目录
一、场景 1、文本编辑器并不是一个好的例子，设备控制器才是 2、设备控制器的demo 二、不用命令模式
三、使用命令模式
四、进一步思考 1、省略对Command的建模可以吗？ 2、命令模式的价值
脱离场景谈设计模式毫无意义。先有某种场景，然后大神们（GoF）对其进行建模，以实现高内聚低耦合。 看了不少讲命令模式的文章，举的例子基本都是文本编辑器。 然而，并没有揭示命令模式的精髓。反而让读者觉得命令模式像过度设计。 生活中，我们手机上会装一个设备控制器 ，里面既可以控制空调的开关，还可以控制电视的开关。我将通过这个例子来阐述命令模式到底解决了什么问题 。 用户选择空调，并选择关闭：空调便关闭了。 用户选择电视机，并选择关闭：电视机便关闭了。 public class AirConditioner {
public void turnOn ( ) {
System . out. println ( "空调已开启" ) ;
}
public void turnOff ( ) {
System . out. println ( "空调已关闭" ) ;
}
}
public class Television {
public void turnOn ( ) {
System . out. println ( "电视已开启" ) ;
}
public void turnOff ( ) {
System . out. println ( "电视已关闭" ) ;
}
}
public class Application {
public static void main ( String [ ] args) {
AirConditioner airConditioner = new AirConditioner ( ) ;
Television television = new Television ( ) ;
Scanner scanner = new Scanner ( System . in) ;
while ( true ) {
String device = scanner. next ( ) ;
if ( device. equals ( "exit" ) ) {
break ;
}
String command = scanner. next ( ) ;
if ( device. equals ( "airConditioner" ) ) {
if ( command. equals ( "turnOn" ) ) {
airConditioner. turnOn ( ) ;
} else if ( command. equals ( "turnOff" ) ) {
airConditioner. turnOff ( ) ;
}
} else if ( device. equals ( "television" ) ) {
if ( command. equals ( "turnOn" ) ) {
television. turnOn ( ) ;
} else if ( command. equals ( "turnOff" ) ) {
television. turnOff ( ) ;
}
}
}
}
}
结果： 随着发展，设备大概率不止空调和电视机，一旦增加新设备，客户端的代码就要有很大的改动。而且，每台设备能执行的命令也不止开启和关闭，一旦新增命令，又要修改客户端的代码。这都违背了“开闭原则”。 另外，各种设备的代码杂糅在一起，违背了“单一职责原则” 。 public interface Receiver {
void turnOn ( ) ;
void turnOff ( ) ;
}
public class AirConditioner implements Receiver {
@Override
public void turnOn ( ) {
System . out. println ( "空调已开启" ) ;
}
@Override
public void turnOff ( ) {
System . out. println ( "空调已关闭" ) ;
}
}
public class Television implements Receiver {
@Override
public void turnOn ( ) {
System . out. println ( "电视已开启" ) ;
}
@Override
public void turnOff ( ) {
System . out. println ( "电视已关闭" ) ;
}
}
public abstract class Command {
protected Receiver receiver;
public void setReceiver ( Receiver receiver) {
this . receiver = receiver;
}
public abstract void execute ( ) ;
}
public class TurnOffCommand extends Command {
@Override
public void execute ( ) {
receiver. turnOff ( ) ;
}
}
public class TurnOnCommand extends Command {
@Override
public void execute ( ) {
receiver. turnOn ( ) ;
}
}
public class DeviceManager {
private final Map < String , Receiver > ;
private final Map < String , Command > ;
private DeviceManager ( ) {
devices = ImmutableMap . of (
DeviceEnum . AIR_CONDITIONER . getValue ( ) , new AirConditioner ( ) ,
DeviceEnum . TELEVISION . getValue ( ) , new Television ( )
) ;
commands = ImmutableMap . of (
CommandEnum . TURN_ON . getValue ( ) , new TurnOnCommand ( ) ,
CommandEnum . TURN_OFF . getValue ( ) , new TurnOffCommand ( )
) ;
}
public static DeviceManager getSingleton ( ) {
return new DeviceManager ( ) ;
}
public void execute ( String device, String commandType) {
Receiver receiver = devices. get ( device) ;
Command command = commands. get ( commandType) ;
command. setReceiver ( receiver) ;
command. execute ( ) ;
}
}
public class Application {
public static void main ( String [ ] args) {
DeviceManager deviceManager = DeviceManager . getSingleton ( ) ;
Scanner scanner = new Scanner ( System . in) ;
while ( true ) {
String device = scanner. next ( ) ;
if ( device. equals ( "exit" ) ) {
break ;
}
String command = scanner. next ( ) ;
deviceManager. execute ( device, command) ;
}
}
}
public class StandByCommand extends Command {
@Override
public void execute ( ) {
receiver. standby ( ) ;
}
}
public interface Receiver {
. . .
void standby ( ) ;
}
public class Washer implements Receiver {
@Override
public void turnOn ( ) {
System . out. println ( "洗衣机已开启" ) ;
}
@Override
public void turnOff ( ) {
System . out. println ( "洗衣机已关闭" ) ;
}
@Override
public void standby ( ) {
System . out. println ( "洗衣机已待机" ) ;
}
}
public class AirConditioner implements Receiver {
. . .
@Override
public void standby ( ) {
System . out. println ( "空调已待机" ) ;
}
}
public class Television implements Receiver {
. . .
@Override
public void standby ( ) {
System . out. println ( "电视已待机" ) ;
}
}
public class DeviceManager {
private final Map < String , Receiver > ;
private final Map < String , Command > ;
private DeviceManager ( ) {
devices = ImmutableMap . of (
. . .
DeviceEnum . WASHER . getValue ( ) , new Washer ( )
) ;
commands = ImmutableMap . of (
. . .
CommandEnum . STAND_BY . getValue ( ) , new StandByCommand ( )
) ;
}
. . .
}
之前写的代码，一行都没改动。只是通过新增代码，便实现了需求： 各个设备、各个命令都很单一，尽可能符合“单一职责”。 public class DeviceManager {
. . .
public void execute ( String device, String commandType) {
Device realDevice = deviceMap. get ( device) ;
realDevice. execute ( commandType) ;
}
. . .
}
public void execute ( String commandType) {
if ( "turnOn" . equals ( commandType) ) {
. . .
} else if ( "turnOff" . equals ( commandType) ) {
. . .
} else {
. . .
}
}
显然，去除对Command的建模后，代码变得冗余了。 当对真实场景建模后，各部分的交互逻辑如下图所示，便可以采用命令模式实现“高内聚、低耦合”的代码设计。
