Using the Command and Adaptor Patterns in Java 8
- Design Patterns in General / The Command Pattern and Java 8
- Hiding Complexity with the Adaptor Pattern / Installing Java 8
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Command and Adaptor are two related design patterns, both of which provide a useful form of indirection that hides unnecessary complexity, while at the same time providing a crisp interface:
- The Command pattern allows for an instruction-style interface: "Run this operation," "Undo this operation," and so on.
- The Adaptor pattern facilitates decoupling between code elements that are functionally related. In using the Adaptor pattern, typically you have a class X that's very complex. Rather than writing calls directly to class X, the Adaptor class acts as an intermediary. In other words, when inevitable changes are needed, only the Adaptor class has to be changed.
Both the Command and Adaptor patterns are very useful in helping to protect legacy code and reduce the need to change it. This excellent attribute underlines the importance of a solid knowledge of design patterns.
But what about the use of patterns in the functional programming Java 8 world? Have patterns become a thing of the past now that Java 8 offers such exotic artifacts as lambda expressions?
I think that patterns remain one of the great additions to modern programming. Patterns allow for design clarity and facilitate coding economy; once you've decided to use a pattern, you can usually reduce the amount of boilerplate coding for a given task. Meanwhile, Java 8 lambda expressions substantially reduce coding overhead. Pattern use in Java 8 and some of the core features of Java 8 itself therefore have similar driving motivations.
To view or use the code examples in this article, download the code file.
Design Patterns in General
So, I'm a great fan of design patterns! Patterns provide for very clear code, and they allow software designers to articulate the software layout in advance of coding. But nowadays, when designer and coder are often the same person, is pattern programming still worth the effort? Why can't we dispense with all this pattern stuff and just get down to coding? Well, I still subscribe to the belief that the sooner you start to code, the longer the task will take. Using patterns allows for a beneficial delay in starting to code, giving you the chance to step away from the programming task for a clearer look at the task.
In addition to delaying coding by giving more time to the design task, patterns remain a pillar of modern software development. Consider a simple case of a reporting system where a client runs reports based on a data feed from another system. This is usually a natural candidate for the Publish-Subscribe pattern. In this pattern, the publisher maintains a list of interested or registered subscribers. Then, when new data is generated by the publisher, the subscribers are notified.
In this way, the Publish-Subscribe pattern reduces the need for potentially expensive polling mechanisms in the client code. In other words, the publisher informs the client when new data is available, without the need for the client to make unnecessary calls to the publisher. Used in this way, Publish-Subscribe mechanisms allow for more efficient systems.
It should therefore come as no surprise that languages such as Python and Java have readily available Publish-Subscribe code frameworks (for example, the Java 7 Observable). Without any more effort than choosing to use the Publish-Subscribe pattern, you implicitly provide a really useful nonfunctional requirement of efficient intersystem data transfer. Standing on the shoulders of giants (the pattern creators) allows your code to reach new heights!
In this article, we'll look at the Command and Adaptor patterns and see how they stack up in the brave new Java 8 world.
To get started, let's have a look at a simple Command pattern interface called Command in Listing 1.
Listing 1: A simple Java Command pattern interface
public interface Command {
public void execute();
}
Listing 1 is about as simple as it gets: just one method called execute() inside an interface. Listing 2 illustrates an extension of the Command interface.
Listing 2: An extension of the Command pattern interface
public interface SwitchableElement extends Command {
public void enableElement();
public void disableElement();
}
Listing 2 models a network device called a switchable element, which is simply a device with one or more physical interfaces that can be enabled and/or disabled. An example of such a device is an Ethernet interface. To see the state (and other details) for an Ethernet interface under Linux, just type the command ifconfig. This command provides a plethora of information for the system Ethernet interfaces, such as the state, the IP address assigned to the interface, the version of IP running, and so on.
Remember: Pretty much all software basically models something in the real world, whether it's a word processing document, an online game, or a global weather simulation.
So, the interface SwitchableElement models the ability to enable and disable such a switchable interface. This is the essence of the command pattern—you use this pattern to "instruct" some element to do something. Likewise, you can code the Command pattern to reverse the instruction, undoing the action.
Going back to the Ethernet interface, if you want to modify the state of an interface from the command line, you just type this:
ifconfig eth1 up/down
Given the above discussion, Listing 3 illustrates an implementation of the interface. Notice that the implementation class in Listing 3 must retain the required state elementState and the previous state (called previousState) in order to support the required undo semantics.
Listing 3: An implementation of the Command pattern interface
public class ManagedElements implements SwitchableElement {
boolean elementState;
boolean previousState;
public ManagedElements(boolean elementState) {
this.elementState = elementState;
System.out.println("ManagedElements initial state: " + elementState);
}
public boolean getElementState() {
return elementState;
}
public void setElementState(boolean elementState) {
this.elementState = elementState;
}
public void enableElement() {
setElementState(true);
}
public void disableElement() {
setElementState(false);
}
public void execute() {
previousState = getElementState();
setElementState(!previousState);
}
/**
* @param args
*/
public static void main(String[] args) {
ManagedElements managedElements = new ManagedElements(false);
managedElements.execute();
System.out.println("ManagedElements post-execute() state: " + managedElements.getElementState());
managedElements.disableElement();
System.out.println("ManagedElements post-disable() state: " +
}
}
In Listing 3, the call to the execute() method serves to modify the interface state from false to true. Then, to implement the undo action, there's a call to managedElements.disableElement(). Listing 4 illustrates an example of running the implementation code.
Listing 4: An implementation of the Command pattern interface
ManagedElements initial state: false ManagedElements post-execute() state: true ManagedElements post-disable() state: false
The sample run in Listing 4 shows the initial state of the interface as false or disabled. The interface state then changes to true once the execute() method has run. The interface state changes back to false following the call to disableElement().
The Command Pattern and Java 8
As we've seen, the Command pattern is still useful! Now, what about its use in Java 8? What if we decided that a new method is needed in the SwitchableElement interface? Java 8 allows for the addition of default methods. This fact ties the implementation code for that new method directly to the interface. Listing 5 illustrates this fact with a new method called by the imaginative name of doSomethingElse(), added to the code from Listing 2.
Listing 5: An extension of the Command pattern interface
public interface SwitchableElement extends Command {
public void enableElement();
public void disableElement();
default public void doSomethingElse() { System.out.println("Hello") };
}
In the context of modeling a network device, an additional command pattern method could be something like backing up the configuration data for that device. This type of operation becomes important in large networks with many devices. Just as backing up devices is useful in this context, the ability to restore device configuration could be another candidate for addition to Listing 5.
The design decision to use the Command pattern for modeling our domain has paid dividends: Now we can easily extend the code as our requirements change. Java 8 also allows us to add default methods directly to the interface. This in turn reduces the programming burden in the Java interface implementation code; that is, Java 8 default interface code doesn't need a corresponding implementation. The combination of the Command pattern and Java 8 is a happy union!
Now, what about the Adaptor pattern?