Design Pattern 简明教程
Design Patterns - Visitor Pattern
在访问者模式中，我们使用一个访问者类，它更改了一个元素类的执行算法。通过这种方式，元素的执行算法可以随着访问者的变化而变化。此模式属于行为模式类别。根据此模式，元素对象必须接受访问者对象，以便访问者对象处理对元素对象的操作。
In Visitor pattern, we use a visitor class which changes the executing algorithm of an element class. By this way, execution algorithm of element can vary as and when visitor varies. This pattern comes under behavior pattern category. As per the pattern, element object has to accept the visitor object so that visitor object handles the operation on the element object.
Implementation
我们准备创建一个定义接纳操作的 ComputerPart 接口。Keyboard、Mouse、Monitor 和 Computer 是实现 ComputerPart 接口的具体类。我们将定义另一个接口 ComputerPartVisitor，它将定义访问者类操作。Computer 使用具体访问者来执行相应操作。
We are going to create a ComputerPart interface defining accept opearation.Keyboard, Mouse, Monitor and Computer are concrete classes implementing ComputerPart interface. We will define another interface ComputerPartVisitor which will define a visitor class operations. Computer uses concrete visitor to do corresponding action.
我们的演示类 VisitorPatternDemo 将使用 Computer 和 ComputerPartVisitor 类来演示访问者模式的使用。
VisitorPatternDemo, our demo class, will use Computer and ComputerPartVisitor classes to demonstrate use of visitor pattern.
Step 1
定义一个接口来表示元素。
Define an interface to represent element.
ComputerPart.java
public interface ComputerPart {
public void accept(ComputerPartVisitor computerPartVisitor);
}Step 2
创建一个扩展上述类的具体类。
Create concrete classes extending the above class.
Keyboard.java
public class Keyboard implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}Monitor.java
public class Monitor implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}Mouse.java
public class Mouse implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}Computer.java
public class Computer implements ComputerPart {
ComputerPart[] parts;
public Computer(){
parts = new ComputerPart[] {new Mouse(), new Keyboard(), new Monitor()};
}
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
for (int i = 0; i < parts.length; i++) {
parts[i].accept(computerPartVisitor);
}
computerPartVisitor.visit(this);
}
}Step 3
确定一个界面来表示访问者。
Define an interface to represent visitor.
ComputerPartVisitor.java
public interface ComputerPartVisitor {
public void visit(Computer computer);
public void visit(Mouse mouse);
public void visit(Keyboard keyboard);
public void visit(Monitor monitor);
}Step 4
创建具体访问者实现上述类。
Create concrete visitor implementing the above class.
ComputerPartDisplayVisitor.java
public class ComputerPartDisplayVisitor implements ComputerPartVisitor {
@Override
public void visit(Computer computer) {
System.out.println("Displaying Computer.");
}
@Override
public void visit(Mouse mouse) {
System.out.println("Displaying Mouse.");
}
@Override
public void visit(Keyboard keyboard) {
System.out.println("Displaying Keyboard.");
}
@Override
public void visit(Monitor monitor) {
System.out.println("Displaying Monitor.");
}
}Step 5
使用 ComputerPartDisplayVisitor 来显示计算机的部分。
Use the ComputerPartDisplayVisitor to display parts of Computer.
VisitorPatternDemo.java
public class VisitorPatternDemo {
public static void main(String[] args) {
ComputerPart computer = new Computer();
computer.accept(new ComputerPartDisplayVisitor());
}
}