Enhanced Class Design

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Enhanced Class Design

• Joe Komar

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Enhanced Class Design Tools

• Abstract classes
• Interfaces
• Packages

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Abstract Classes

• Typically represents a concept in hierarchy
• Cannot be instantiated
• Contain abstract methods
• Some methods typically not abstract
• Expect the children to finish the definition of
  the class
• Format abstract methodname()
• defines what parameters are accepted and what is
  returned

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Abstract Classes

• The modifiers final and static cannot be used
  with abstract methods
• If there is one or more abstract methods in a
  class is must be declared abstract
• e.g., abstract class Food
• Children must override the abstract methods of
  the abstract parent
• If dont override all abstract methods, the child
  must be declared abstract

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Printer Example
File
Binary_File
Text_File
Image_File
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Printer Example
public class Printer public static void
main (String args) byte logo_data
41, 42, 49, 44 Text_File report new
Text_File ("Sand Reconner", 66, "One
two three") Image_File logo new
Image_File ("Number 1", 45,
logo_data) Print_Logger
daily new Print_Logger()
daily.log(report) daily.log(logo)
// method main // class Printer
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Printer Example (contd)
abstract class File protected String id
protected int size public File (String
file_id, int file_size) id file_id
size file_size // constructor File
public String name() return id
// method name abstract public String
print() // class File
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Printer Example (contd)
class Text_File extends File protected
String text public Text_File (String id, int
size, String file_contents) super(id,
size) text file_contents
//constructor Text_File public String
print() return text // print
method // class Text_File
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Printer Example (contd)
class Binary_File extends File protected
byte data public Binary_File (String
id, int size, byte file_data)
super(id, size) data file_data
//constructor Binary_File public String
print() return "" //method
print // class Binary_File
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Printer Example (contd)
class Image_File extends Binary_File
public Image_File (String id, int size, byte
file_data) super(id, size, file_data)
// constructor Image_File public
String print() return new String
(data) //method print //class Image_File
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Printer Example (contd)
class Print_Logger public void log (File
file) System.out.println (file.name()
" " file.print()) // method
log //class Print_Logger
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Printer Example Output
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Abstract Classes
Crust
Toppings
Traditional
Deep Dish
Mushroom
Sausage
Cheese
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Interfaces

• Collection of constants and abstract methods
• Not classes
• Can be used in the definition of a class
• interface interface-name constants and methods
• class class-name implements interface-name
  implementation of abstract methods

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Interfaces

• All methods in an interface are, by default,
  public and abstract
• All constants are public and final
• Constants can be used in the class as if they
  were declared locally
• Interfaces can be linked in a hierarchy (extends)
  like classes

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Interfaces

• Interface names can be used as a class type in
  a formal parameter
• Any class that implements that interface will be
  accepted as the actual parameter
• A class can implement more than one interface
• Interfaces allow for multiple inheritance

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Printer2 Example
public class Printer2 public static void
main (String args) byte logo_data
41, 42, 49, 44 Text_File report new
Text_File ("Sand Reconner", 66, "One
two three") Image_File logo new
Image_File ("Number 1", 45,
logo_data) Print_Logger
daily new Print_Logger()
daily.log(report) daily.log(logo) //
method main // class Printer2
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Printer2 Example
class File protected String id
protected int size public File (String
file_id, int file_size) id file_id
size file_size // constructor File
public String name() return id
// method name // class File
No Print() method Not abstract any longer
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Printer2 Example
class Text_File extends File implements Printable
protected String text public Text_File
(String id, int size, String file_contents)
super(id, size) text
file_contents //constructor Text_File
public String print() return text
// print method // class Text_File
Implements the print() method name() method in
File
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Printer2 Example
class Binary_File extends File protected
byte data public Binary_File (String
id, int size, byte file_data)
super(id, size) data file_data
//constructor Binary_File // class
Binary_File
Doesnt implement Printable Parent to Image_File
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Printer2 Example
class Image_File extends Binary_File implements
Printable public Image_File (String
id, int size, byte file_data)
super(id, size, file_data) // constructor
Image_File public String print()
return new String (data) //method
print //class Image_File
Implements print() name() in File class
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Printer2 Example
interface Printable String name()
String print() // interface
Printable class Print_Logger public void
log (Printable file) System.out.println
(file.name() " " file.print()) //
method log //class Print_Logger
Names name() and print() methods to be implemented
interface Name
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Printer2 Example Output
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Abstract Classes and Interfaces

• Used together become a powerful tool
• Multiple Inheritance
• Encapsulate and hide information
• Specify what must be done not how
• Allow systems to be built by many programmers
• Basically, represent the heart of object
  orientation

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Packages

• Used to group similar classes together under one
  name
• Identify files and classes for the package
• Put compiled versions into a subdirectory
  associated with that package name
• Include in the classpath environment variable
• Include in the standard path
• Name at compile and run time

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Simple_IO Example
package Simple_IO import java.io. public class
Reader public static int read() throws
IOException BufferedReader stdin new
BufferedReader (new InputStreamReader
(System.in)) String value
stdin.readLine() return Integer.parseInt
(value) // method read public static
String read_line() throws IOException
BufferedReader stdin new BufferedReader
(new InputStreamReader (System.in))
return stdin.readLine() // method
read_line //class Reader
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Simple_IO Example
package Simple_IO public class Writer
public static void write (int value)
System.out.println (value) // method
write public static void write_line (String
line) System.out.println (line)
// method write_line // class Writer
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Compiling and Running

• Assume that Simple_IO directory on A
• Place Reader.java and Writer.java in that
  directory and compile as normal creating the
  .class files
• You can continue to add classes to this directory
• To use them use import Simple_IO.

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Example use of a package
import java.io.IOException import
Simple_IO. class Simple_IO_Test public
static void main (String args) throws
IOException int value Reader.read()
String line Reader.read_line()
Writer.write (value) Writer.write_line
(line) // method main //class
Simple_IO_Test
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Finding the Directory

• Got your own system -- add directories in the
  standard classpath
• At UST, store directories on diskette, then to
  compile and run
• javac -classpath a\classpath prog.java
• java -classpath a\classpath prog

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Package Considerations

• If you have a conflict with method names, use the
  fully qualified name
• Simple_IO.Reader.read()
• Simple_IO.Writer.write()
• Dont then need to use an import statement
• Note that these packages you create need to exist
  on the system on which the program runs

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Summary

• Abstract classes -- if any method is abstract,
  the entire class is abstract
• Abstract methods must all be implemented
  somewhere in the hierarchy
• Interfaces are useful in implementing more than
  one hierarchy
• Packages are ways of grouping similar and/or
  interdependent classes under one name