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Chapter 9 Strings and Text I/O Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 1 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 (§9.2-9.4). 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). Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. • 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 more than 40 methods for manipulating strings. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 3 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 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 4 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. 0132130807 5 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. 0132130807 6 animation Trace Code After executing String s= “Java”; Content cannot be changed String s = "Java"; s = "HTML"; After executing s = “HTML” String are immutable; once created, their contents cannot be changed. After executing s = "HTML"; After executing String s = "Java"; s : String String object for "Java" Contents cannot be changed s : String This string object is now unreferenced String object for "Java" : String String object for "HTML" Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 7 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. 0132130807 8 Examples String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); s1 : String s3 Interned string object for "Welcome to Java" String s3 = "Welcome to Java"; System.out.println("s1 == s2 is " + (s1 == s2)); s2 System.out.println("s1 == s3 is " + (s1 == s3)); display s1 == s2 is false s1 == s3 is true : String A string object for "Welcome to Java" 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. 0132130807 9 animation Trace Code String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); s1 : String Interned string object for "Welcome to Java" String s3 = "Welcome to Java"; Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 10 Trace Code String s1 = "Welcome to Java"; s1 String s2 = new String("Welcome to Java"); : String Interned string object for "Welcome to Java" String s3 = "Welcome to Java"; s2 : String A string object for "Welcome to Java" Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 11 Trace Code String s1 = "Welcome to Java"; s1 : String s3 String s2 = new String("Welcome to Java"); Interned string object for "Welcome to Java" String s3 = "Welcome to Java"; s2 : String A string object for "Welcome to Java" Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 12 String Comparisons java.lang.String +equals(s1: String): boolean Returns true if this string is equal to string s1. +equalsIgnoreCase(s1: String): boolean Returns true if this string is equal to string s1 caseinsensitive. +compareTo(s1: String): int 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. +compareToIgnoreCase(s1: String): int Same as compareTo except that the comparison is caseinsensitive. +regionMatches(toffset: int, s1: String, Returns true if the specified subregion of this string exactly offset: int, len: int): boolean matches the specified subregion in string s1. +regionMatches(ignoreCase: boolean, Same as the preceding method except that you can specify toffset: int, s1: String, offset: int, whether the match is case-sensitive. len: int): boolean +startsWith(prefix: String): boolean Returns true if this string starts with the specified prefix. +endsWith(suffix: String): boolean Returns true if this string ends with the specified suffix. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 13 String Comparisons 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. 0132130807 14 String Comparisons, cont. 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) { //== // 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. 0132130807 compare 15 String Length, Characters, and Combining Strings The String class provides the methods for obtaining length, retrieving individual characters, and concate strings as shown below: java.lang.String +length(): int Returns the number of characters in this string. +charAt(index: int): char Returns the character at the specified index from this string. +concat(s1: String): String Returns a new string that concatenate this string with string s1. string. 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. 0132130807 16 Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) starts from 0 Index Indices 0 1 2 3 4 5 6 message W e l c o m e message.charAt(0) 7 8 9 t o message.length() is 15 10 11 12 13 14 J a v a message.charAt(14) Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 17 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. 0132130807 18 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"; Indices 0 1 2 3 4 5 6 message W e l c o m e 7 8 9 t o message.substring(0, 11) 10 11 12 13 14 J a v a message.substring(11) Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 19 Converting, Replacing, and Splitting Strings The String class provides the methods for converting, replacing, and splitting string, as show below java.lang.String +toLowerCase(): String Returns a new string with all characters converted to lowercase. +toUpperCase(): String Returns a new string with all characters converted to uppercase. +trim(): String Returns a new string with blank characters trimmed on both sides. +replace(oldChar: char, newChar: char): String Returns a new string that replaces all matching character in this string with the new character. +replaceFirst(oldString: String, Returns a new string that replaces the first matching substring in newString: String): String this string with the new substring. +replaceAll(oldString: String, Returns a new string that replace all matching substrings in this newString: String): String string with the new substring. +split(delimiter: String): Returns an array of strings consisting of the substrings split by the String[] delimiter. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 20 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. 0132130807 21 Splitting a String The split method can be sued to extract tokens from a string with specified a character that identified within the string beginning or end. String[] tokens = "Java#HTML#Perl".split("#", 0); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " "); displays Java HTML Perl Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 22 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. 0132130807 23 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. 0132130807 24 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. 0132130807 25 Finding a Character or a Substring in a String java.lang.String +indexOf(ch: char): int Returns the index of the first occurrence of ch in the string. Returns -1 if not matched. +indexOf(ch: char, fromIndex: int): int Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched. +indexOf(s: String): int Returns the index of the first occurrence of string s in this string. Returns -1 if not matched. +indexOf(s: String, fromIndex: Returns the index of the first occurrence of string s in this string int): int after fromIndex. Returns -1 if not matched. +lastIndexOf(ch: int): int Returns the index of the last occurrence of ch in the string. Returns -1 if not matched. +lastIndexOf(ch: int, Returns the index of the last occurrence of ch before fromIndex fromIndex: int): int in this string. Returns -1 if not matched. +lastIndexOf(s: String): int Returns the index of the last occurrence of string s. Returns -1 if not matched. +lastIndexOf(s: String, Returns the index of the last occurrence of string s before fromIndex: int): int fromIndex. Returns -1 if not matched. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 26 Finding a Character or a Substring in a String "Welcome "Welcome "Welcome "Welcome "Welcome 11. "Welcome "Welcome to to to to to Java".indexOf('W') returns 0. Java".indexOf('x') returns -1. Java".indexOf('o', 5) returns 9. Java".indexOf("come") returns 3. Java".indexOf("Java", 5) returns to Java".indexOf("java", 5) returns -1. to Java".lastIndexOf('a') returns 14. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 27 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. 0132130807 28 The Character Class Java provides a wrapper class for every primitive type such as Character, Boolean, Byte, Short, Integer, Long, Float, , and more. java.lang.Character +Character(value: char) Constructs a character object with char value +charValue(): char Returns the char value from this object +compareTo(anotherCharacter: Character): int Compares this character with another +equals(anotherCharacter: Character): boolean Returns true if this character equals to another +isDigit(ch: char): boolean Returns true if the specified character is a digit +isLetter(ch: char): boolean Returns true if the specified character is a letter +isLetterOrDigit(ch: char): boolean Returns true if the character is a letter or a digit +isLowerCase(ch: char): boolean Returns true if the character is a lowercase letter +isUpperCase(ch: char): boolean Returns true if the character is an uppercase letter +toLowerCase(ch: char): char Returns the lowercase of the specified character +toUpperCase(ch: char): char Returns the uppercase of the specified character Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 29 Case Study: Checking Palindromes This program presents whether a string is a palindrom 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. Video- Checking Palindromes Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 30 Command-Line Argument Case Study: Calculator program 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. 0132130807 31 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. 0132130807 32 Problem: Counting Each Letter in a String import java.util.Scanner; public class CountEachLetter { This example gives a program /** Main method */ that counts the number of public static void main(String[] args) { // Create a Scanner occurrence of each letter in a Scanner input = new Scanner(System.in); string. Assume the letters are not // Prompt the user to enter a string case-sensitive. 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 Enter a string: Sarah North for (int i = 0; i < counts.length; i++) { a appears 2 times if (counts[i] != 0) h appears 2 times System.out.println((char)('a' + i) + " appears " + n appears 1 time counts[i] + ((counts[i] == 1) ? " time" : " times")); o appears 1 time } r appears 2 times } s appears 1 time /** Count each letter in the string */ t appears 1 time 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; } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All 33 } rights reserved. 0132130807 Run CountEachLetter StringBuilder and StringBuffer StringBuilder/StringBuffer class is an alternative to the String class. The 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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 34 String Builder and String Buffer 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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 35 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. 0132130807 36 Modifying Strings in the Builder java.lang.StringBuilder +append(data: char[]): StringBuilder Appends a char array into this string builder. +append(data: char[], offset: int, len: int): StringBuilder Appends a subarray in data into this string builder. +append(v: aPrimitiveType): StringBuilder Appends a primitive type value as a string to this builder. +append(s: String): StringBuilder Appends a string to this string builder. +delete(startIndex: int, endIndex: int): StringBuilder Deletes characters from startIndex to endIndex. +deleteCharAt(index: int): StringBuilder Deletes a character at the specified index. +insert(index: int, data: char[], offset: int, len: int): StringBuilder Inserts a subarray of the data in the array to the builder at the specified index. +insert(offset: int, data: char[]): StringBuilder Inserts data into this builder at the position offset. +insert(offset: int, b: aPrimitiveType): StringBuilder Inserts a value converted to a string into this builder. +insert(offset: int, s: String): StringBuilder Inserts a string into this builder at the position offset. +replace(startIndex: int, endIndex: int, s: String): StringBuilder Replaces the characters in this builder from startIndex to endIndex with the specified string. +reverse(): StringBuilder Reverses the characters in the builder. +setCharAt(index: int, ch: char): void Sets a new character at the specified index in this builder. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 37 Using append since StringBuffer/StringBuilder objects are mutable, we can make changes to the value stored in the object. What this effectively means is that string operations such as append would be more efficient if performed using StringBuffer/StringBuilder objects than String objects. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 38 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. 0132130807 39 The toString, capacity, length, setLength, and charAt Methods java.lang.StringBuilder +toString(): String Returns a string object from the string builder. +capacity(): int Returns the capacity of this string builder. +charAt(index: int): char Returns the character at the specified index. +length(): int Returns the number of characters in this builder. +setLength(newLength: int): void Sets a new length in this builder. +substring(startIndex: int): String Returns a substring starting at startIndex. +substring(startIndex: int, endIndex: int): String Returns a substring from startIndex to endIndex-1. +trimToSize(): void Reduces the storage size used for the string builder. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 40 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 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Run 41 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: public class A { public static void main(String[] args) { String[] strings = {"New York", "Boston", "Atlanta"}; B.main(strings); } } class B { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 42 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 Run java Calculator 2 - 3 java Calculator 2 / 3 java Calculator 2 “*” 3 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 43 Java - parseInt() Method 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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 44 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 9 5.0 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 1092 45 Companion Website 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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 46 Companion Website Matching Strings "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("Java.*") "Java is cool".matches("Java.*") "Java is powerful".matches("Java.*") Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 47 Companion Website Regular Expression Syntax Regular Expression x . (ab|cd) [abc] [^abc] Matches Example a specified character x any single character a, b, or c a, b, or c any character except a, b, or c a through z any character except a through z a through e or m through p intersection of a-e with c-p Java matches Java Java matches J..a ten matches t(en|im] Java matches Ja[uvwx]a Java matches Ja[^ars]a \d \D \w \W \s \S a a a a a a Java2 matches "Java[\\d]" $Java matches "[\\D][\\D]ava" Java matches "[\\w]ava" $Java matches "[\\W][\\w]ava" "Java 2" matches "Java\\s2" Java matches "[\\S]ava" p* zero or more occurrences of pattern p one or more occurrences of pattern p zero or one occurrence of pattern p exactly n occurrences of pattern p at least n occurrences of pattern p between n and m occurrences (inclusive) [a-z] [^a-z] [a-e[m-p]] [a-e&&[c-p]] p+ p? p{n} p{n,} p{n,m} digit, same as [1-9] non-digit word character non-word character whitespace character non-whitespace char Java matches [A-M]av[a-d] Java matches Jav[^b-d] Java matches [A-G[I-M]]av[a-d] Java matches [A-P&&[I-M]]av[a-d] Java matches "[\\w]*" Java matches "[\\w]+" Java matches "[\\w]?Java" Java matches "[\\w]?ava" Java matches "[\\w]{4}" Java matches "[\\w]{3,}" Java matches "[\\w]{1,9}" Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 48 Companion Website Replacing and Splitting Strings java.lang.String +matches(regex: String): boolean Returns true if this string matches the pattern. +replaceAll(regex: String, replacement: String): String Returns a new string that replaces all matching substrings with the replacement. +replaceFirst(regex: String, replacement: String): String Returns a new string that replaces the first matching substring with the replacement. +split(regex: String): String[] Returns an array of strings consisting of the substrings split by the matches. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 49 Companion Website Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; String s = "Java Java Java".replaceFirst("v\\w", "wi") ; String[] s = "Java1HTML2Perl".split("\\d"); Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 50 Obtaining file properties and manipulating file java.io.File +File(pathname: String) Creates a File object for the specified pathname. The pathname may be a directory or a file. +File(parent: String, child: String) Creates a File object for the child under the directory parent. child may be a filename or a subdirectory. +File(parent: File, child: String) Creates a File object for the child under the directory parent. parent is a File object. In the preceding constructor, the parent is a string. +exists(): boolean Returns true if the file or the directory represented by the File object exists. +canRead(): boolean Returns true if the file represented by the File object exists and can be read. +canWrite(): boolean Returns true if the file represented by the File object exists and can be written. +isDirectory(): boolean Returns true if the File object represents a directory. +isFile(): boolean Returns true if the File object represents a file. +isAbsolute(): boolean Returns true if the File object is created using an absolute path name. +isHidden(): boolean Returns true if the file represented in the File object is hidden. The exact definition of hidden is system-dependent. On Windows, you can mark a file hidden in the File Properties dialog box. On Unix systems, a file is hidden if its name begins with a period character '.'. +getAbsolutePath(): String Returns the complete absolute file or directory name represented by the File object. +getCanonicalPath(): String Returns the same as getAbsolutePath() except that it removes redundant names, such as "." and "..", from the pathname, resolves symbolic links (on Unix platforms), and converts drive letters to standard uppercase (on Win32 platforms). +getName(): String Returns the last name of the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getName() returns test.dat. +getPath(): String Returns the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat. +getParent(): String Returns the complete parent directory of the current directory or the file represented by the File object. For example, new File("c:\\book\\test.dat").getParent() returns c:\book. +lastModified(): long Returns the time that the file was last modified. +delete(): boolean Deletes this file. The method returns true if the deletion succeeds. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All +renameTo(dest: File): boolean Renames this file. The method returns true if the operation succeeds. rights reserved. 0132130807 51 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. Figure 16.1 shows a sample run of the program on Windows, and Figure 16.2 a sample run on Unix. TestFileClass Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Run 52 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. 0132130807 53 Writing Data Using PrintWriter java.io.PrintWriter +PrintWriter(filename: String) Creates a PrintWriter for the specified file. +print(s: String): void Writes a string. +print(c: char): void Writes a character. +print(cArray: char[]): void Writes an array of character. +print(i: int): void Writes an int value. +print(l: long): void Writes a long value. +print(f: float): void Writes a float value. +print(d: double): void Writes a double value. +print(b: boolean): void Writes a boolean value. Also contains the overloaded println methods. A println method acts like a print method; additionally it prints a line separator. The line separator string is defined by the system. It is \r\n on Windows and \n on Unix. The printf method was introduced in §3.6, “Formatting Console Output and Strings.” Also contains the overloaded printf methods. . WriteData Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Run 54 Reading Data Using Scanner java.util.Scanner +Scanner(source: File) Creates a Scanner that produces values scanned from the specified file. +Scanner(source: String) Creates a Scanner that produces values scanned from the specified string. +close() Closes this scanner. +hasNext(): boolean Returns true if this scanner has another token in its input. +next(): String Returns next token as a string. +nextByte(): byte Returns next token as a byte. +nextShort(): short Returns next token as a short. +nextInt(): int Returns next token as an int. +nextLong(): long Returns next token as a long. +nextFloat(): float Returns next token as a float. +nextDouble(): double Returns next token as a double. +useDelimiter(pattern: String): Scanner Sets this scanner’s delimiting pattern. ReadData Run Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 55 Decimal to Binary Program end of the chapter – 9.10 Decimal to binary – write a method that parse a decimal number into a binary number as a string. The method header is as follows: Program public static String decimalToBinary(int value) Write a test program that prompts the user to enter a decimal integer value and displays the corresponding binary value. Video – Exercise 9-10 – Decimal to binary Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 56 (GUI) File Dialogs ReadFileUsingJFileChooser Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Run 57