ビジターモードVisitor

//Purpose.  Visitor design pattern

//1. Add an accept(Visitor) method to the "element" hierarchy
//2. Create a "visitor" base class w/ a visit() method for every "element" type
//3. Create a "visitor" derived class for each "operation" to do on "elements"
//4. Client creates "visitor" objects and passes each to accept() calls

interface Element {
	public void accept(Visitor v); // first dispatch // 1. accept(Visitor)
} // interface

class This implements Element {
	public void accept(Visitor v) {
		v.visit(this);
	} // 1. accept(Visitor)

	public String thiss() {
		return "This";
	} // implementation
}

class That implements Element {
	public void accept(Visitor v) {
		v.visit(this);
	}

	public String that() {
		return "That";
	}
}

class TheOther implements Element {
	public void accept(Visitor v) {
		v.visit(this);
	}

	public String theOther() {
		return "TheOther";
	}
}

interface Visitor { // 2. Create a "visitor"
	public void visit(This e); // //// second dispatch // base class with a

	public void visit(That e); // visit() method for

	public void visit(TheOther e); // every "element"
} // type

class UpVisitor implements Visitor { // 3. Create a "visitor"
	public void visit(This e) { // derived class for
		System.out.println("do Up on " + e.thiss());
	} // each "operation"

	public void visit(That e) { // to perform on
		System.out.println("do Up on " + e.that());
	} // "elements"

	public void visit(TheOther e) {
		System.out.println("do Up on " + e.theOther());
	}
}

class DownVisitor implements Visitor {
	public void visit(This e) {
		System.out.println("do Down on " + e.thiss());
	}

	public void visit(That e) {
		System.out.println("do Down on " + e.that());
	}

	public void visit(TheOther e) {
		System.out.println("do Down on " + e.theOther());
	}
}

class VisitorDemo {
	public static Element[] list = { new This(), new That(), new TheOther() };

	public static void main(String[] args) {
		UpVisitor up = new UpVisitor(); // 4. Client creates
		DownVisitor down = new DownVisitor(); // "visitor" objects
		for (int i = 0; i < list.length; i++)
			// and passes each
			list[i].accept(up); // to accept() calls
		for (int i = 0; i < list.length; i++)
			list[i].accept(down);
	}
}