Introduction to Java Programming - Chapter 8: Strings and Text I/O

Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 1 Chapter 8 Strings and Text I/O Chapter 7 Objects and Classes Chapter 8 Strings and Text I/O Chapter 9 Inheritance and Polymorphism Chapter 6 Arrays §10.2, “Abstract Classes” §10.4, “Interfaces” Chapter 13 Graphics Chapter 14 Event-Driven Programming Chapter 17 Exceptions and Assertions Chapter 18 Binary I/O Exception and binary I/O can be covered after Chapter 9 GUI can be covered after §10.2, “Abstract Classes” Chapter 12 GUI Basics Chapter 11 Object-Oriented Design Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 2 Objectives  To use the String class to process fixed strings (§8.2).  To use the Character class to process a single character (§8.3).  To use the StringBuilder/StringBuffer class to process flexible strings (§8.4).  To know the differences between the String, StringBuilder, and StringBuffer classes (§8.2-8.4).  To learn how to pass strings to the main method from the command line (§8.5).  (Optional) To use the regular expressions to represent patterns for matching, replacing, and splitting strings (§8.6).  To discover file properties, delete and rename files using the File class (§8.7).  To write data to a file using the PrintWriter class (§8.8.1).  To read data from a file using the Scanner class (§8.8.2). Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 4 java.lang.String +String() +String(value: String) +String(value: char[]) +charAt(index: int): char +compareTo(anotherString: String): int +compareToIgnoreCase(anotherString: String): int +concat(anotherString: String): String +endsWith(suffix: String): boolean +equals(anotherString: String): boolean +equalsIgnoreCase(anotherString: String): boolean +getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin): void +indexOf(ch: int): int +indexOf(ch: int, fromIndex: int): int +indexOf(str: String): int +indexOf(str: String, fromIndex: int): int +lastIndexOf(ch: int): int +lastIndexOf(ch: int, fromIndex: int): int +lastIndexOf(str: String): int +lastIndexOf(str: String, fromIndex: int): int +regionMatches(toffset: int, other: String, offset: int, len: int): boolean +length(): int +replace(oldChar: char, newChar: char): String +startsWith(prefix: String): boolean +subString(beginIndex: int): String +subString(beginIndex: int, endIndex: int): String +toCharArray(): char[] +toLowerCase(): String +toString(): String +toUpperCase(): String +trim(): String +copyValueOf(data: char[]): String +valueOf(c: char): String +valueOf(data: char[]): String +valueOf(d: double): String +valueOf(f: float): String +valueOf(i: int): String +valueOf(l: long): String Constructs an empty string Constructs a string with the specified string literal value Constructs a string with the specified character array Returns the character at the specified index from this string Compares this string with another string Compares this string with another string ignoring case Concat this string with another string Returns true if this string ends with the specified suffix Returns true if this string is equal to anther string Checks if this string equals anther string ignoring case Copies characters from this string into the destination character array Returns the index of the first occurrence of ch Returns the index of the first occurrence of ch after fromIndex Returns the index of the first occurrence of str Returns the index of the first occurrence of str after fromIndex Returns the index of the last occurrence of ch Returns the index of the last occurrence of ch before fromIndex Returns the index of the last occurrence of str Returns the index of the last occurrence of str before fromIndex Returns true if the specified subregion of this string exactly matches the specified subregion of the string argument Returns the number of characters in this string Returns a new string with oldChar replaced by newChar Returns true if this string starts with the specified prefix Returns the substring from beginIndex Returns the substring from beginIndex to endIndex-1. Returns a char array consisting characters from this string Returns a new string with all characters converted to lowercase Returns a new string with itself Returns a new string with all characters converted to uppercase Returns a string with blank characters trimmed on both sides Returns a new string consisting of the char array data Returns a string consisting of the character c Same as copyValueOf(data: char[]): String Returns a string representing the double value Returns a string representing the float value Returns a string representing the int value Returns a string representing the long value Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 5 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"; Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 6 Strings Are Immutable A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML"; Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 7 Trace Code String s = "Java"; s = "HTML"; : String String object for "Java" s After executing String s = "Java"; After executing s = "HTML"; : String String object for "Java" : String String object for "HTML" Contents cannot be changed This string object is now unreferenced s animation Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 8 Trace Code String s = "Java"; s = "HTML"; : String String object for "Java" s After executing String s = "Java"; After executing s = "HTML"; : String String object for "Java" : String String object for "HTML" Contents cannot be changed This string object is now unreferenced s animation Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 9 Interned Strings Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. You can also use a String object’s intern method to return an interned string. For example, the following statements: Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 10 Examples display s1 == s is false s2 == s is true s == 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. String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s.intern(); String s3 = "Welcome to Java"; System.out.println("s1 == s is " + (s1 == s)); System.out.println("s2 == s is " + (s2 == s)); System.out.println("s == s3 is " + (s == s3)); : String Interned string object for "Welcome to Java" : String A string object for "Welcome to Java" s s1 s2 s3 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 11 Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s.intern(); String s3 = "Welcome to Java"; : String Interned string object for "Welcome to Java" s animation Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 12 Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s.intern(); String s3 = "Welcome to Java"; : String Interned string object for "Welcome to Java" : String A string object for "Welcome to Java" s s1 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 13 Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s.intern(); String s3 = "Welcome to Java"; : String Interned string object for "Welcome to Java" : String A string object for "Welcome to Java" s s1 s2 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 14 Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s.intern(); String s3 = "Welcome to Java"; : String Interned string object for "Welcome to Java" : String A string object for "Welcome to Java" s s1 s2 s3 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 15 Finding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7) Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 16 Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index)  Index starts from 0 W e l c o m e t o J a v a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 message Indices message.charAt(0) message.charAt(14) message.length() is 15 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 17 String Concatenation String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 same as (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5); Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 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"; W e l c o m e t o J a v a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 message Indices message.substring(0, 11) message.substring(11) Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 19 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 } Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 20 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 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 21 String Conversions The contents of a string cannot be changed once the string is created. But you can convert a string to a new string using the following methods:  toLowerCase  toUpperCase  trim  replace(oldChar, newChar) Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 22 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 23 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’. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 24 Example: Finding Palindromes Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward. CheckPalindrome Run Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 25 The Character Class java.lang.Character +Character(value: char) +charValue(): char +compareTo(anotherCharacter: Character): int +equals(anotherCharacter: Character): boolean +isDigit(ch: char): boolean +isLetter(ch: char): boolean +isLetterOrDigit(ch: char): boolean +isLowerCase(ch: char): boolean +isUpperCase(ch: char): boolean +toLowerCase(ch: char): char +toUpperCase(ch: char): char Constructs a character object with char value Returns the char value from this object Compares this character with another Returns true if this character equals to another Returns true if the specified character is a digit Returns true if the specified character is a letter Returns true if the character is a letter or a digit Returns true if the character is a lowercase letter Returns true if the character is an uppercase letter Returns the lowercase of the specified character Returns the uppercase of the specified character Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 26 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 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 27 Example: Counting Each Letter in a String This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive. CountEachLetter Run Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 28 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. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 29 StringBuilder vs. StringBuffer The StringBuilder class, introduced in JDK 1.5, is similar to StringBuffer except that the update methods in StringBuffer are synchronized. Use StringBuffer if it may be accessed by multiple tasks concurrently. Using StringBuilder is more efficient if it is accessed by a single task. The constructors and methods in StringBuffer and StringBuilder are almost the same. This book covers StringBuffer. You may replace StringBuffer by StringBuilder. The program can compile and run without any other changes. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 30 The StringBuffer Class The StringBuffer class is an alternative to the String class. In general, a string buffer can be used wherever a string is used. StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer. However, the value of a String object is fixed once the string is created. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 31 java.lang.StringBuffer +StringBuffer() +StringBuffer(capacity: int) +StringBuffer(str: String) +append(data: char[]): StringBuffer +append(data: char[], offset: int, len: int): StringBuffer +append(v: aPrimitiveType): StringBuffer +append(str: String): StringBuffer +capacity(): int +charAt(index: int): char +delete(startIndex: int, endIndex: int): StringBuffer +deleteCharAt(int index): StringBuffer +insert(index: int, data: char[], offset: int, len: int): StringBuffer +insert(offset: int, data: char[]): StringBuffer +insert(offset: int, b: aPrimitiveType): StringBuffer +insert(offset: int, str: String): StringBuffer +length(): int +replace(int startIndex, int endIndex, String str): StringBuffer +reverse(): StringBuffer +setCharAt(index: int, ch: char): void +setLength(newLength: int): void +substring(startIndex: int): String +substring(startIndex: int, endIndex: int): String Constructs an empty string buffer with capacity 16 Constructs a string buffer with the specified capacity Constructs a string buffer with the specified string Appends a char array into this string buffer Appends a subarray in data into this string buffer Appends a primitive type value as string to this buffer Appends a string to this string buffer Returns the capacity of this string buffer Returns the character at the specified index Deletes characters from startIndex to endIndex Deletes a character at the specified index Inserts a subarray of the data in the array to the buffer at the specified index Inserts data to this buffer at the position offset Inserts a value converted to string into this buffer Inserts a string into this buffer at the position offset Returns the number of characters in this buffer Replaces the characters in this buffer from startIndex to endIndex with the specified string Reveres the characters in the buffer Sets a new character at the specified index in this buffer Sets a new length in this buffer Returns a substring starting at startIndex Returns a substring from startIndex to endIndex Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 32 StringBuffer Constructors  public StringBuffer() No characters, initial capacity 16 characters.  public StringBuffer(int length) No characters, initial capacity specified by the length argument.  public StringBuffer(String str) Represents the same sequence of characters as the string argument. Initial capacity 16 plus the length of the string argument. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 33 Appending New Contents into a String Buffer StringBuffer strBuf = new StringBuffer(); strBuf.append("Welcome"); strBuf.append(' '); strBuf.append("to"); strBuf.append(' '); strBuf.append("Java"); Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 34 Example: Checking Palindromes Ignoring Non-alphanumeric Characters PalindromeIgnoreNonAlphanumeric Run Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 35 Main Method Is Just a Regular Method 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]); } } 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 36 Command-Line Parameters class TestMain { public static void main(String[] args) { ... } } java TestMain arg0 arg1 arg2 ... argn Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 37 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. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 38 Example: Using Command-Line Parameters Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers. Calculator java Calculator 2 + 3 java Calculator 2 - 3 Run java Calculator 2 / 3 java Calculator 2 “*” 3 Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 39 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 40 Matching Strings matches is a method in the String class. "Java".matches("Java"); //returns true "Java".equals("Java"); // returns true Yet, following calls also return true "Java is fun".matches("Java.*") "Java is cool".matches("Java.*") "Java is powerful".matches("Java.*") Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 41 Regular Expression Syntax Regular Expression Matches Example x a specified character x Java matches Java . any single character Java matches J..a (ab|cd) a, b, or c ten matches t(en|im] [abc] a, b, or c Java matches Ja[uvwx]a [^abc] any character except Java matches Ja[^ars]a a, b, or c [a-z] a through z Java matches [A-M]av[a-d] [^a-z] any character except Java matches Jav[^b-d] a through z [a-e[m-p]] a through e or Java matches m through p [A-G[I-M]]av[a-d] [a-e&&[c-p]] intersection of a-e Java matches with c-p [A-P&&[I-M]]av[a-d] \d a digit, same as [1-9] Java2 matches "Java[\\d]" \D a non-digit $Java matches "[\\D][\\D]ava" \w a word character Java matches "[\\w]ava" \W a non-word character $Java matches "[\\W][\\w]ava" \s a whitespace character "Java 2" matches "Java\\s2" \S a non-whitespace char Java matches "[\\S]ava" p* zero or more Java matches "[\\w]*" occurrences of pattern p p+ one or more Java matches "[\\w]+" occurrences of pattern p p? zero or one Java matches "[\\w]?Java" occurrence of pattern p Java matches "[\\w]?ava" p{n} exactly n Java matches "[\\w]{4}" occurrences of pattern p p{n,} at least n Java matches "[\\w]{3,}" occurrences of pattern p p{n,m} between n and m Java matches "[\\w]{1,9}" occurrences (inclusive) Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 42 Replacing and Splitting Strings java.lang.String +matches(regex: String): boolean +replaceAll(regex: String, replacement: String): String +replaceFirst(regex: String, replacement: String): String +split(regex: String): String[] Returns true if this string matches the pattern. Returns a new string that replaces all matching substrings with the replacement. Returns a new string that replaces the first matching substring with the replacement. Returns an array of strings consisting of the substrings split by the matches. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 43 Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; s become “Jawi Jawi Jawi”. String s = "Java Java Java".replaceFirst("v\\w", "wi") ; s become “Jawi Java Java”. String[] s = "Java1HTML2Perl".split("\\d"); s[0] holds: “Java” s[1] holds: “HTML” s[2] holds: “Perl” Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 44 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. Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 45 java.io.File +File(pathname: String) +File(parent: String, child: String) +File(parent: File, child: String) +exists(): boolean +canRead(): boolean +canWrite(): boolean +isDirectory(): boolean +isFile(): boolean +isAbsolute(): boolean +isHidden(): boolean +getAbsolutePath(): String +getCanonicalPath(): String +getName(): String +getPath(): String +getParent(): String +lastModified(): long +delete(): boolean +renameTo(dest: File): boolean Creates a File object for the specified pathname. The pathname may be a directory or a file. Creates a File object for the child under the directory parent. child may be a filename or a subdirectory. 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. Returns true if the file or the directory represented by the File object exists. Returns true if the file represented by the File object exists and can be read. Returns true if the file represented by the File object exists and can be written. Returns true if the File object represents a directory. Returns true if the File object represents a file. Returns true if the File object is created using an absolute path name. 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 '.'. Returns the complete absolute file or directory name represented by the File object. 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). 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. 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. 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. Returns the time that the file was last modified. Deletes this file. The method returns true if the deletion succeeds. R names this file. The method returns true if the operation succeeds. Obtaining file properties and manipulating file Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 46 Example: Using the File Class 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 47 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, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 48 Writing Data Using PrintWriter WriteData Run java.io.PrintWriter +PrintWriter(filename: String) +print(s: String): void +print(c: char): void +print(cArray: char[]): void +print(i: int): void +print(l: long): void +print(f: float): void +print(d: double): void +print(b: boolean): void Also contains the overloaded println methods. Also contains the overloaded printf methods. Creates a PrintWriter for the specified file. Writes a string. Writes a character. Writes an array of character. Writes an int value. Writes a long value. Writes a float value. Writes a double value. Writes a boolean value. 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.” Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 49 Reading Data Using Scanner java.util.Scanner +Scanner(source: File) +Scanner(source: String) +close() +hasNext(): boolean +next(): String +nextByte(): byte +nextShort(): short +nextInt(): int +nextLong(): long +nextFloat(): float +nextDouble(): double +useDelimiter(pattern: String): Scanner Creates a Scanner that produces values scanned from the specified file. Creates a Scanner that produces values scanned from the specified string. Closes this scanner. Returns true if this scanner has another token in its input. Returns next token as a string. Returns next token as a byte. Returns next token as a short. Returns next token as an int. Returns next token as a long. Returns next token as a float. Returns next token as a double. Sets this scanner’s delimiting pattern. ReadData Run Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 50 Example: Replacing Text Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows: java ReplaceText sourceFile targetFile oldString newString For example, invoking java ReplaceText FormatString.java t.txt StringBuilder StringBuffer replaces all the occurrences of StringBuilder by StringBuffer in FormatString.java and saves the new file in t.txt. ReplaceText Run Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 51 Scanning Primitive Type Values If a token is a primitive data type value, you can use the methods nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain it. For example, the following code adds all numbers in the string. Note that the delimiter is space by default. JDK 1.5 Feature String s = "1 2 3 4"; Scanner scanner = new Scanner(s); int sum = 0; while (scanner.hasNext()) sum += scanner.nextInt(); System.out.println("Sum is " + sum); Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6 52 Console Input Using Scanner Another important application of the Scanner class is to read input from the console. For example, the following code reads an int value from the keyboard: JDK 1.5 Feature System.out.print("Please enter an int value: "); Scanner scanner = new Scanner(System.in); int i = scanner.nextInt();