Interpreter - Design Patterns Java - page 204

public void execute(Context c)
{
Iterator i = c.machines.iterator();
while (i.hasNext())
{
Machine m = (Machine) i.next();
c.assign(variable, m);
body.execute(c);
}
}
This code iterates over the machines in the provided context, assigning a variable to each
machine and executing the loop's body. The idea here is for clients to create a body that
depends on the variable, as in a normal for loop. Consider a short Java program that shuts
down all the machines in the MachineLine context:
package com.oozinoz.robot.interpreter;
import com.oozinoz.machine.*;
public class ShowForCommand
{
public static void main(String[] args)
{
Context c = MachineLine.createContext();
Variable m = new Variable("m");
ForMachines fm =
new ForMachines(m, new ShutdownCommand(m));
fm.execute(c);
}
}
The body of the for command shuts down machine "m" . The for command assigns this
variable to each machine in the provided context and executes the body. The program
produces a log of the command execution:
Mixer1201 shutting down
Fuser1101 shutting down
StarPress1401 shutting down
ShellAssembler1301 shutting down
UnloadBuffer1501 shutting down
Implementors of the Term interface must implement eval() so that it returns an object of
type Machine . However, the Term hierarchy includes two classes that are effectively
Boolean operators that return null to mean false and return a Machine object to mean true.
Figure 25.4 shows the Boolean subclasses of Term .