Adapter 2pp

The
Adapter
Pattern

Design Patterns In Java Bob Tarr

The Adapter Pattern

l Intent
é Convert the interface of a class into another interface clients expect.
Adapter lets classes work together that couldn't otherwise because of
incompatible interfaces.
l Also Known As
é Wrapper
l Motivation
é Sometimes a toolkit or class library can not be used because its interface is
incompatible with the interface required by an application
é We can not change the library interface, since we may not have its source
code
é Even if we did have the source code, we probably should not change the
library for each domain-specific application

Design Patterns In Java
The Adapter Pattern Bob Tarr
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The Adapter Pattern

l Motivation
é Example:

3

The Adapter Pattern

l Structure
é A class adapter uses multiple inheritance to adapt one interface to another:

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The Adapter Pattern

l Structure
é An object adapter relies on object composition:

5

The Adapter Pattern

l Applicability
Use the Adapter pattern when
é You want to use an existing class, and its interface does not match the one
you need
é You want to create a reusable class that cooperates with unrelated classes
with incompatible interfaces
l Implementation Issues
é How much adapting should be done?
Ý Simple interface conversion that just changes operation names and order of
arguments
Ý Totally different set of operations
é Does the adapter provide two-way transparency?
Ý A two-way adapter supports both the Target and the Adaptee interface. It
allows an adapted object (Adapter) to appear as an Adaptee object or a Target
object

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Adapter Pattern Example 1

l The classic round pegs and square pegs!
l Here's the SquarePeg class:
/**
* The SquarePeg class.
* This is the Target class.
*/
public class SquarePeg {
public void insert(String str) {
System.out.println("SquarePeg insert(): " + str);
}
}

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Adapter Pattern Example 1 (Continued)

l And the RoundPeg class:
/**
* The RoundPeg class.
* This is the Adaptee class.
*/
public class RoundPeg {
public void insertIntoHole(String msg) {
System.out.println("RoundPeg insertIntoHole(): " + msg);
}
}

l If a client only understands the SquarePeg interface for inserting
pegs using the insert() method, how can it insert round pegs? A
peg adapter!

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l Here is the PegAdapter class:
/**
* The PegAdapter class.
* This is the Adapter class.
* It adapts a RoundPeg to a SquarePeg.
* Its interface is that of a SquarePeg.
*/
public class PegAdapter extends SquarePeg {
private RoundPeg roundPeg;

public PegAdapter(RoundPeg peg) {this.roundPeg = peg;}

public void insert(String str) {roundPeg.insertIntoHole(str);}

}

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l Typical client program:
// Test program for Pegs.
public class TestPegs {
public static void main(String args[]) {

// Create some pegs.
RoundPeg roundPeg = new RoundPeg();
SquarePeg squarePeg = new SquarePeg();

// Do an insert using the square peg.
squarePeg.insert("Inserting square peg...");

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5


// Now we'd like to do an insert using the round peg.
// But this client only understands the insert()
// method of pegs, not a insertIntoHole() method.
// The solution: create an adapter that adapts
// a square peg to a round peg!
PegAdapter adapter = new PegAdapter(roundPeg);
adapter.insert("Inserting round peg...");
}

}

l Client program output:
SquarePeg insert(): Inserting square peg...
RoundPeg insertIntoHole(): Inserting round peg...

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l Notice in Example 1 that the PegAdapter adapts a RoundPeg to a
SquarePeg. The interface for PegAdapter is that of a SquarePeg.
l What if we want to have an adapter that acts as a SquarePeg or a
RoundPeg? Such an adapter is called a two-way adapter.
l One way to implement two-way adapters is to use multiple
inheritance, but we can't do this in Java
l But we can have our adapter class implement two different Java
interfaces!

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l Here are the interfaces for round and square pegs:
/**
*The IRoundPeg interface.
*/
public interface IRoundPeg {
public void insertIntoHole(String msg);
}

/**
*The ISquarePeg interface.
*/
public interface ISquarePeg {
public void insert(String str);
}

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l Here are the new RoundPeg and SquarePeg classes. These are
essentially the same as before except they now implement the
appropriate interface.
// The RoundPeg class.
public class RoundPeg implements IRoundPeg {
public void insertIntoHole(String msg) {
System.out.println("RoundPeg insertIntoHole(): " + msg);
}
}

// The SquarePeg class.
public class SquarePeg implements ISquarePeg {
public void insert(String str) {
System.out.println("SquarePeg insert(): " + str);
}
}

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l And here is the new PegAdapter:
/**
* The PegAdapter class.
* This is the two-way adapter class.
*/
public class PegAdapter implements ISquarePeg, IRoundPeg {
private RoundPeg roundPeg;
private SquarePeg squarePeg;

public PegAdapter(RoundPeg peg) {this.roundPeg = peg;}

public PegAdapter(SquarePeg peg) {this.squarePeg = peg;}

public void insert(String str) {roundPeg.insertIntoHole(str);}

public void insertIntoHole(String msg){squarePeg.insert(msg);}
}
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l A client that uses the two-way adapter:
// Test program for Pegs.
public class TestPegs {
public static void main(String args[]) {
// Create some pegs.
RoundPeg roundPeg = new RoundPeg();
SquarePeg squarePeg = new SquarePeg();

// Do an insert using the square peg.
squarePeg.insert("Inserting square peg...");

// Create a two-way adapter and do an insert with it.
ISquarePeg roundToSquare = new PegAdapter(roundPeg);
roundToSquare.insert("Inserting round peg...");

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// Do an insert using the round peg.
roundPeg.insertIntoHole("Inserting round peg...");

// Create a two-way adapter and do an insert with it.
IRoundPeg squareToRound = new PegAdapter(squarePeg);
squareToRound.insertIntoHole("Inserting square peg...");
}
}

l Client program output:

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l This example comes from Roger Whitney, San Diego State
University
l Situation: A Java class library exists for creating CGI web server
programs. One class in the library is the CGIVariables class
which stores all CGI environment variables in a hash table and
allows access to them via a get(String evName) method. Many
Java CGI programs have been written using this library. The
latest version of the web server supports servlets, which provide
functionality similar to CGI programs, but are considerably more
efficient. The servlet library has an HttpServletRequest class
which has a getX() method for each CGI environment variable.
We want to use servlets. Should we rewrite all of our existing
Java CGI programs??

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l Solution : Well, we'll have to do some rewriting, but let's attempt
to minimize things. We can design a CGIAdapter class which has
the same interface (a get() method) as the original CGIVariables
class, but which puts a wrapper around the HttpServletRequest
class. Our CGI programs must now use this CGIAdapter class
rather than the original CGIVariables class, but the form of the
get() method invocations need not change.

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l Here's a snippet of the CGIAdapter class:
public class CGIAdapter {
Hashtable CGIVariables = new Hashtable(20);

public CGIAdapter(HttpServletRequest CGIEnvironment) {
CGIVariables.put("AUTH_TYPE", CGIEnvironment.getAuthType());
CGIVariables.put("REMOTE_USER",
CGIEnvironment.getRemoteUser());
etc.
}

public Object get(Object key) {return CGIvariables.get(key);}

}

l Note that in this example, the Adapter class (CGIAdapter) itself
constructs the Adaptee class (CGIVariables)
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l Consider a utility class that has a copy() method which can make
a copy of an vector excluding those objects that meet a certain
criteria. To accomplish this the method assumes that all objects
in the vector implement the Copyable interface providing the
isCopyable() method to determine if the object should be copied
or not.
Copyable Interface
VectorUtilities
isCopyable()

BankAccount

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l Here’s the Copyable interface:
public interface Copyable {
public boolean isCopyable();
}

l And here’s the copy() method of the VectorUtilities class:
public static Vector copy(Vector vin) {
Vector vout = new Vector();
Enumeration e = vin.elements();
while (e.hasMoreElements()) {
Copyable c = (Copyable) e.nextElement();
if (c.isCopyable())
vout.addElemet(c);
}
return vout;
}

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l But what if we have a class, say the Document class, that does not
implement the Copyable interface. We want to be able perform a
selective copy of a vector of Document objects, but we do not
want to modify the Document class at all. Sounds like a job for
(TA-DA) an adapter!
l To make things simple, let’s assume that the Document class has
a nice isValid() method we can invoke to determine whether or
not it should be copied

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l Here’s our new class diagram:

Copyable Interface
VectorUtilities
isCopyable()

Document
DocumentAdapter
isValid()

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l And here is our DocumentAdapter class:
public class DocumentAdapter implements Copyable {

private Document d;

public DocumentAdapter(Document d) {
document = d;
}

public boolean isCopyable() {
return d.isValid();
}

}

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l Do you see any Adapter pattern here?
public class ButtonDemo {

public ButtonDemo() {
Button button = new Button("Press me");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
doOperation();
}
});
}
public void doOperation() { whatever }

}

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l Button objects expect to be able to invoke the actionPerformed()
method on their associated ActionListener objects. But the
ButtonDemo class does not have this method! It really wants the
button to invoke its doOperation() method. The anonymous inner
class we instantiated acts as an adapter object, adapting
ButtonDemo to ActionListener!

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Adapter 2pp

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Adapter 2pp