String Variables

1
String Variables

• String variables have always presented a
  challenge for programming languages.
• They have varying sizes, and for internal storage
  purposes, the computer would prefer to predict in
  advance the amount of storage needed for the
  value of a variable.
• As a result, strings are often a special case
  in a programming language.

2
Strings in Java

• Strings in Java are also a reference type.
• They are similar to an array of characters in
  that they represent an ordered sequence of
  characters with positions numbered from 0.
• String constants are enclosed in double quotes.
• Example
• String message "Hello World!"
• System.out.println( message )
• The class String provides many useful methods.
• See page 287 of the text for a longer list of
  useful methods.
• The complete list is at http//java.sun.com/j2se/1
  .4.2/docs/api/

3
Comparison with character arrays

• Although a String has many similarities to an
  array of characters, there are some important
  differences.
• You dont need to use new to create a string
  unless you want to use one of the String class
  constructors
• String s1 "Hello!"
• String s2 new String("Hello!")
• You DO NOT use to access the characters in the
  string.
• The contents of a String cannot be changed after
  it has been created.
• You can have a String variable refer to a
  different string, but you cannot change
  characters within the original string.
• There is NO equivalent to
• char x new char 'h','e','l','l','o'
• x2 'q'

4
Conversion to/from character arrays

• To convert from a character array to a String
• char x new char 'h','e','l','l','o'
• String s1 new String( x )
• To convert from a String to a character array
• char x aString.toCharArray( )

5
Useful String methods

• Suppose we have
• String message "Hello World!"
• To find the length of a string
• int theStringLength message.length()
• To find the character at position i (numbered
  from 0)
• int i 4
• char theChar message.charAt( i )

6
Useful String methods

• To change any primitive data type to a String
• int anInteger 17
• String aString String.valueOf( anInteger )
• To append one string after another
  (concatenation)
• String joinedString string1 string2

7
Useful String methods

• To find a particular character or String within a
  String
• First occurrence of 'x'
• int location aString.indexOf('x')
• First occurrence of "world" starting from
  position pos
• int location2 aString.indexOf( "world", pos )
• There is also a corresponding method lastIndexOf(
  ) occurrence of "world" starting from position
  pos

8
Useful String methods

• Substrings
• The substring method returns a String consisting
  of the characters starting from the first
  position inclusive up to but NOT including the
  second position
• String str "abcdefghijklmno"
• String str2 str.substring( 2, 5 )
• Result is "cde
• Case changes toLowerCase( ), toUpperCase( )

9
Comparing Strings

• A String is a reference type and so they are NOT
  compared with .
• The String class has a method compareTo() to
  compare 2 strings.
• The characters in each string are compared one at
  a time from left to right, using the collating
  sequence.
• The comparison stops after a character comparison
  results in a mismatch, or one string ends before
  the other.
• If str1 lt str2, then compareTo() returns an int lt
  0
• If str1 gt str2, then compareTo() returns an int gt
  0
• If the character at every index matches, and the
  strings are the same length, the method returns 0

10
Comparing Strings

• What is the value of result for these examples?
• Example 1
• String str1 "abcde"
• String str2 "abcfg"
• int result str1.compareTo(str2)
• Example 2
• String str1 "abcde"
• String str2 "ab"
• int result str1.compareTo(str2)

11
The class StringBuffer

• This class is used for modifiable strings.
• The main use is for collecting large amounts of
  text that is built from smaller strings.
• Methods
• append( aValue )
• Appends a string representation of aValue to the
  end of the buffer.
• aValue could be a string or a value of any
  primitive type int, double, boolean, etc.
• insert( position, aValue )
• Inserts a string at a specific position in the
  buffer.

12
The class StringTokenizer

• This class is useful for separating a large
  string into smaller parts.
• Tokens are the desired parts of the string
• For example words.
• The class uses delimiters to identify the
  points at which a string must be split into
  tokens.
• By default, the delimiters are the set of white
  space characters blank, tab, new-line.
• You can supply your own list of characters as the
  set of delimiters.
• You can also choose whether a delimiter is
  returned or not as a token.
• Normally, they are not returned as tokens

13
StringTokenizer methods

• The string tokenizer is set up to return one
  token at a time, as long as there are tokens
  remaining.
• Each time a token is returned, the tokenizer
  internally moves to the next token.
• Create a tokenizer
• import java.util.StringTokenizer
• StringTokenizer st new StringTokenizer( aString
  )
• Method to count the number of tokens
• int numberOfTokens st.countTokens( )
• Method that returns true if there are more tokens
  remaining
• boolean moreTokens st.hasMoreTokens( )
• Method to return the next token
• String aString st.nextToken( )

14
StringTokenizer constructors

• Create a string tokenizer that does not include
  the delimiters as tokens
• StringTokenizer st new StringTokenizer( aString
  )
• Create a string tokenizer than uses each
  character of the second string as a delimiter
• StringTokenizer st2 new StringTokenizer(
  aString, " .,!\n\t" )
• Create a string tokenizer that returns the
  delimiters as tokens
• StringTokenizer st3 new StringTokenizer(
  aString, " \n\t", true )