Wrapper Classes

1
Wrapper Classes

• What are Wrapper Classes
• Converting Data Types to Objects
• Converting Objects to Data Types
• Strings and Wrapper Objects
• Robert Stumpf, Ph.D.
• CIS Department

2
What are Wrapper Classes

• It is possible to have an int as a data type
• Also it is possible to have an Integer Object
• Java has the benefits of both storage methods
• Data types are efficient while Objects have extra
  behaviors

3
What are Wrapper Classes

• All data types have the ability to be inside an
  object with a corresponding name
• This is called wrapping a data type inside an
  object
• A primary reason for this is many classes
  (Collections) for example can only contain objects

4
What are Wrapper Classes

• Java has both data types and objects
• boolean Boolean
• char Char
• int Integer
• double Double
• etc..

5
What are Wrapper Classes

• Thus the corresponding class name begins with an
  upper case letter
• A reason extra behaviors are needed is to make it
  easy to convert data types to objects and objects
  to data types

6
Converting Data Types to Objects

• To convert from data type to object
• Boolean bb new Boolean (true)
• Character cc new Character (a)
• Integer ii new Integer (1)
• Double dd new Double (1.0)

7
Converting Data Types to Objects

• One can convert from data types to strings using
  the built in toString method
• For example
• String s Integer.toString (20)
• or
• String s Double.toString (20.0)

8
Converting Data Types to Objects

• One can also convert from data types to strings
  with concatenation
• For example
• s Answer is 20
• This is equivalent to
• s Answer is Integer.toString (20)
• This is used frequently in println
• System.out.println (Answer is 20 )

9
Converting Objects to Data Types

• To convert from object to data type
• Boolean bb new Boolean (true)
• boolean b bb.booleanValue ( )
• Character cc new Character (a)
• char c cc.charValue ( )
• Integer ii new Integer (1)
• int i ii.intValue ( )
• Double dd new Double (1.0)
• double d dd.doubleValue ( )

10
Converting Objects to Data Types

• To convert from String to data type
• String s new String (1)
• int i Integer.parseInt ( s )
• String s new String (1)
• long l Long.parseLong ( s )
• Cant be done for float or double !
• Note that this is a static (class) method
• Interesting enough Netscape implements parseFloat
  in Java Script

11
Strings and Wrapper Objects

• To convert from String to object
• String s new String (1)
• Integer ii Integer.valueOf ( s )
• String s new String (1.0)
• Double dd Double.valueOf ( s )
• Note that these are static (class) methods as
  they are sent to the class, not the object

12
Strings and Wrapper Objects

• To convert from object to String
• Boolean bb new Boolean (true)
• String s bb.toString ( )
• Character cc new Character (a)
• String s cc.toString ( )
• Integer ii new Integer (1)
• String s ii.toString ( )
• Double dd new Double (1.0)
• String s dd.toString ( )

13
Summary

• Arithmetic is done in data types only
• Objects exist because many utility classes use
  only objects as arguments
• The complicated part is remembering how to
  convert from one form to another
• This is twice as complicated when one considers
  Strings
• Most books treat this subject very lightly

14
Thank You

• No day in which you learn something is a
  complete loss.Thank you
• Robert Stumpf
• email rvstumpf_at_csupomona.edu
• url http//www.csupomona.edu/rvstumpf