设计模式2--装饰模式（the decorator design pattern）

允许多次实现一种方法，避免每次改动代码。

没有使用装饰模式：

Computer description()

Computer new_description()

Computer newer_description()

使用了装饰模式：

Computer description()

Computer description()

Disk description()

Computer description()

Disk description()

Monitor description()

public class Computer
{
public Computer(){}

public String description() {
return "You're getting a computer.";
}
}

public class Computer
{
public Computer(){}

public String description() {
return "You're getting a computer and a disk.";
}
}

public class Computer
{
public Computer(){}

public String description() {
return "You're getting a computer and a disk and a monitor.";
}
}

public class Computer
{
public Computer(){}

public String description() {
return "computer";
}
}

public abstract class ComponentDecorator extends Computer
{
public abstract String description();
}

public class Disk extends ComponentDecorator
{
Computer computer;

public Disk(Computer c)
{
computer = c;
}

public String description()
{
return computer.description()+" and a disk";
}
}

public class Monitor extends ComponentDecorator
{
Computer computer;

public Monitor(Computer c)
{
computer = c;
}

public String description()
{
return computer.description()+" and a monitor";
}
}

public class Test
{
public static void main(String args[]) {
Computer computer = new Computer();
computer = new Disk(computer);
computer = new Monitor(computer);
computer = new Monitor(computer);
System.out.println("You're getting a " + computer.description() + ".");
}
}