放宽要求是什么意思?例如,在一个简单的方法中,它接收两个Integers并将它们一起添加到另一个Integer中,如何放松该方法中的需求?
package client
import even.Even
import natural.Natural
void f(Integer x,Integer y) {
Integer z = add(x,y);
}
Even和Natural都扩展了Integer类,如果这是相关的。所有三个类都有不同的add()方法,Integer是唯一一个在客户端的方法f()形式上有它的类。
答案 0 :(得分:1)
有趣的是 - 我在上一篇 post 中写了这篇文章:
通过使用 Visitor pattern (最初在“设计模式”一书中描述),可以在“单一调度”语言中实现“多个调度”行为。
从上面的链接中查看Java代码示例:
interface CarElementVisitor {
void visit(Wheel wheel);
void visit(Engine engine);
void visit(Body body);
void visit(Car car);
}
interface CarElement {
void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}
class Wheel implements CarElement {
private String name;
public Wheel(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
public void accept(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 Engine implements CarElement {
public void accept(CarElementVisitor visitor) {
visitor.visit(this);
}
}
class Body implements CarElement {
public void accept(CarElementVisitor visitor) {
visitor.visit(this);
}
}
class Car implements CarElement {
CarElement[] elements;
public Car() {
//create new Array of elements
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(CarElementVisitor visitor) {
for(CarElement elem : elements) {
elem.accept(visitor);
}
visitor.visit(this);
}
}
class CarElementPrintVisitor implements CarElementVisitor {
public void visit(Wheel wheel) {
System.out.println("Visiting " + wheel.getName() + " wheel");
}
public void visit(Engine engine) {
System.out.println("Visiting engine");
}
public void visit(Body body) {
System.out.println("Visiting body");
}
public void visit(Car car) {
System.out.println("Visiting car");
}
}
class CarElementDoVisitor implements CarElementVisitor {
public void visit(Wheel wheel) {
System.out.println("Kicking my " + wheel.getName() + " wheel");
}
public void visit(Engine engine) {
System.out.println("Starting my engine");
}
public void visit(Body body) {
System.out.println("Moving my body");
}
public void visit(Car car) {
System.out.println("Starting my car");
}
}
public class VisitorDemo {
public static void main(String[] args) {
CarElement car = new Car();
car.accept(new CarElementPrintVisitor());
car.accept(new CarElementDoVisitor());
}
}