The String Class Trace Code Constructing a String String s = - - PDF document

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May 18, 2023 129 likes •229 views

The String Class Trace Code Constructing a String String s = "Java"; String message2 = new String("Welcome to Java); s = "HTML"; String s1 = new String(); Since strings are used frequently, Java provides a

SLIDE 1

The String Class

 Constructing a String  Since strings are used frequently, Java provides a

shorthand initializer for creating a string:

String message2 = new String("Welcome to Java“); String s1 = new String(); String message1 = "Welcome to Java“; String s2 = “”;

Chapter 8 Strings and Text I/O

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";

Trace Code

4 : 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

String s = "Java"; s = "HTML";

Trace Code

5 : 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

String s = "Java"; s = "HTML";

String Comparisons

java.lang.String +equals(s1: String): boolean +equalsIgnoreCase(s1: String): boolean +compareTo(s1: String): int +compareToIgnoreCase(s1: String): int +regionMatches(toffset: int, s1: String,

ffset: int, len: int): boolean

+regionMatches(ignoreCase: boolean, toffset: int, s1: String, offset: int, len: int): boolean +startsWith(prefix: String): boolean +endsWith(suffix: String): boolean Returns true if this string is equal to string s1. Returns true if this string is equal to string s1 case- insensitive. 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. Same as compareTo except that the comparison is case- insensitive. Returns true if the specified subregion of this string exactly matches the specified subregion in string s1. Same as the preceding method except that you can specify whether the match is case-sensitive. Returns true if this string starts with the specified prefix. Returns true if this string ends with the specified suffix.

SLIDE 2

String Comparisons

equals

String s1 = new String("Welcome“); String s2 = new String(“welcome”); if (s1.equals(s2)){

// s1 and s2 have the same contents

} if (s1 == s2) {

}

// s1 and s2 have the same reference

String Comparisons, cont.

compareTo 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

String Length, Characters, and Combining Strings

java.lang.String +length(): int +charAt(index: int): char +concat(s1: String): String Returns the number of characters in this string. Returns the character at the specified index from this string. Returns a new string that concatenate this string with string s1.

Finding String Length

Finding string length using the length() method:

message = "Welcome"; message.length() Retrieving Individual Characters in a String

 Use message.charAt(index)  Index starts from 0 W e l c

t o J a v a 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

Do not use message[0] !!!

String Concatenation

String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);

SLIDE 3

Extracting Substrings

java.lang.String +subString(beginIndex: int): String +subString(beginIndex: int, endIndex: int): String Returns this string’s substring that begins with the character at the specified beginIndex and extends to the end of the string, as shown in Figure 8.6. Returns this string’s substring that begins at the specified beginIndex and extends to the character at index endIndex – 1, as shown in Figure 8.6. Note that the character at endIndex is not part of the substring.

Extracting Substrings

 Using the substring method in the String class 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)

String s1 = "Welcome to Java"; String s2 = s1.substring(0, 11) + "HTML";

Converting, Replacing, and Splitting Strings

java.lang.String +toLowerCase(): String +toUpperCase(): String +trim(): String +replace(oldChar: char, newChar: char): String +replaceFirst(oldString: String, newString: String): String +replaceAll(oldString: String, newString: String): String +split(delimiter: String): String[] Returns a new string with all characters converted to lowercase. Returns a new string with all characters converted to uppercase. Returns a new string with blank characters trimmed on both sides. Returns a new string that replaces all matching character in this string with the new character. Returns a new string that replaces the first matching substring in this string with the new substring. Returns a new string that replace all matching substrings in this string with the new substring. Returns an array of strings consisting of the substrings split by the delimiter.

Examples

"Welcome".toLowerCase() "Welcome".toUpperCase() "Welcome".replace('e', 'A’)

“welcome” “WELCOME” “WAlcomA”

Splitting a String

String str = “Cat-Dog-Mouse”; String[] tokens = str.split(“-”); System.out.println(tokens[2]); System.out.println(“eats”); System.out.println(tokens[0]); {“Cat”, “Dog”, “Mouse”}

Finding a Character or a Substring in a String

18 java.lang.String +indexOf(ch: char): int +indexOf(ch: char, fromIndex: int): int +indexOf(s: String): int +indexOf(s: String, fromIndex: int): int +lastIndexOf(ch: int): int +lastIndexOf(ch: int, fromIndex: int): int +lastIndexOf(s: String): int +lastIndexOf(s: String, fromIndex: int): int Returns the index of the first occurrence of ch in the string. Returns -1 if not matched. Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched. Returns the index of the first occurrence of string s in this string. Returns -1 if not matched. Returns the index of the first occurrence of string s in this string after fromIndex. Returns -1 if not matched. Returns the index of the last occurrence of ch in the string. Returns -1 if not matched. Returns the index of the last occurrence of ch before fromIndex in this string. Returns -1 if not matched. Returns the index of the last occurrence of string s. Returns -1 if not matched. Returns the index of the last occurrence of string s before

fromIndex. Returns -1 if not matched.

SLIDE 4

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.

Problem: Finding Palindromes

 Objective: Checking whether a string is a palindrome: a

string that reads the same forward and backward.

 CheckPalindrome.java

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

Problem: Counting Each Letter in a String

 This example gives a program that counts the number of

ccurrence of each letter in a string.

 Assume the letters are not case-sensitive.  CountEachLetter.java

Review

 String class – an immutable sequence of characters  Character class – a character

Review questions

 Suppose s is a string with the value "java". What will be

assigned to x if you execute the following code? char x = s.charAt(4);

A. 'a'
B. 'v'
C. Nothing will be assigned to x, because the execution causes

the runtime error StringIndexOutofBoundsException.

SLIDE 5

Review questions

 Suppose s1 and s2 are two strings. Which of the

following statements or expressions is incorrect?

A. String s3 = s1 - s2;
B. boolean b = s1.compareTo(s2);
C. char c = s1[0];
D. char c = s1.charAt(s1.length());

Review questions

 Analyze the following code.

class Test { public static void main(String[] args) { String s; System.out.println("s is " + s); } }

A. The program has a compilation error because s is not initialized, but it is

referenced in the println statement.

B. The program has a runtime error because s is not initialized, but it is referenced

in the println statement.

C. The program has a runtime error because s is null in the println statement.
D. The program compiles and runs fine.

Outline

 StringBuilder/StringBuffer class  File class and File I/O

StringBuilder/StringBuffer

 An alternative to the String class – a mutable string  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

 A String object is fixed once the string is created.

StringBuilder Constructors

java.lang.StringBuilder

+StringBuilder() +StringBuilder(capacity: int) +StringBuilder(s: String) Constructs an empty string builder with capacity 16. Constructs a string builder with the specified capacity. Constructs a string builder with the specified string.

Modifying Strings in the Builder

30 java.lang.StringBuilder

+append(data: char[]): StringBuilder +append(data: char[], offset: int, len: int): StringBuilder +append(v: aPrimitiveType): StringBuilder +append(s: String): StringBuilder +delete(startIndex: int, endIndex: int): StringBuilder +deleteCharAt(index: int): StringBuilder +insert(index: int, data: char[], offset: int, len: int): StringBuilder +insert(offset: int, data: char[]): StringBuilder +insert(offset: int, b: aPrimitiveType): StringBuilder +insert(offset: int, s: String): StringBuilder +replace(startIndex: int, endIndex: int, s: String): StringBuilder +reverse(): StringBuilder +setCharAt(index: int, ch: char): void Appends a char array into this string builder. Appends a subarray in data into this string builder. Appends a primitive type value as a string to this builder. Appends a string to this string builder. Deletes characters from startIndex to endIndex. Deletes a character at the specified index. Inserts a subarray of the data in the array to the builder at the specified index. Inserts data into this builder at the position offset. Inserts a value converted to a string into this builder. Inserts a string into this builder at the position offset. Replaces the characters in this builder from startIndex to endIndex with the specified string. Reverses the characters in the builder. Sets a new character at the specified index in this builder.

SLIDE 6

The toString, capacity, length, setLength, and charAt Methods

java.lang.StringBuilder

+toString(): String +capacity(): int +charAt(index: int): char +length(): int +setLength(newLength: int): void +substring(startIndex: int): String +substring(startIndex: int, endIndex: int): String +trimToSize(): void Returns a string object from the string builder. Returns the capacity of this string builder. Returns the character at the specified index. Returns the number of characters in this builder. Sets a new length in this builder. Returns a substring starting at startIndex. Returns a substring from startIndex to endIndex-1. Reduces the storage size used for the string builder.

Problem: Checking Palindromes Using StringBuilder

 Rewrite the palindromes program that checks whether a

string is a palindrome by using StringBuilder class

 CheckPalindromeStringBuilder.java

The File Class

 File class is used to obtain file properties and to delete

and rename files

 The File class is intended to provide an abstraction that

deals with most of the machine-dependent complexities

f files and path names in a machine-independent fashion

 The File class is a wrapper class for the file name and its

directory path.

 It is not for reading and writing file contents.

Obtaining file properties and manipulating file

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

bject. 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

bject.

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. Renames this file. The method returns true if the operation succeeds.

Problem: Explore File Properties

 Let’s test the File class  TestFileClass.java

File 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, we need to use I/O classes:

Scanner and PrintWriter

SLIDE 7

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.”

Writing Data Using PrintWriter

PrinterWriter Example

 Write student scores to a file named “scores.txt”  WriteData.java

Reading data using Scanner class

 Read data from keyboard Scanner input = new Scanner(System.in);  Read data from a file Scanner input = new Scanner(new File(filename));

Reading Data Using Scanner

40 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.

Scanner example

 Reading data from the score file named “scores.txt”  ReadData.java

Problem: Replacing Text

Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows:

java ReplaceText sourceFile targetFile oldString newString

ReplaceText.java

SLIDE 8

Review questions



The following program displays __________. public class Test { public static void main(String[] args) { String s = "Java"; StringBuilder buffer = new StringBuilder(s); change(buffer); System.out.println(buffer); } private static void change(StringBuilder buffer) { buffer.append(" and HTML"); } }

A. Java
B. Java and HTML
C. and HTML
D. nothing is displayed

Review questions



Which class contains the method for checking whether a file exists?

A. File
B. PrintWriter
C. Scanner
D. System



Which class do you use to write data into a text file?

A. File
B. PrintWriter
C. Scanner
D. System



Which class do you use to read data into a text file?

A. File
B. PrintWriter
C. Scanner
D. System

Practice problems

 String class

 8.1

 StringBuilder class

 8.17

 Command line arguments

 8.20

 File class

 8.23

 File I/O

 8.29