Design Patterns Java™ Workbook

Chapter 24. Command

CHALLENGE 24.2

Fill in the code for the anonymous subclasses of ActionListener, overriding

actionPerformed(), noting that this method expects an ActionEvent

argument.

When you outfit a menu with commands, you are plugging your commands into a context
provided by another developer. In other cases of COMMAND, you will take the role of the
context developer, providing the context in which commands will execute. For example, you
might want to provide a timing service that records how long methods take to execute.

Using Command to Supply a Service.....................................................................................................

Suppose that you want to let developers time how long a method takes to execute. Figure 24.1
shows a Command interface and a time() utility that times the execution of a command.

Figure 24.1. The time() method returns the number of milliseconds that a command

takes to execute.

The code for time() captures the system time before and after executing the command and
returns the difference:

public static long time(Command c)
{
long t1 = System.currentTimeMillis();
c.execute();
long t2 = System.currentTimeMillis();
return t2 - t1;
}

Suppose that you decide to test the time() method with an automated testing tool. One
reasonable test is that a command that sleeps for, say, 2,000 milliseconds should take about
2,000 milliseconds to execute: