Characters and Strings

1
Chapter 9

• Characters and Strings

2
Topics

• Character primitives
• Character Wrapper class
• More String Methods
• String Comparison
• String Buffer
• String Tokenizer

3
Character Data

• In Java, single characters are represented using
  the primitive data type char.
• Character constants are written as symbols
  enclosed in single quotes
• char ch1 'X'

4
Character Encoding

• Characters are stored in memory as integer values
• Each character has a unique "code" that is used
  to represent it.
• Having a standard encoding allows different
  computers to share information easily.
• Several encoding schemes have been used
• ASCII
• EBCDIC (not used any more)
• Unicode

5
ASCII Codes

• ASCII stands for American Standard Code for
  Information Interchange.
• ASCII is one of the document coding schemes
  widely used today.
• each character represented by 8 bits ( values
  0-255)
• ASCII works well for English-language documents
  because all characters and punctuation marks are
  included in the ASCII codes.
• ASCII does not represent the full character sets
  of other languages.

6
Unicode

• 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.
• Each character is represented with 2 bytes (16
  bits).

7
Character Class

• Like the other primitive types, there is a
  wrapper class for the char type.
• Useful Methods
• charValue( ) returns a char that is the value
  stored in the Character
• getNumericValue( ) returns the Unicode value for
  the char

8
Static CharacterMethods

• boolean methods
• isDigit( char)
• isLetter( char)
• isSpace( char)
• isUpperCase( char)
• isLowerCase(char)
• other methods
• char toLowerCase( char)
• toUpperCase( char)

9
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.
• We access individual characters of a string by
  calling the charAt method of the String object.

10
Strings

• Each character in a string has an index that we
  can use to access the character.
• We know for learning about arrays that Java uses
  zero-based indexing
• the first characters index is 0, the second is
  1, and so on.
• To refer to the first character of the word name,
  we say
• name.charAt(0)

11
String Indexing

• An indexed expression is used to refer to
  individual characters in a string.

12
Strings

• Since String is a class, we can create an
  instance of a class by using the new method.
• The statements we have used so far, such as
• String name1 Kona
• works as a shorthand for
• String name1 new String(Kona)
• But this shorthand works for the String class
  only.

13
Other String Methods

• String toLowerCase()
• String toUpperCase()
• String replace( char oldChar,
• char newChar)
• and many more

14
String Comparison

• String comparison may be done in several ways.
• The methods equals and equalsIgnoreCase compare
  string values one is case-sensitive and one is
  not.
• The method compareTo returns an int value
• Zero (0) if the strings are equal.
• A negative integer if the first string is less
  than the second.
• A positive integer if the first string is greater
  than the second.

15
String Equality

• Comparing String objects is similar to comparing
  other objects.
• The equality test () is true if the contents of
  the variables are the same.
• For a reference data type, the equality test is
  true if both variables refer to the same object,
  because they both contain the same address.
• The equals method is true if the String objects
  to which the two variables refer contain the same
  string value.
• the same sequence of characters

16
Equality Testing

• The difference between the equality test and the
  equals method.

17
Equality Testing
18
Equality Testing for Strings

• As long as a new String object is created using
  the new operator, the rule for comparing objects
  applies to comparing strings.
• String str new String (Java)
• If the new operator is not used, a String
  reference refers to the literal string constant.
• String str Java
• There will be only one copy of any literal string

19
Memory Diagram

• The difference between using and not using the
  new operator for String.

20
Immutability

• When a String object is created, it cannot be
  changed.
• Look at the documentation - there are no mutator
  methods
• We say Strings are immutable.
• Manipulating the content of a String requires the
  creation of a new String object.
• If you are doing a lot of manipulation this is
  not very efficient.

21
StringBuffer Class

• 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 more efficiently
  using the StringBuffer class.

22
StringBuffer Example

• StringBuffer word new StringBuffer(Java)
• word.setCharAt(0, D)
• word.setCharAt(1, i )
• changes the string from Java to Diva.

23
Ch9ReplaceVowelsWithX.java

• This example reads a sentence and replaces all
  vowels in the sentence with the character X.
• tempStringBuffer
• new StringBuffer(inSentence)
• numberOfCharacters tempStringBuffer.length()
• for (int index 0 index lt numberOfCharacters
  index)
• if ( isVowel( letter)
• tempStringBuffer.setCharAt(index,'X')
  

24
StringBuffer Operations

• We cannot input StringBuffer objects.
• We must input String objects and convert them to
  StringBuffer objects.
• We use the append method to append a String or
  StringBuffer object to the end of a StringBuffer
  object.
• The method append can also take an argument of
  the primitive data type.
• Any primitive data type argument is converted to
  a string before it is appended to a StringBuffer
  object.

25
StringBuffer Insertion

• We can insert a string at a specified position by
  using the insert method.
• ltStringBuffergt.insert(ltinsertIndexgt, ltvaluegt)
• where ltinsertIndexgt must be greater than or
  equal to 0 and less than or equal to the length
  of ltStringBuffergt, and the ltvaluegt can be an
  object or a primitive.
• For example, executing
• StringBuffer str
• new StringBuffer(Java is great)
• str.insert(8, really)
• changes the string "Java is great" to "Java is
  really great".

26
String Tokenizer

• A StringTokenizer object can be used to break a
  string up into smaller substrings (tokens)
• By default, the tokenizer separates the string at
  white space
• The default StringTokenizer constructor takes the
  original string to be separated as a parameter
• StringTokenizer st
• new StringTokenizer( text)
• An alternate constructor allows you to specify
  different separators (delimiters).
• StringTokenizer st
• new StringTokenizer( text, delims)

27
String Tokenizer Methods

• The hasMoreTokens method returns true if there
  are more tokens in the string.
• Each call to the nextToken method returns the
  next token in the string.
• If you need to know how many tokens there are,
  you can call countTokens