Strings in Java

1
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.

2
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'

3
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( )

4
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 )

5
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

6
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

7
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( )

8
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

9
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)