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The String Class
1
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
2
Strings Are Immutable
3
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
6
java.lang.String
+equals(s1: String): boolean
+equalsIgnoreCase(s1: String): boolean
+compareTo(s1: String): int
+compareToIgnoreCase(s1: String): int
+regionMatches(toffset: int, s1: String, offset: 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.
String Comparisons
7
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.
8
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
9
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
10
Finding string length using the length() method:
message = "Welcome";
message.length()
Retrieving Individual Characters in a String
11
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
Do not use message[0] !!!
String Concatenation
12
String s3 = s1.concat(s2);
String s3 = s1 + s2;
s1 + s2 + s3 + s4 + s5
(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);
Extracting Substrings
13
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
14
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
15
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
16
"Welcome".toLowerCase()
"Welcome".toUpperCase()
"Welcome".replace('e', 'A’)
“welcome”
“WELCOME”
“WAlcomA”
Splitting a String
17
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.
Finding a Character or a Substring in a String
19
"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
20
Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward.
CheckPalindrome.java
The Character Class
21
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
22
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.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.
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
28
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
29
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.
The toString, capacity, length, setLength, and charAt Methods
31
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
32
Rewrite the palindromes program that checks whether a string is a palindrome by using StringBuilder class
CheckPalindromeStringBuilder.java
The File Class
33
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 of 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
34
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.
Renames this file. The method returns true if the operation succeeds.
Problem: Explore File Properties
35
Let’s test the File class
TestFileClass.java
File I/O
36
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
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
37
PrinterWriter Example Write student scores to a file named “scores.txt”
WriteData.java
Reading data using Scanner class Read data from keyboardScanner input = new Scanner(System.in);
Read data from a fileScanner 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
42
Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passedas command-line arguments as follows:
java ReplaceText sourceFile targetFile oldString newString
ReplaceText.java
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. JavaB. Java and HTMLC. and HTMLD. nothing is displayed
Review questions Which class contains the method for checking whether a file exists?
A. FileB. PrintWriterC. ScannerD. System
Which class do you use to write data into a text file?
A. FileB. PrintWriterC. ScannerD. System
Which class do you use to read data into a text file?
A. FileB. PrintWriterC. ScannerD. 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
