  Chapter 6 (Arrays) 11/24/ COSC-1336, Lecture 6

Thank you for your attention! Questions? 11/24/ COSC-1336, Lecture 6