Chapter 8

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Chapter 8 Strings
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1
Objectives
F
F
F
F
F
F
F
F
F
To use the String class to process fixed strings (§9.2).
To use the Character class to process a single character (§9.3).
To use the StringBuilder/StringBuffer class to process flexible
strings (§9.4).
To distinguish among the String, StringBuilder, and StringBuffer
classes (§9.2-9.4).
To learn how to pass arguments to the main method from the
command line (§9.5).
To discover file properties and to delete and rename files using
the File class (§9.6).
To write data to a file using the PrintWriter class (§9.7.1).
To read data from a file using the Scanner class (§9.7.2).
(GUI) To open files using a dialog box (§9.8).
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Motivations
Often you encounter the problems that involve string
processing and file input and output. Suppose you need to
write a program to replace all occurrences of a word with a
new word in a file. How do you solve this problem?
This chapter introduces strings and text files,
which will enable you to solve this problem.
• A string is a sequence of characters.
• Strings are treated as an array of characters, but
in Java string is an object.
• The String class has 11 constructors and more
than 40 methods for manipulating strings.
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3
The String Class
F
Constructing a String:
– String message = "Welcome to Java“;
– String message = new String("Welcome to Java“);
– String s = new String();
F
F
F
F
F
F
F
F
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
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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";
Here are some more examples of how strings can be used:
System.out.println("abc");
String cde = "cde";
System.out.println("abc" + cde);
String c = "abc".substring(2,3);
String d = cde.substring(1, 2);
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5
Strings Are Immutable
A String object is immutable; its contents cannot be
changed. Does the following code change the contents
of the string? The answer is no.
String s = "Java";
/*creates a String object with content “Java” and assigns
its reference to s. */
s = "HTML";
// create a new String object with content “HTML” and assigns to s.
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6
animation
Trace Code
After executing String s= “Java”;
Content cannot be changed
String s = "Java";
s = "HTML";
After executing s = “HTML”
String are immutable; once
created, their contents cannot be
changed.
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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7
Interned Strings
Take a look a the String deeper.
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. For example, the following
statements:
String s1 = “Welcome to Java”;
String s2 = new String (“Welcome to Java”);
String s3 = “Welcome to Java”;
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8
Examples
String s1 = "Welcome to Java";
String s2 = new String("Welcome to Java");
s1
: String
s3
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
System.out.println("s1 == s2 is " + (s1 == s2)); s2
System.out.println("s1 == s3 is " + (s1 == s3));
display
s1 == s2 is false
s1 == s3 is true
: 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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9
animation
Trace Code
String s1 = "Welcome to Java";
String s2 = new String("Welcome to Java");
s1
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
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10
Trace Code
String s1 = "Welcome to Java";
s1
String s2 = new String("Welcome to Java");
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
s2
: String
A string object for
"Welcome to Java"
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11
Trace Code
String s1 = "Welcome to Java";
s1
: String
s3
String s2 = new String("Welcome to Java");
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
s2
: String
A string object for
"Welcome to Java"
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12
String Comparisons
java.lang.String
+equals(s1: String): boolean
Returns true if this string is equal to string s1.
+equalsIgnoreCase(s1: String):
boolean
Returns true if this string is equal to string s1 caseinsensitive.
+compareTo(s1: String): int
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.
+compareToIgnoreCase(s1: String):
int
Same as compareTo except that the comparison is caseinsensitive.
+regionMatches(toffset: int, s1: String, Returns true if the specified subregion of this string exactly
offset: int, len: int): boolean
matches the specified subregion in string s1.
+regionMatches(ignoreCase: boolean, Same as the preceding method except that you can specify
toffset: int, s1: String, offset: int,
whether the match is case-sensitive.
len: int): boolean
+startsWith(prefix: String): boolean
Returns true if this string starts with the specified prefix.
+endsWith(suffix: String): boolean
Returns true if this string ends with the specified suffix.
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String Comparisons
F
Statements display true and false.
String s1 = new String("Welcome“);
String s2 = "welcome";
String s3 =“Welcome to C++”;
system.out.println(s1.equals(s2)); //true
system.out.println(s1.equals(s3)); //false
if (s1.equals(s2)){
// s1 and s2 have the same contents
}
if (s1 == s2) {
/* Check to see if only s1 and s2 are equal and refer to
the same object. s1 and s2 have the same reference*/
}
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String Comparisons, cont.
F
compareTo(Object object)
String s1 = new String("Welcome“);
String s2 = "welcome";
if (s1.compareTo(s2) > 0) {
//compareTo method can also be used to compare two strings
// 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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compare
15
String Length, Characters, and
Combining Strings
The String class provides the methods for obtaining length,
retrieving individual characters, and concate strings as shown
below:
java.lang.String
+length(): int
Returns the number of characters in this string.
+charAt(index: int): char
Returns the character at the specified index from this string.
+concat(s1: String): String
Returns a new string that concatenate this string with string s1.
string.
You can get the length of a string by invoking its length() method.
Example:
message.length() //returns the length of the string message.
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16
Retrieving Individual Characters
in a String
F
Do not use message[0]
F
Use message.charAt(index)
F
Index starts from 0
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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17
String Concatenation
We can use the concate method to concatenate two strings.
The below example, cancate strings s1, s2, into s3.
String s3 = s1.concat(s2);
Also plus (+) sing works to concate two or more strings.
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, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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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";
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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Converting, Replacing, and Splitting
Strings
The String class provides the methods for converting, replacing,
and splitting string, as show below
java.lang.String
+toLowerCase(): String
Returns a new string with all characters converted to lowercase.
+toUpperCase(): String
Returns a new string with all characters converted to uppercase.
+trim(): String
Returns a new string with blank characters trimmed on both sides.
+replace(oldChar: char,
newChar: char): String
Returns a new string that replaces all matching character in this
string with the new character.
+replaceFirst(oldString: String, Returns a new string that replaces the first matching substring in
newString: String): String
this string with the new substring.
+replaceAll(oldString: String, Returns a new string that replace all matching substrings in this
newString: String): String
string with the new substring.
+split(delimiter: String):
Returns an array of strings consisting of the substrings split by the
String[]
delimiter.
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Examples
"Welcome".toLowerCase() returns a new string, welcome.
"Welcome".toUpperCase() returns a new string,
WELCOME.
" Welcome ".trim() returns a new string, Welcome.
"Welcome".replace('e', 'A') returns a new string, WAlcomA.
"Welcome".replaceFirst("e", "AB") returns a new string,
WABlcome.
"Welcome".replace("e", "AB") returns a new string,
WABlcomAB.
"Welcome".replace("el", "AB") returns a new string,
WABcome.
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Splitting a String
The split method can be sued to extract tokens from
a string with specified a character that identified
within the string beginning or end.
String[] tokens = "Java#HTML#Perl".split("#", 0);
for (int i = 0; i < tokens.length; i++)
System.out.print(tokens[i] + " ");
displays
Java HTML Perl
Just be careful to avoid certain special characters: [ ] . * ? \ ( ) | { }
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Matching, Replacing and Splitting by Patterns
You can match, replace, or split a string by specifying a pattern.
This is an extremely useful and powerful feature, commonly
known as regular expression. Regular expression is complex to
beginning students. For this reason, two simple patterns are
used in this section. Please refer to Supplement III.F, “Regular
Expressions,” for further studies.
"Java".matches("Java");
"Java".equals("Java");
"Java is fun".matches("Java.*");
"Java is cool".matches("Java.*");
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Matching, Replacing and Splitting by Patterns
The replaceAll, replaceFirst, and split methods can be used
with a regular expression. For example, the following statement
returns a new string that replaces $, +, or # in "a+b$#c" by the
string NNN.
String s = "a+b$#c".replaceAll("[$+#]", "NNN");
System.out.println(s);
Here the regular expression [$+#] specifies a pattern that
matches $, +, or #. So, the output is aNNNbNNNNNNc.
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Matching, Replacing and Splitting by Patterns
The following statement splits the string into an array of strings
delimited by some punctuation marks.
String[] tokens = "Java,C?C#,C++".split("[.,:;?]");
for (int i = 0; i < tokens.length; i++)
System.out.println(tokens[i]);
displays
Java C C# C++
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Finding a Character or a Substring
in a String
java.lang.String
+indexOf(ch: char): int
Returns the index of the first occurrence of ch in the string.
Returns -1 if not matched.
+indexOf(ch: char, fromIndex:
int): int
Returns the index of the first occurrence of ch after fromIndex in
the string. Returns -1 if not matched.
+indexOf(s: String): int
Returns the index of the first occurrence of string s in this string.
Returns -1 if not matched.
+indexOf(s: String, fromIndex: Returns the index of the first occurrence of string s in this string
int): int
after fromIndex. Returns -1 if not matched.
+lastIndexOf(ch: int): int
Returns the index of the last occurrence of ch in the string.
Returns -1 if not matched.
+lastIndexOf(ch: int,
Returns the index of the last occurrence of ch before fromIndex
fromIndex: int): int
in this string. Returns -1 if not matched.
+lastIndexOf(s: String): int
Returns the index of the last occurrence of string s. Returns -1 if
not matched.
+lastIndexOf(s: String,
Returns the index of the last occurrence of string s before
fromIndex: int): int
fromIndex. Returns -1 if not matched.
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Finding a Character or a
Substring in a String
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
11.
"Welcome
"Welcome
to
to
to
to
to
Java".indexOf('W') returns 0.
Java".indexOf('x') returns -1.
Java".indexOf('o', 5) returns 9.
Java".indexOf("come") returns 3.
Java".indexOf("Java", 5) returns
to Java".indexOf("java", 5) returns -1.
to Java".lastIndexOf('a') returns 14.
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Convert Character and Numbers to
Strings
String are not arrays, but a string can be converted
into an array and vice versa.
To convert a string to an array of characters, use
the toCharArray method.
Convert string “Java” to an array;
char[] chars = “Java”.toCharArray();
So, char[0] is ‘J’, char[1] is ‘a’, char[2] ‘v’, char[3]
‘a’.
You can also getChars (int srcBegin, int srcEnd)
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The Character Class
Java provides a wrapper class for every primitive type such as
Character, Boolean, Byte, Short, Integer, Long, Float, , and more.
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
You can create a character object from a char value
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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Problem: 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
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31
StringBuilder and StringBuffer
StringBuilder/StringBuffer class is
an alternative to the String class.
FThe
FIn
general, a StringBuilder/StringBuffer can be
used wherever a string is used.
F 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.
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String Builder and String Buffer
F
The most important difference between String
and StringBuffer/StringBuilder in java is that
– String object is immutable whereas
– StringBuffer/StringBuilder objects are mutable.
By immutable, we mean that the value stored in the
String object cannot be changed.
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Internally a new String object is created to do the
changes.
So suppose you declare a String object:
String myString = “Hello”;
Next, you want to append “Guest” to the same String.
What you can do is the following:
myString = myString + ” Guest”;
When you print the contents of myString the output
will be “Hello Guest”. Although we made use of the
same object(myString), internally a new object was
created in the process.
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Modifying Strings in the Builder
java.lang.StringBuilder
+append(data: char[]): StringBuilder
Appends a char array into this string builder.
+append(data: char[], offset: int, len: int):
StringBuilder
Appends a subarray in data into this string builder.
+append(v: aPrimitiveType): StringBuilder
Appends a primitive type value as a string to this
builder.
+append(s: String): StringBuilder
Appends a string to this string builder.
+delete(startIndex: int, endIndex: int):
StringBuilder
Deletes characters from startIndex to endIndex.
+deleteCharAt(index: int): StringBuilder
Deletes a character at the specified index.
+insert(index: int, data: char[], offset: int,
len: int): StringBuilder
Inserts a subarray of the data in the array to the builder
at the specified index.
+insert(offset: int, data: char[]):
StringBuilder
Inserts data into this builder at the position offset.
+insert(offset: int, b: aPrimitiveType):
StringBuilder
Inserts a value converted to a string into this builder.
+insert(offset: int, s: String): StringBuilder
Inserts a string into this builder at the position offset.
+replace(startIndex: int, endIndex: int, s:
String): StringBuilder
Replaces the characters in this builder from startIndex
to endIndex with the specified string.
+reverse(): StringBuilder
Reverses the characters in the builder.
+setCharAt(index: int, ch: char): void
Sets a new character at the specified index in this
builder.
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Using append
F
F
since StringBuffer/StringBuilder objects are
mutable, we can make changes to the value
stored in the object.
What this effectively means is that string
operations such as append would be more
efficient if performed using
StringBuffer/StringBuilder objects than
String objects.
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Examples
stringBuilder.append("Java");
stringBuilder.insert(11, "HTML and ");
stringBuilder.delete(8, 11) changes the builder to Welcome
Java.
stringBuilder.deleteCharAt(8) changes the builder to
Welcome o Java.
stringBuilder.reverse() changes the builder to avaJ ot
emocleW.
stringBuilder.replace(11, 15, "HTML")
changes the builder to Welcome to HTML.
stringBuilder.setCharAt(0, 'w') sets the builder to welcome
to Java.
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The toString, capacity, length,
setLength, and charAt Methods
java.lang.StringBuilder
+toString(): String
Returns a string object from the string builder.
+capacity(): int
Returns the capacity of this string builder.
+charAt(index: int): char
Returns the character at the specified index.
+length(): int
Returns the number of characters in this builder.
+setLength(newLength: int): void
Sets a new length in this builder.
+substring(startIndex: int): String
Returns a substring starting at startIndex.
+substring(startIndex: int, endIndex: int):
String
Returns a substring from startIndex to endIndex-1.
+trimToSize(): void
Reduces the storage size used for the string builder.
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Problem: 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
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39
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]);
}
}
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Problem: Calculator
F
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
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Java - parseInt() Method
Description:
This method is used to get the primitive data type of a certain String.
parseXxx() is a static method and can have one argument or
two.
Syntax:
All the variant of this method are given below:
static int parseInt(String s)
static int parseInt(String s, int radix)
Parameters:
Here is the detail of parameters:
String s : This is a string representation of decimal.
int radix : This would be used to convert String s into integer.
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Return Value :
•parseInt(String s): This returns an integer (decimal only).
•parseInt(int i): This returns an integer, given a string
representation of decimal, binary, octal, or hexadecimal (radix equals
10, 2, 8, or 16 respectively) numbers as input.
•Example:
public class Test{
public static void main(String args[]){
int x = Integer.parseInt("9");
double c = Double.parseDouble("5");
int b = Integer.parseInt("444",10);
System.out.println(x);
System.out.println(c);
System.out.println(b);
}
}
Result
9
5.0
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444
43
Companion
Website
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.
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Companion
Website
Matching Strings
"Java".matches("Java");
"Java".equals("Java");
"Java is fun".matches("Java.*")
"Java is cool".matches("Java.*")
"Java is powerful".matches("Java.*")
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Companion
Website
Regular Expression Syntax
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}"
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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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.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Examples
String s = "Java Java Java".replaceAll("v\\w", "wi") ;
String s = "Java Java Java".replaceFirst("v\\w", "wi") ;
String[] s = "Java1HTML2Perl".split("\\d");
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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