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VisitorDemo.java
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VisitorDemo.java
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/*
* Visitor Pattern Demo
* source: https://en.wikipedia.org/wiki/Visitor_pattern
*
*/
interface CarElement {
void accept(CarElementVisitor visitor);
}
interface CarElementVisitor {
void visit(Body body);
void visit(Car car);
void visit(Engine engine);
void visit(Wheel wheel);
}
class Body implements CarElement {
public void accept(final CarElementVisitor visitor) {
visitor.visit(this);
}
}
class Car implements CarElement {
CarElement[] elements;
public Car() {
this.elements = new CarElement[] {
new Wheel("front left"), new Wheel("front right"),
new Wheel("back left"), new Wheel("back right"),
new Body(), new Engine()
};
}
public void accept(final CarElementVisitor visitor) {
for (CarElement elem : elements) {
elem.accept(visitor);
}
visitor.visit(this);
}
}
class Engine implements CarElement {
public void accept(final CarElementVisitor visitor) {
visitor.visit(this);
}
}
class Wheel implements CarElement {
private String name;
public Wheel(final String name) {
this.name = name;
}
public String getName() {
return name;
}
public void accept(final CarElementVisitor visitor) {
/*
* accept(CarElementVisitor) in Wheel implements
* accept(CarElementVisitor) in CarElement, so the call
* to accept is bound at run time. This can be considered
* the *first* dispatch. However, the decision to call
* visit(Wheel) (as opposed to visit(Engine) etc.) can be
* made during compile time since 'this' is known at compile
* time to be a Wheel. Moreover, each implementation of
* CarElementVisitor implements the visit(Wheel), which is
* another decision that is made at run time. This can be
* considered the *second* dispatch.
*/
visitor.visit(this);
}
}
class CarElementDoVisitor implements CarElementVisitor {
public void visit(final Body body) {
System.out.println("Moving my body");
}
public void visit(final Car car) {
System.out.println("Starting my car");
}
public void visit(final Wheel wheel) {
System.out.println("Kicking my " + wheel.getName() + " wheel");
}
public void visit(final Engine engine) {
System.out.println("Starting my engine");
}
}
class CarElementPrintVisitor implements CarElementVisitor {
public void visit(final Body body) {
System.out.println("Visiting body");
}
public void visit(final Car car) {
System.out.println("Visiting car");
}
public void visit(final Engine engine) {
System.out.println("Visiting engine");
}
public void visit(final Wheel wheel) {
System.out.println("Visiting " + wheel.getName() + " wheel");
}
}
public class VisitorDemo {
public static void main(final String[] args) {
final Car car = new Car();
car.accept(new CarElementPrintVisitor());
car.accept(new CarElementDoVisitor());
}
}