Download Chapter 9: Objects and Classes

Chapter 9
Strings and Text I/O
1
Objectives






To use the String class to process fixed strings.
To use the Character class to process a single character.
To discover file properties, delete and rename files using the File
class .
To write data to a file using the PrintWriter class.
To read data from a file using the Scanner class.
(Optional GUI) To add components to a frame.
2
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
3
java.lang.String
+String()
Constructs an empty string
+String(value: String)
Constructs a string with the specified string literal value
+String(value: char[])
Constructs a string with the specified character array
+charAt(index: int): char
Returns the character at the specified index from this string
+compareTo(anotherString: String): int
Compares this string with another string
+compareToIgnoreCase(anotherString: String): int Compares this string with another string ignoring case
+concat(anotherString: String): String
Concat this string with another string
+endsWith(suffix: String): boolean
Returns true if this string ends with the specified suffix
+equals(anotherString: String): boolean
Returns true if this string is equal to anther string
+equalsIgnoreCase(anotherString: String): boolean Checks if this string equals anther string ignoring case
+getChars(int srcBegin, int srcEnd, char[] dst, int Copies characters from this string into the destination character
dstBegin): void
array
+indexOf(ch: int): int
Returns the index of the first occurrence of ch
+indexOf(ch: int, fromIndex: int): int
Returns the index of the first occurrence of ch after fromIndex
+indexOf(str: String): int
Returns the index of the first occurrence of str
+indexOf(str: String, fromIndex: int): int
Returns the index of the first occurrence of str after fromIndex
+lastIndexOf(ch: int): int
Returns the index of the last occurrence of ch
+lastIndexOf(ch: int, fromIndex: int): int
Returns the index of the last occurrence of ch before fromIndex
+lastIndexOf(str: String): int
Returns the index of the last occurrence of str
+lastIndexOf(str: String, fromIndex: int): int
Returns the index of the last occurrence of str before fromIndex
+regionMatches(toffset: int, other: String, offset:
int, len: int): boolean
Returns true if the specified subregion of this string exactly
matches the specified subregion of the string argument
+length(): int
Returns the number of characters in this string
+replace(oldChar: char, newChar: char): String
Returns a new string with oldChar replaced by newChar
+startsWith(prefix: String): boolean
Returns true if this string starts with the specified prefix
+subString(beginIndex: int): String
Returns the substring from beginIndex
+subString(beginIndex: int, endIndex: int): String
Returns the substring from beginIndex to endIndex-1.
+toCharArray(): char[]
Returns a char array consisting characters from this string
+toLowerCase(): String
Returns a new string with all characters converted to lowercase
+toString(): String
Returns a new string with itself
+toUpperCase(): String
Returns a new string with all characters converted to uppercase
+trim(): String
Returns a string with blank characters trimmed on both sides
+copyValueOf(data: char[]): String
Returns a new string consisting of the char array data
+valueOf(c: char): String
Returns a string consisting of the character c
+valueOf(data: char[]): String
Same as copyValueOf(data: char[]): String
+valueOf(d: double): String
Returns a string representing the double value
+valueOf(f: float): String
Returns a string representing the float value
+valueOf(i: int): String
Returns a string representing the int value
+valueOf(l: long): String
Returns a string representing the long value
4
Constructing Strings
String newString = new String(stringLiteral);
String message = new String("Welcome to Java");
•A
String variable holds a reference to a String object that
stores a string value.
•Since
strings are used frequently, Java provides a shorthand
initializer for creating a string:
String message = "Welcome to Java";
5
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";
6
animation
Trace Code
String s = "Java";
s = "HTML";
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"
7
animation
Trace Code
String s = "Java";
s = "HTML";
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"
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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:
9
Examples
String s = "Welcome to Java";
s
s2
String s1 = new String("Welcome to Java");
s3
: String
Interned string object for
"Welcome to Java"
String s2 = s1.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));
display
s1 == s is false
s2 == s is true
s == s3 is true
s1
: 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.
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animation
Trace Code
String s = "Welcome to Java";
String s1 = new String("Welcome to Java");
s
: String
Interned string object for
"Welcome to Java"
String s2 = s1.intern();
String s3 = "Welcome to Java";
11
Trace Code
String s = "Welcome to Java";
s
: String
Interned string object for
"Welcome to Java"
String s1 = new String("Welcome to Java");
String s2 = s1.intern();
String s3 = "Welcome to Java";
s1
: String
A string object for
"Welcome to Java"
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Trace Code
String s = "Welcome to Java";
s
s2
: String
Interned string object for
"Welcome to Java"
String s1 = new String("Welcome to Java");
String s2 = s1.intern();
String s3 = "Welcome to Java";
s1
: String
A string object for
"Welcome to Java"
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Trace Code
String s = "Welcome to Java";
s
s2
String s1 = new String("Welcome to Java");
s3
: String
Interned string object for
"Welcome to Java"
String s2 = s1.intern();
String s3 = "Welcome to Java";
s1
: String
A string object for
"Welcome to Java"
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Finding String Length
Finding string length using the length()
method:
message = "Welcome";
message.length() (returns 7)
15
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)
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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);
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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)
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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
}
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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
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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
– E.g. “Welcome”.toLowerCase() returns a new string, welcome.

toUpperCase
– E.g. “Welcome”.toUpperCase() returns a new string, WELCOME.

trim
– Returns a new string by eliminating blank characters from both
ends of the string.

replace(oldChar, newChar)
– Use to replace all occurrences of a character in the string with a
new character.
21
Finding a Character or a
Substring in a String
"Welcome to Java".indexOf('W') returns 0.
"Welcome to Java".indexOf('x') returns -1.
-Return
the index of the first character in the string that matches the specified character.
"Welcome to Java".indexOf('o', 5) returns 9.
-Returns the index of the first character in the starting from the specified index that
matches the specified character.
"Welcome to Java".indexOf("come") returns 3.
Returns the index of the first character of the substring in the string that matches the
specified string.
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"Welcome to Java".indexOf("Java", 5) returns
11.
"Welcome to Java".indexOf("java", 5) returns 1.
- Returns the index of the first character of the substring in the
string starting from the specified index that matches the
specified string.
"Welcome to Java".lastIndexOf('a') returns 14.
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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’.
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Convert String to Array
char[] chars = “Java”.toCharArray();
chars[0] is ‘J’, chars[1] is ‘a’, chars[2] is ‘v’ and
chars[3] is ‘a’.
 getChars(int srcBegin, int srcEnd, char[] dst, int
dstBegin)
char[] dst = {‘J’,’a’,’v’,’a’,’1’,’3’,’0’,’1’};
“CS3720”.getChars(2, 6, dst, 4);

dst becomes {‘J’,’a’,’v’,’a’,’3’,’7’,’2’,’0’}
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String str = new String(new char[]{‘J’,’a’, ‘v’, ‘a’});
Or
String str = String.valueOf(new char[] {‘J’,’a’, ‘v’, ‘a’});
 String.valueOf(5.44)
converts 5.44 to String,
‘5’, ‘.’,’4’ and’4’.
 Double.parseDouble(str)
or
Integer.parseInt(str) to convert string double
value or an int value.
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Example:
Finding Palindromes
Objective:
Checking whether a string
is a palindrome: a string that reads the
same forward and backward.
CheckPalindrome
Run
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1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
import javax.swing.JOptionPane;
public class CheckPalindrome {
public static void main(String[] args) {
String s = JOptionPane.showInputDialog(“Enter a string:”);
String output = “”;
if (isPalindrome(s))
output = s + “ is a palindrome”;
else
output = s + “ is not a palindrome”;
JOptionPane.showMessageDialog(null, output);
}
public static boolean isPalindrome(String s) {
int low = 0;
int high = s.length()-1;
while (low < high) {
if (s.charAt(low) != s.charAt(high))
return false;
low++;
high--;
}
return true;
}}
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The Character Class
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
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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
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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
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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.
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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.
+renameTo(dest: File): boolean
Renames this file. The method returns true if the operation succeeds.
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Example: Using the File Class
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
Run
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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.
35
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
Run
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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
37
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
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