Jay Robert Fernandez, M'Sc'

1
Xavier University Computer Center Sun
Microsystems, Philippines
Java sa Eskwela (JsE) Program

• Jay Robert Fernandez, M.Sc.
• Mark Joseph Atienza
• Instructors

2
Chapter 10
Strings and Characters
3
Chapter 10

• Outline
• Fundamentals of Characters and Strings
• String Constructors
• String Methods length, charAt and getChars
• Comparing Strings
• Locating Characters and Substrings in Strings
• Extracting Substrings from Strings
• Concatenating Strings
• Miscellaneous String Methods
• Using String Method valueOf

4
Chapter 10

• Outline
• String Method intern
• StringBuffer Class
• StringBuffer Constructors
• StringBuffer Methods length, capacity, setLength
  and ensureCapacity
• StringBuffer Methods charAt, setCharAt, getChars
  and reverse
• StringBuffer append Methods
• StringBuffer Insertion and Deletion Methods

5
Objectives

• To be able to create and manipulate
  non-modifiable character string objects of class
  String
• To able to create and manipulate modifiable
  character string objects of class StringBuffer
• To be able to create and manipulate objects of
  class Character

6
String and Characters

• Characters
• building block of Java source programs
• groups of meaningful characters interpreted by
  computer as instructions
• character constant integer represented as a
  character in single quotes (Unicode character
  set)
• 'z' is integer value of z

7
String and Characters

• Strings
• group of characters treated as a single unit
• may include letters, digits, special characters
  (,- ...)
• Strings are objects of class String

8
String and Characters

• Strings (continued)
• String literal (string constants or anonymous
  String objects)
• sequence of characters written in double quotes
• "John Q. Doe" or "111222333"
• Java treats all anonymous String objects with
  same contents as one object with many references

9
String and Characters

• Strings (continued)
• assigning strings
• may assign in declaration
• String color "blue"
• color is a String reference
• "blue" is an anonymous String object

10
String and Characters

• Constructors for class String
• s1 new String()
• s1 is an empty string
• s2 new String( anotherString )
• s2 is a copy of anotherString
• s3 new String( charArray )
• s3 contains all the characters in charArray

11
String and Characters

• Constructors for class String
• s4 new String( charArray, offset, numElements )
• s4 contains numElements characters from
  charArray, starting at location offset
• s5 new String( byteArray, offset, numElements)
• as above, but with a byteArray
• s6 new String( byteArray )
• s6 contains copy of entire byteArray

12
String and Characters

• StringBuffer
• dynamically resizable, modifiable string (more
  later)
• StringBuffer buffer new StringBuffer("Hello
  there")
• can initialize Strings with StringBuffers
• s7 new String( myStringBuffer )

13
String and Characters

• String methods
• s1.length()
• returns length of string
• be careful, Strings do not have an instance
  variable length like arrays
• s1.length is an error
• s1.charAt( index )
• returns char at location index
• numbered like arrays (start at position 0)

14
String and Characters

• String methods (continued)
• s1.getChars( start, end, charArray, offset )
• copies characters in s1 from index start to index
  end to location offset in charArray

15
String and Characters

• Comparing Strings
• all characters represented as numeric codes
  (Appendix D)
• when computer compares strings, compares numeric
  codes
• lexicographical comparison - compare Unicode
  values
• lowercase different than uppercase

16
String and Characters

• Comparison methods
• s1.equals( "Hi" )
• returns true if s1 lexicographically equal to
  "Hi"
• capitalization (case) matters
• s1.equalsIgnoreCase( otherString )
• tests for equality, case does not matter
• "hello" equal to "HELLO"

17
String and Characters

• Comparison methods (continued)
• s1.compareTo( otherString )
• uses lexicographical comparison
• returns 0 if strings equal
• returns negative number if s1 lt otherString
• returns positive number if s1 gt otherString

18
String and Characters

• Comparison methods (continued)
• s1.regionMatches( offset, otherString, start, end
  )
• compares s1 from location offset to otherString
  from location start to end
• s1.regionMatches( ignoreCase, offset,
  otherString, start, end )
• as above, but if ignoreCase is true, case is
  ignored

19
String and Characters

• operator
• when used with primitive data types, returns true
  if equal
• when used with references, returns true if point
  to same location in memory
• remember, Java treats all anonymous String
  objects with same contents as one object with
  many references

20
String and Characters

• operator
• s1 "hello" //uses anonymous object
• s2 new String("hello")
• s1 "hello" //true
• s2 "hello" //false

21
String and Characters

• Search methods
• s1.indexOf( integerRep )
• integerRep - integer representation of a
  character ('z')
• returns index of the first occurrence of the
  character in the string (returns -1 if not found)
• s1.indexOf( integerRep, startSearch )
• as above, but begins search at position
  startSearch

22
String and Characters

• Search methods (continued)
• s1.lastIndexOf( integerRep )
• returns index of last occurrence of character in
  string
• s1.lastIndexOf( integerRep, startSearch )
• as above, but searches backwards from position
  startSearch

23
String and Characters

• Search methods (continued)
• can use indexOf and lastIndexOf with Strings
• search for substrings within a string
• identical usage as with characters, i.e.
• s1.indexOf( "hi" )
• s1.lastIndexOf( "this", 24 )

24
String and Characters

• substring methods
• return a String object
• s1.substring( startIndex )
• returns a substring from startIndex to the end of
  the string
• s1.substring( start, end )
• returns a substring from location start up to,
  but not including, location end
• if index out of bounds, StringIndexOutOfBoundsExce
  ption generated

25
String and Characters

• Method concat
• concatenates two Strings, returns new String
  object
• original Strings are not modified
• s1.concat( s2 )
• returns concatenation of s1 and s2
• if no argument, returns s1

26
String and Characters

• Miscellaneous methods
• s1.replace( char1, char2 )
• returns a new String, replacing all instances of
  char1 with char2
• use integer representations 'z', '1', etc.
• original String not changed
• if no instances of char1, original string returned

27
String and Characters

• Miscellaneous methods (continued)
• s1.toUpperCase()
• returns new String object with all letters
  capitalized (if no changes needed, original
  string returned)
• original string unchanged
• s1.toLowerCase() similar
• s1.trim()
• returns new String, removing all whitespace
  characters at beginning or end
• original unchanged

28
String and Characters

• Miscellaneous methods (continued)
• s1.toString()
• all objects can be converted to Strings with
  toString
• if not overridden, default version creates a
  String with object's name and hashcode
• s1.toCharArray()
• creates a new character array containing copy of
  characters in s1

29
String and Characters

• static methods of class String
• take arguments and convert them to Strings
• String.valueOf( charArray )
• returns new String object containing characters
  in charArray
• String.valueOf( charArray, startIndex, numChars )
• as above, but starts from position startIndex and
  copies numChars characters

30
String and Characters

• static methods of class String
• other versions of valueOf take boolean, char,
  int, long, float, double, and Object
• Objects converted to Strings with toString

31
String and Characters

• Method intern
• improves string comparison performance
• returns reference guaranteed to have same
  contents
• multiple interns generate references to same
  object
• Strings can be compared with (same location in
  memory) instead of equals
• much faster than comparing character by character

32
String and Characters

• class String
• once a String is created, it cannot be changed
• class StringBuffer
• can create modifiable Strings
• capable of storing number of characters specified
  by capacity
• if capacity exceeded, automatically expands to
  hold new characters
• can use and operators

33
String and Characters

• StringBuffer constructors
• sb1 new StringBuffer()
• creates empty StringBuffer, capacity of 16
  characters
• sb2 new StringBuffer(capacity)
• creates empty StringBuffer with specified
  capacity
• sb3 new StringBuffer(myString)
• creates a StringBuffer containing myString, with
  capacity myString.length() 16

34
String and Characters

• StringBuffer methods
• length returns length
• capacity returns capacity
• setLength( newLength )
• changes length of StringBuffer to newLength
• if too long, truncates excess characters
• if too short, fills with null character (numeric
  representation of 0)

35
String and Characters

• StringBuffer methods
• ensureCapacity(size)
• expands capacity to a minimum of size
• if size less than current capacity, then capacity
  is unchanged

36
String and Characters

• StringBuffer methods
• charAt( index ) returns character at position
  index
• setCharAt( index, char )
• sets character at position index to char
• getChars( start, end, charArray, loc )
• copies from position start up to (not including)
  end into location loc in charArray
• reverse reverses contents of StringBuffer

37
String and Characters

• Tokenization
• breaking statements into pieces
• class StringTokenizer breaks a String into
  component tokens
• tokens separated by delimiters, typically white
  space characters
• other characters may be used

38
String and Characters

• Class StringTokenizer
• one constructor StringTokenizer( myString )
• initialize StringTokenizer with a String
• default delimiter " \n\t\r" (space, newline, tab,
  carriage return)
• other constructors allow you to specify delimiter

39
String and Characters

• Methods
• countTokens()
• determines number of tokens in String
• nextToken()
• returns next token as a String
• nextToken( delimiterString )
• changes delimiter while tokenizing a String
• hasMoreTokens()
• determines if there are more tokens to be
  tokenized