1. Computer Science 209
The Adapter Pattern

2. Hardware Adapters
Three-prong to two-prong

3. Hardware Adapters
Firewire 800

4. Hardware Adapters
Thunderbolt to Firewire 800

5. Hardware Adapters
Thunderbolt 2 to Thunderbolt to Firewire 800

6. The Context of the Adapter Pattern
I want to use an existing class (the adaptee) without modifying it
The context for using this class requires conformance to an different interface (the target)
The target and adaptee interfaces are conceptually related

7. Solution of the Adapter Pattern
I define an adapter class that implements the target interface
The adapter class contains a reference to the adaptee and translates target methods to adaptee methods
The client wraps an adapter around an adaptee

8. Example Problem
I want to add an icon as a component to a GUI’s container
An icon is similar to a component (both are involved with painting and determining dimensions of a rectangular area)
I want to get my icon to behave just like a component but I don’t want to change my icon itself

9. Solution Define an adapter class that extends JComponent
This class contains a reference to my icon
Override the paintComponent and getPreferredSize methods to paint the icon and get its dimensions

10. The Icon Interface
import java.awt.*;
public interface Icon{
public int getIconWidth();
public int getIconHeight();
public void paintIcon(Component c, Graphics g, int x, int y); }
The component that displays an icon needs to be able to get its width and height and to paint the icon at a given position
paintIcon can use the Component parameter to obtain properties like the background color that might be useful

11. Using a New Icon for Stars
import javax.swing.JOptionPane;
public class IconTest{
public static void main(String[] args){
JOptionPane.showMessageDialog(null,
"Here is a Star",
"Message",
JOptionPane.INFORMATION_MESSAGE,
new StarIcon(100)); }

12. Implementing the Icon Interface
import java.awt.*;
import javax.swing.Icon;
public class StarIcon implements Icon{
private int size;
public StarIcon(int size) {this.size = size;}
public int getIconWidth() {return size;}
public int getIconHeight() {return size;}
public void paintIcon(Component c, Graphics g, int x, int y){
// Code for drawing a star }

13. Using a New Icon for Stars
import javax.swing.JOptionPane;
public class IconTest{
public static void main(String[] args){
JOptionPane.showMessageDialog(null,
"Here is a Star",
"Message",
JOptionPane.INFORMATION_MESSAGE,
new StarIcon(100)); }

14. Code for IconAdapter import javax.swing.*; import java.awt.*;
public class IconAdapter extends JComponent{
private Icon icon;
public IconAdapter(Icon icon){
this.icon = icon; }
public void paintComponent(Graphics g){
icon.paintIcon(this, g, 0, 0);
public Dimension getPreferredSize(){
return new Dimension(icon.getIconWidth(),
icon.getIconHeight());

15. Using IconAdapter import javax.swing.*; import java.awt.*;
public class IconAdapterTest{
public static void main(String[]args){
Icon icon = new StarIcon();
JComponent component = new IconAdapter(icon);
JFrame frame = new JFrame();
Container pane = frame.getContentPane();
pane.add(component);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true); }

16. Distinct Responsibilities
<<Interface>>
Target
Adapter
Adaptee
targetMethod()
adapteeMethod()
Calls adapteeMethod()

17. Another Example: Input Streams
Input streams return data as bytes
We want characters or integers or doubles or strings
Turn an input stream into a scanner, which reads the bytes from the input stream and returns values of these types

18. Wrapping an Input Stream in a Scanner
java.util.Scanner reader = new Scanner(System.in);
<int, double, String> thing = reader.next<Int, Double, Line>();
The adaptee is an InputStream (System.in)
The target is a Scanner
The adapter is a Scanner

19. Java Collection Interfaces
List
Set
Map
SortedSet
SortedMap

20. The Collection Interface
boolean add(E element)
boolean addAll(Collection<E> c)
void clear()
boolean contains(E element)
boolean containsAll(Collection<E> c)
boolean equals(Object o)
int hashCode()
boolean isEmpty()
Iterator iterator()
boolean remove(Element E)
boolean removeAll(Collection<E> c)
boolean retainAll(Collection<E> c)
int size()
Object[] toArray()
Object[] toArray(Object[] a)
The mutators (in green) are optional operations

21. AbstractCollection
All Collection methods are implemented in the class AbstractCollection
Subclasses can override these methods

22. Example: contains public boolean contains(E element){
Iterator<E> iter = this.iterator();
while (iter.hasNext())
if (element.equals(iter.next()))
return true;
return false; }
System.out.println(list.contains("hi there"));

23. Java Collection Interfaces
TrueStack
List
Set
Map
TrueQueue
SortedSet
SortedMap
TrueStack and TrueQueue could extend Collection
But then they must support the add, size, and iterator methods

24. Or We Could Adapt, Instead
Define an adapter class that extends AbstractCollection and contains a reference to the adaptee collection
Override the add, size, and iterator methods in the adapter class

25. Example: Adapting a Binary Search Tree
public BSTAdapter extends AbstractCollection<E>{
private BST<E> tree;
public BSTAdapter(BST<E> tree){
this.tree = tree; }
public int size() {return tree.size()}
public Iterator<E> iterator() {return tree.iterator()}
public boolean add(E element) {return tree.add(element)}
BSTAdapter<E> adapter = new BSTAdapter<E>(tree);
adapter.addAll(list);
adapter.addAll(sortedSet);