Decorator

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Decorator

• Josh Voils and Ralph Rodkey

2
GoF Definition of Intent

• Attach additional responsibilities to an object
  dynamically
• Provide an alternative to subclassing for
  extending functionality

3
GoF Example

• We want to add a border and a scroll bar to a
  TextView object
• Lets use a decorator!

aTextView
Instance diagram for our example
4
The UML
5
Challenge A

• Why use a Decorator instead of subclassing?

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Streams

• Java Streams are an example of a decorator
• Challenge B
• Draw an analogy between the OutputStream
  hierarchy and the TextView hierarchy

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Oozinoz Writer

• A writer is a stream that reads characters
• Java Provides the filterWriter class to
  facilitate the creation of new filters
• Here Metsker uses a FilterWriter to create an
  OozinozFilter

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OozinozFilter Code

• package com.oozinoz.io
• import java.io.
• public abstract class OozinozFilter extends
  FilterWriter
• protected OozinozFilter(Writer out)
• super(out)
• public void write(char cbuf, int off, int
  len) throws IOException
• for (int i 0 i lt len i)
• write(cbufi)
• public abstract void write(int c) throws
  IOException
• public void write(String s, int off, int len)
  throws IOException
• write(s.toCharArray(), off, len)

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O.W. (cont)

• From the abstract OozinozFilter he creates some
  custom filters

package com.oozinoz.io import java.io. public
class UpperCaseFilter extends OozinozFilter
public UpperCaseFilter(Writer out)
super(out) public void write(int c)
throws IOException out.write(Character.toU
pperCase((char) c))
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Application of Filters
package com.oozinoz.applications import
java.io. import com.oozinoz.io. public class
ShowFilters public static void main(String
args) throws IOException BufferedReader
in new BufferedReader(new FileReader(args0))
Writer out new FileWriter(args1)
out new WrapFilter(new BufferedWriter(out),
40) out new TitleCaseFilter(out)
while (true) String s
in.readLine() if (s null)
break out.write(s
"\n") out.close()
in.close()
This program line-wraps text in an input file and
puts it in title case
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Challenge 27.1

• If you want to direct output to System.out
  instead of to a file, you can create a Writer
  object that directs its output to System.out
• Writer out new PrintWriter(System.out)
• Write a snippet of code to define a Writer object
  that wraps text at 15 characters, centers the
  text, sets the text to random casing, and directs
  the output to System.out.

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Challenge 27.1 Answer

• One answer is
• Writer out new PrintWriter(System.out)
• out new WrapFilter(new BufferedWriter(out),
  15)
• ((WrapFilter) out).setCenter(true)
• out new RandomCaseFilter(out)
• Alternatively
• WrapFilter out
• new WrapFilter(
• new BufferedWriter(
• new RandomCaseFilter(
• new PrintWriter(System.out))),
• 15)
• out.setCenter(true)

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Challenge 27.5 (sorta)
package com.oozinoz.applications import
javax.swing. import javax.swing.border. import
java.awt. import com.oozinoz.ui. public
class ShowBorders protected static JButton
createButton(String label) JButton b
new JButton(label) b.setFont(SwingFacade.
getStandardFont()) b.setFocusPainted(fals
e) return b public static
void main(String args) JButton ignite
createButton("Ignite") JButton launch
createButton("Launch")
launch.setBorder( new
CompoundBorder( new
LineBorder(Color.black, 4), new
CompoundBorder( new
BevelBorder(BevelBorder.RAISED),
new EmptyBorder(10, 10, 10, 10))))
JPanel p new JPanel() p.add(ignite)
p.add(launch) SwingFacade.launch(p,
" Borders")

• Why are swing borders not decorators?

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Questions? Comments?