Characters, String and Regular expressions

1
Characters, String and Regular expressions
2
Characters

• char data type is used to represent a single
  character.
• Characters are stored in a computer memory using
  some form of encoding.
• Java uses Unicode, which includes ASCII, for
  representing char constants.

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ASCII Encoding
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Unicode Encoding

• The Unicode Worldwide Character Standard
  (Unicode) supports the interchange, processing,
  and display of the written texts of diverse
  languages.
• Java uses the Unicode standard for representing
  char constants.

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Character Processing
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Strings

• A string is a sequence of characters that is
  treated as a single value.
• Instances of the String class are used to
  represent strings in Java.

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String declaration

• Create a String object
• String ltvariable namegt
• ltvariable namegtnew String(ltvalue of a
  stringgt)
• Create a String literal
• String ltvariable namegt
• ltvariable namegt ltvalue of a string

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Example

• String word1
• word1 new String(Java)
• OR
• String word1
• word1 Java

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Examples
We can do this because String objects
are immutable.
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String constructor

• No-argument constructor
• One-argument constructor
• A String object
• One-argument constructor
• A char array
• Three-argument constructor
• A char array
• An integer specifies the starting position
• An integer specifies the number of characters to
  access

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Review

• To compute how many characters the string
  myString contains, we use
• myString.size
• myString.size()
• myString.length
• myString.length()

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Review

• To compute how many characters the string
  myString contains, we use
• myString.size
• myString.size()
• myString.length
• myString.length()

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Review

• Java uses this to represent characters of diverse
  languages
• ASCII
• UNICODE
• EDBIC
• BINARY

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Review

• Java uses this to represent characters of diverse
  languages
• ASCII
• UNICODE
• EDBIC
• BINARY

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Accessing Individual Elements

• Individual characters in a String accessed with
  the charAt method.

name.charAt( 3 )
name
The method returns the character at position 3.
This variable refers to the whole string.
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Other Useful String Operators
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Compute Length of a string

• Method length()
• Returns the length of a string
• Example
• String strVar
• strVar new String(Java)
• int len strVar.length()

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Substring

• Method
• Extract a substring from a given string by
  specifying the beginning and ending positions
• Example
• String strVar, strSubStr
• strVar new String(Exam after Easter)
• strSubStr strVar.substring(0,4)

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Index position of a substring within another
string

• Method
• Find an index position of a substring within
  another string.
• Example
• String strVar1 Google it
• String strVar2 Google
• int index
• index strVar1.indexOf(strVar2)

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String concatenation

• Method
• Create a new string from two strings by
  concatenating the two strings.
• Example
• String strVar1 Google
• String strVar2 Search Engine
• String sumStr
• sumStr strVar1.concat(strVar2)

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Review

• What method is used to refer to individual
  character in a String
• getBytes
• indexOf
• getChars
• charAt

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Review

• What method is used to refer to individual
  character in a String
• getBytes
• indexOf
• getChars
• charAt

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Review

• To compare two strings in Java, we use
• equals method
• !
• ltgt

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Review

• To compare two strings in Java, we use
• equals method
• !
• ltgt

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String comparison

• Methods
• equals
• equalsIgnoreCase
• compareTo
• compareToIgnoreCase

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String comparison

• equals, and equalsIgnoreCase
• Example
• String string1 COMPSCI
• String string2 compsci
• boolean isEqual
• isEqual string1.equals(string2)

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Common error

• Comparing references with can lead to logic
  errors, because compares the references to
  determine whether they refer to the same object,
  not whether two objects have the same contents.
  When two identical (but separate) objects are
  compared with , the result will be false. When
  comparing objects to determine whether they have
  the same contents, use method equals.

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String comparison

• compareTo
• Example
• String string1 Adam
• String string2 AdamA
• int compareResult
• compareResult string1.compareTo(string2)

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String comparison

• - string1.compareTo(string2)
• Compares two strings lexicographically
• will return 0 if two strings are equal
• will return negative value if string1 is less
  than string 2
• will return positive value if string1 is
  greater than string 2
• The comparison is based on the Unicode value of
  each character in the strings

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String comparison

• The comparison is based on the Unicode value of
  each character in the strings
• let k be the smallest index valid for both
  strings
• compareTo returns the difference of the two
  character values at position k in the two string
  -- that is, the value
• character at the position k of string 1
  character at the position k of string 2

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regionMatches

• regionMatches(boolean ignoreCase, int toffset,
  String other, int ooffset, int len)
• A substring of this String object is compared to
  a substring of the argument other.
• The result is true if these substrings represent
  character sequences that are the same, ignoring
  case if and only if ignoreCase is true.

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The String Class is Immutable

• In Java a String object is immutable
• This means once a String object is created, it
  cannot be changed, such as replacing a character
  with another character or removing a character
• The String methods we have used so far do not
  change the original string. They created a new
  string from the original. For example, substring
  creates a new string from a given string.

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Review

• If x.equals(y) is true, then xy is always true
• True
• False

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Review

• If x.equals(y), then xy is always true
• True
• False

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The StringBuffer Class

• In many string processing applications, we would
  like to change the contents of a string. In other
  words, we want it to be mutable.
• Manipulating the content of a string, such as
  replacing a character, appending a string with
  another string, deleting a portion of a string,
  and so on, may be accomplished by using the
  StringBuffer class.

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StringBuffer Example
Changing a string Java to Diva
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Delete a substring from a StringBuffer object

• StringBuffer word new StringBuffer(CCourse)
• word.delete(0,1)

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Append a string

• StringBuffer word new StringBuffer(CS )
• word.append(Course)

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Insert a string

• StringBuffer word new StringBuffer(MCS
  Course)
• word.insert(4,220)

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Convert from StringBuffer to String

• StringBuffer word new StringBuffer(Java)
• word.setCharAt(0,D)
• word.setCharAt(1,i)
• System.out.println(word.toString())

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Review

• Both the String and StringBuffer classes include
  the charAt and setCharAt methods
• True
• False

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Review

• Both the String and StringBuffer classes include
  the charAt and setCharAt methods
• True
• False

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Review

• What will be the value of str after the following
  statements are executed
• String str
• StringBuffer strBuf
• str "Decaffeinated"
• strBuf new StringBuffer(str.substring(2,7))
• strBuf.setCharAt(1,'o')
• strBuf.append('e')
• str strBuf.toString()

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StringBuffer methods

• Method length
• Return StringBuffer length
• Method capacity
• Return StringBuffer capacity
• Method setLength
• Increase or decrease StringBuffer length
• Method ensureCapacity
• Set StringBuffer capacity
• Guarantee that StringBuffer has minimum capacity

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Class StringTokenizer

• Tokenizer
• Partition String into individual substrings
• Use delimiter
• Typically whitespace characters (space, tab,
  newline, etc)
• Java offers java.util.StringTokenizer

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Outline
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Pattern Example

• Suppose students are assigned a three-digit code
• The first digit represents the major (5 indicates
  computer science)
• The second digit represents either in-state (1),
  out-of-state (2), or international (3)
• The third digit indicates campus housing
• On-campus dorms are numbered 1-7.
• Students living off-campus are represented by the
  digit 8.

51231-7
The 3-digit pattern to represent computer science
majors living on-campus is
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Regular Expressions, Class Pattern and Class
Matcher

• Regular expression
• Sequence of characters and symbols
• Useful for validating input and ensuring data
  format
• Facilitate the construction of a compiler
• Regular-expression operations in String
• Method matches
• Matches the contents of a String to regular
  expression
• Returns a boolean indicating whether the match
  succeeded

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Regular Expressions, Class Pattern and Class
Matcher

• Predefine character classes
• Escape sequence that represents a group of
  character
• Digit
• Numeric character
• Word character
• Any letter, digit, underscore
• Whitespace character
• Space, tab, carriage return, newline, form feed

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Predefined character classes.
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Regular Expressions

• Other patterns
• Square brackets ()
• Match characters that do not have a predefined
  character class
• E.g., aeiou matches a single character that is
  a vowel
• Dash (-)
• Ranges of characters
• E.g., A-Z matches a single uppercase letter
• Not include the indicated characters
• E.g., Z matches any character other than Z

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Regular expression

• Quantifiers
• Plus ()
• Match one or more occurrences
• E.g., A
• Matches AAA but not empty string
• Asterisk ()
• Match zero or more occurrences
• E.g., A
• Matches both AAA and empty string
• Others in Fig. 29.22

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Quantifiers used in regular expressions.

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Regular Expression Examples
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Regular expression

• Replacing substrings and splitting strings
• String method replaceAll
• Replace text in a string with new text
• String method replaceFirst
• Replace the first occurrence of a pattern match
• String method split
• Divides string into several substrings

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Regular expression

• Class Pattern
• Represents a regular expression
• Class Match
• Contains a regular-expression pattern and a
  CharSequence
• Interface CharSequence
• Allows read access to a sequence of characters
• String and StringBuffer implement CharSequence

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Regular Expression Examples
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Matching

• Searches for 2 character pattern whose first
  character may be any uppercase letter between A
  and G, and whose second character may be any
  number except 4
• Searches for a character pattern that may be any
  alphabet except p,q,r,s, or t
• A. str.matches(a-zA-Zpqrst)
• B. str.matches(A-G0-9 4)

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Matching

• Searches for 2 character pattern whose first
  character may be any uppercase letter between A
  and G, and whose second character may be any
  number except 4
• Searches for a character pattern that may be any
  alphabet except p,q,r,s, or t
• A. str.matches(a-zA-Zpqrst)
• B. str.matches(A-G0-94)

B
A
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Review

• Which character sequence would be used to
  designate a character pattern of a fixed length
  of three digits
• 30-9
• 0-90-90-9
• 0-93

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Review

• Which character sequence would be used to
  designate a character pattern of a fixed length
  of three digits
• 30-9
• 0-90-90-9
• 0-93

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Review

• Choose the correct argument for the following
  code that searches for any number between 100 and
  999 in a given string.
• str.matches( )
• 0-90-91-9
• 0-90-90-9
• 1-90-90-9

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Review

• Choose the correct argument for the following
  code that searches for any number between 100 and
  999 in a given string.
• str.matches( )
• 0-90-91-9
• 0-90-90-9
• 1-90-90-9