Characters, Strings and the String Buffer

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Characters, Strings and the String Buffer

• Jim Burns

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Identifying problems that can occur when you
manipulate string data

• String is not a simple data type like int, float,
  or double
• String creates an instance of a class, the class
  String
• As such it contains a reference or an address and
  not the actual string
• So you cannot do equality comparisons of two
  different instances of String, because you are
  simply testing if the addresses are the same

3
An example of incorrect code

• import javax.swing.JOptionPane
• public class TryToCompareStrings
• public static void main(String args)
• String aName "Carmen"
• String anotherName
• anotherName JOptionPane.showInputDialog(nu
  ll,
• "Enter your name")
• if(aName anotherName)
• JOptionPane.showMessageDialog(null,
  aName
• " equals " anotherName)
• else
• JOptionPane.showMessageDialog(null,
  aName
• " does not equal " anotherName)
• System.exit(0)

4
Correct Code

• import javax.swing.JOptionPane
• public class CompareStrings
• public static void main(String args)
• String aName "Carmen"
• String anotherName
• anotherName JOptionPane.showInputDialog(nu
  ll,
• "Enter your name")
• if(aName.equals(anotherName))
• JOptionPane.showMessageDialog(null,
  aName
• " equals " anotherName)
• else
• JOptionPane.showMessageDialog(null,
  aName
• " does not equal " anotherName)
• System.exit(0)

5
Three classes for working with strings

• Charactera class whose instances can hold a
  single character valueprovides methods that can
  manipulate or inspect single-character data
• Stringa class for working with fixed-string
  datathat is unchanging data composed of multiple
  characters, strings that are immutable
• StringBuffera class for storing and manipulating
  changeable data composed of mulltiple characters

6
Manipulating Characters

• We know the char data type can hold any single
  character
• Character class provides the following methods
• isUpperCase(), toUpperCase(), isLowerCase(),
  toLowerCase(), isDigit(), isLetter(),
  isLetterOrDigit(), isWhitespace()
• Methods that begin with is perform tests
  delivering true or false values
• Methods that begin with to perform conversions

7
Declaring a String Object

• We know that characters enclosed within double
  quotation marks are literal strings
• Weve learned to print these strings using
  println() and showMessageDialog()
• An literal string is an unnamed object, or
  anonymous object, of the String class
• A String variable is simply a named object of the
  same class.
• The class String is defined in java.lang.String,
  which is automatically imported into every
  program you write

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Declaring a String variable

• When you declare a String variable, the String
  itselfthat is, the series of characters
  contained in the Stringis distinct from the
  variable you use to refer to it.
• Can initialize a String variable with or without
  a String constructor

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With or without a String Constructor

• With the constructor
• String aGreeting new String(Hello)
• Without the constructor
• String aGreeting Hello
• Unlike other classes, you can create a String
  object without using the keyword new or
  explicitly calling the class constructor

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Comparing String Values

• Consider the following two statements
• String aGreeting hello
• aGreeting Bonjour
• These statements are syntactically correct. What
  happens is that the address contained in
  aGreeting is changed to point to Bonjour rather
  than hello, both of which are contained at
  different locations in memory. Eventually, the
  garbage collector discards the hello characters.

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Comparing String Values

• The String class provides methods for comparing
  strings
• In the example above the sign is comparing
  memory addresses, not the actual strings.
• The String class equals() method evaluates the
  contents of two String objects to determine if
  they are equivalent.

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• Import javax.swing.JOptionPane
• Public class CompareStrings
• public static void main(String args)
• String aName Carmen, anotherName
• anotherName JOptionPane. showInputDialog(null
  , Enter your name)
• if(aName.equals(anotherName))
• JOptionPane.showMessageDialog(null, aName
  equals anotherName)
• else
• JOptionPane.showMessageDialog(null, aName
  does not equal anotherName))
• System.exit(0)

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The equals() method

• From the code above, we can see that the equals()
  method returns a Boolean value of true or false

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equalsIgnoreCase() Method

• Similar to the equals() method
• Ignores case
• String aName Roger
• If(aName.equalsIgnoreCase(roGER))
• evaluates to true

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compareTo() method

• Returns an integer that is the numeric difference
  between the first two non-matching characters

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Using other String Methods

• toUpperCase() and toLowerCase() convert any
  String to its uppercase and lowercase equivalent
• Length() returns the length of a String

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• import javax.swing.
• public class BusinessLetter
• public static void main(String args)
• String name
• String firstName ""
• String familyName ""
• int x
• char c
• name JOptionPane.showInputDialog(null,
• "Please enter customer's first and last
  name")
• x 0
• while(x lt name.length())
• if(name.charAt(x) ' ')
• firstName name.substring(0, x)
• familyName name.substring(x 1,
  name.length())

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• x 0
• while(x lt name.length())
• if(name.charAt(x) ' ')
• firstName name.substring(0, x)
• familyName name.substring(x 1,
  name.length())
• x name.length()
• x

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Concatenation

• You know you can concatenate strings to strings
  as in System.out.println(firstName
  lastName)

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Concatenationnumbers to strings by using

• The following is permissible
• Int myAge 25
• String aString My age is myAge
• Another example would be
• String anotherString
• float someFloat 12.34f
• anotherString someFloat

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Concatenation by using the toString() method

• String theString
• Int someInt 10
• theString Integer.toString(someInt)
• String aString
• double someDouble 8.25
• aString Double.toString(someDouble)

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Converting Strings to Numbers

• Use a wrapper like the Integer class which is a
  part of java.lang
• A wrapper is a class that is wrapped around a
  simpler element
• Int anInt Integer.parseInt(649) stores the
  value 649 in the variable anInt

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• public class TestCharacter
• public static void main(String args)
• char aChar 'C'
• System.out.println("The character is "
  aChar)
• if(Character.isUpperCase(aChar))
• System.out.println(aChar " is
  uppercase")
• else
• System.out.println(aChar " is not
  uppercase")
• if(Character.isLowerCase(aChar))
• System.out.println(aChar " is
  lowercase")
• else
• System.out.println(aChar " is not
  lowercase")
• aChar Character.toLowerCase(aChar)
• System.out.println("After toLowerCase(),
  aChar is " aChar)
• aChar Character.toUpperCase(aChar)
• System.out.println("After toUpperCase(),
  aChar is " aChar)
• if(Character.isLetterOrDigit(aChar))

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Test Character App

• public class TestCharacter
• public static void main(String args)
• char aChar 'C'
• System.out.println("The character is "
  aChar)
• if(Character.isUpperCase(aChar))
• System.out.println(aChar " is
  uppercase")
• else
• System.out.println(aChar " is not
  uppercase")
• if(Character.isLowerCase(aChar))
• System.out.println(aChar " is
  lowercase")
• else

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• System.out.println(aChar " is not lowercase")
• aChar Character.toLowerCase(aChar)
• System.out.println("After toLowerCase(),
  aChar is " aChar)
• aChar Character.toUpperCase(aChar)
• System.out.println("After toUpperCase(),
  aChar is " aChar)
• if(Character.isLetterOrDigit(aChar))
• System.out.println(aChar " is a letter or
  digit")
• else
• System.out.println(aChar " is neither a
  letter nor a digit")
• if(Character.isWhitespace(aChar))
• System.out.println(aChar " is
  whitespace")
• else
• System.out.println(aChar " is not
  whitespace")

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Learning about the StringBuffer Class

• Some strings are not constants, not immutable
• String someChars Goodbye
• someChars Goodbye Everybody
• someChars Goodbye Everybody
• You cannot change the string Goodbye
• To overcome these limitations, you can use the
  StringBuffer class
• The StringBuffer class was invented to
  accommodate strings that are not immutable
  (constants)

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The StringBuffer Class

• Uses a buffer that is much larger than any one
  string actual size of the buffer is the capacity
• Provides methods that can change individual
  characters within a string
• Must initialize StringBuffer objects as follows
• StringBuffer eventString new StringBuffer(Hello
  there)
• Cannot use StringBuffer eventString Hello
  there

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Methods in the StringBuffer Class

• setLength() will change the length of a String in
  a StringBuffer object
• capacity() will find the capacity of an object
• Has four constructors
• public StringBuffer() constructs a StringBuffer
  with no characters and a default size of 16
  characters
• public StringBuffer(int Capacity) creates a
  StringBuffer with no characters and a capacity
  defined by the parameter
• public StringBuffer(String s) contains the same
  characters as those stored in the String object s

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Still more methods in the StringBuffer Class

• Append() lets you add characters to the end of a
  StringBuffer object
• Insert() lets you add characters at a specific
  location within a StringBuffer object
• StringBuffer someBuffer new StringBuffer(Happy
  Birthday)
• someBuffer.insert(6,30th )
• Produces Happy 30th Birthday

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Still more methods in StringBuffer

• setCharAt() method allows you to change a single
  character at a specified location
• someBuffer.setCharAt(6,4)
• Changes someBuffer to Happy 40th Birthday
• Can use charAt() method will return the character
  at an offset number of positions from the first
  character
• StringBuffer text new StringBuffer(Java
  Programming)
• Then text.charAt(5) returns the character P.

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• package Strings
• import javax.swing.
• public class RepairName
• /
• _at_param args
• /
• public static void main(String args)
• String name, saveOriginalName
• int stringLength
• int i
• char c
• name JOptionPane.showInputDialog(null, "Please
  enter your first and last name")
• saveOriginalName name
• stringLength name.length()
• for (i0 i lt stringLength i)
• c name.charAt(i)