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 use the StringBuilder/StringBuffer class to process flexible
strings.
To know the differences between the String, StringBuilder, and
StringBuffer classes.
To learn how to pass strings to the main method from the
command line.
(Optional) To use the regular expressions to represent patterns
for matching, replacing, and splitting strings.
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"
8
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.
10
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"
12
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"
13
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"
14
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)
16
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);
17
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)
18
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
}
19
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
20
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.
22
"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.
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’.
24
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’}
25
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.
26
Example:
Finding Palindromes
Objective:
Checking whether a string
is a palindrome: a string that reads the
same forward and backward.
CheckPalindrome
Run
27
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;
}}
28
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
29
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
30
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
31
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.
32
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.
33
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.
34
java.lang.StringBuffer
+StringBuffer()
Constructs an empty string buffer with capacity 16
+StringBuffer(capacity: int)
Constructs a string buffer with the specified capacity
+StringBuffer(str: String)
Constructs a string buffer with the specified string
+append(data: char[]): StringBuffer
Appends a char array into this string buffer
+append(data: char[], offset: int, len: int): StringBuffer
Appends a subarray in data into this string buffer
+append(v: aPrimitiveType): StringBuffer
Appends a primitive type value as string to this buffer
+append(str: String): StringBuffer
Appends a string to this string buffer
+capacity(): int
Returns the capacity of this string buffer
+charAt(index: int): char
Returns the character at the specified index
+delete(startIndex: int, endIndex: int): StringBuffer
Deletes characters from startIndex to endIndex
+deleteCharAt(int index): StringBuffer
Deletes a character at the specified index
+insert(index: int, data: char[], offset: int, len: int):
StringBuffer
Inserts a subarray of the data in the array to the buffer at
the specified index
+insert(offset: int, data: char[]): StringBuffer
Inserts data to this buffer at the position offset
+insert(offset: int, b: aPrimitiveType): StringBuffer
Inserts a value converted to string into this buffer
+insert(offset: int, str: String): StringBuffer
Inserts a string into this buffer at the position offset
+length(): int
Returns the number of characters in this buffer
+replace(int startIndex, int endIndex, String str):
StringBuffer
Replaces the characters in this buffer from startIndex to
endIndex with the specified string
+reverse(): StringBuffer
Reveres the characters in the buffer
+setCharAt(index: int, ch: char): void
Sets a new character at the specified index in this buffer
+setLength(newLength: int): void
Sets a new length in this buffer
+substring(startIndex: int): String
Returns a substring starting at startIndex
+substring(startIndex: int, endIndex: int): String
Returns a substring from startIndex to endIndex
35
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.
36
Appending New Contents
into a String Buffer
StringBuffer strBuf = new StringBuffer();
strBuf.append("Welcome");
strBuf.append(' ');
strBuf.append("to");
strBuf.append(' ');
strBuf.append("Java");
37
 strBuf
contains “Welcome to Java”
– strBuf.insert(11, “HTML and “);
– Insert string “HTML and “ at position 11.
 Now
strBuf contains
– “Welcome to HTML and Java”
38
 strBuf.delete(8,
11) changes the buffer to
“Welcome Java”
 strBuf.deleteCharAt(8) changes the buffer
to “Welcome o Java”
 strBuf.reverse() changer the buffer to “avaJ
ot emocleW”
 strBuf.replace(11, 15, “HTML”) changes
the buffer to “Welcome to HTML”
 strBuf.setCharAt(), ‘w’) sets the buffer to
“welcome to Java”;
39
Example:
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
Run
40
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]);
}
}
41
Command-Line Parameters
class TestMain {
public static void main(String[] args) {
...
}
}
java TestMain arg0 arg1 arg2 ... argn
Don’t need double quotes because separated by
space
 If a string contains a space, use double quotes to
separate them.

42
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.
43
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.
java Calculator 2 + 3
Calculator
Run
java Calculator 2 - 3
java Calculator 2 / 3
java Calculator 2 “*” 3
44
Optional
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.
45
Optional
Matching Strings
"Java".matches("Java");
"Java".equals("Java");
They are similar and give value true.
However, the matches method is more powerful
"Java is fun".matches("Java.*")
"Java is cool".matches("Java.*")
"Java is powerful".matches("Java.*")
All true
The substring “.*” matches any zero or more characters.
46
Regular Expression Syntax
Optional
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}"
47
Optional
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.
48
Examples
String s = "Java Java Java".replaceAll("v\\w", "wi") ;
Output  Jawi Jawi Jawi
String s = "Java Java Java".replaceFirst("v\\w", "wi") ;
Output  Jawi Java Java
String[] s = "Java1HTML2Perl".split("\\d");
49
Pedagogical NOTE
The previous edition of this book introduced text I/O
using many subclasses of java.io.Writer and
java.io.Reader. These classes are lower-level and difficult
to learn. The java.util.PrintWriter and java.util.Scanner
classes are higher-level and easy to use. All the programs
written using the lower-level text I/O classes can be
revised using the PrintWriter and Scanner classes.
Therefore, the PrintWriter and Scanner classes supersede
the lower-level text I/O classes. If students need to know
these lower-level text I/O classes, please refer to
Supplement V.I, “Text I/O Using Reader and Writer.”
50
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.
51
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.
52
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
53
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.
54
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
55
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
56
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
57
The StringTokenizer Class
java.util.StringTokenizer
+StringTokenizer(s: String)
Constructs a string tokenizer for the string.
+StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string
with the specified delimiters.
String)
+StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string
with the delimiters and returnDelims.
String, returnDelimiters: boolean)
Returns the number of remaining tokens.
+countTokens(): int
+hasMoreTokens(): boolean
Returns true if there are more tokens left.
+nextToken(): String
Returns the next token.
+nextToken(delimiters: String): String Returns the next token using new delimiters.
58
Examples 1
String s = "Java is cool.";
StringTokenizer tokenizer = new StringTokenizer(s);
System.out.println("The total number of tokens is " +
tokenizer.countTokens());
while (tokenizer.hasMoreTokens())
System.out.println(tokenizer.nextToken());
The code displays
The total number of tokens is 3
Java
is
cool.
59
Examples 2
String s = "Java is cool.";
StringTokenizer tokenizer = new StringTokenizer(s, "ac");
System.out.println("The total number of tokens is " +
tokenizer.countTokens());
while (tokenizer.hasMoreTokens())
System.out.println(tokenizer.nextToken());
The code displays
The total number of tokens is 4
J
v
is
ool.
60
Examples 3
String s = "Java is cool.";
StringTokenizer tokenizer = new StringTokenizer(s, "ac", ture);
System.out.println("The total number of tokens is " +
tokenizer.countTokens());
while (tokenizer.hasMoreTokens())
System.out.println(tokenizer.nextToken());
The code displays
The total number of tokens is 7
J
a
v
a
is
c
ool.
61
No no-arg Constructor in
StringTokenizer
The StringTokenizer class does not have a no-arg
constructor. Normally it is a good programming
practice to provide a no-arg constructor for each
class. On rare occasions, however, a no-arg
constructor does not make sense. StringTokenizer
is such an example. A StringTokenizer object
must be created for a string, which should be
passed as an argument from a constructor.
62
JDK 1.5
Feature
The Scanner Class
The delimiters are single characters in StringTokenizer. You can
use the new JDK 1.5 java.util.Scanner class to specify a word as a
delimiter.
String s = "Welcome to Java! Java is fun! Java is cool!";
Scanner scanner = new Scanner(s);
scanner.useDelimiter("Java");
Creates an instance of Scanner for the
string.
Sets “Java” as a delimiter.
while (scanner.hasNext())
System.out.println(scanner.next());
Welcome to
!
is fun!
is cool!
hasNext() returns true if there are
still more tokens left.
The next() method returns a
token as a string.
Output
63
JDK 1.5
Feature
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.
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);
64
JDK 1.5
Feature
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:
System.out.print("Please enter an int value: ");
Scanner scanner = new Scanner(System.in);
int i = scanner.nextInt();
NOTE: StringTokenizer can specify several single characters as
delimiters. Scanner can use a single character or a word as the
delimiter. So, if you need to scan a string with multiple single
characters as delimiters, use StringTokenizer. If you need to use a
word as the delimiter, use Scanner.
65
Optional
GUI
Containers and Layout Managers
 Objective:
Add a button to the frame.
HoldComponents
Run
66
Optional
GUI
Using FlowLayout Manager
 Objective:
Add a button to the frame of
FlowLayout.
UseFlowLayout
Run
67
Optional
GUI
Adding Multiple Components
 Objective:
Add two buttons to the frame of
FlowLayout.
TwoButtons
Run
68