java - 关于java中虚方法的问题

Question

简而言之：我希望以下代码打印“sub”：

Element e = new SubElement();
print(e);
... 

private static void print(Element e) {
    System.out.println("e");
}

private static void print(SubElement e) {
    System.out.println("sub");
}

而且我不想更改打印（元素 e）。所以没有什么像

private static void print(Element e) {
    if (e instanceof SubElement) {
        print((SubElement) e);
    } else {
        System.out.println("e");
    }
}

我想做的是

print(e.getClass().cast(e));

自动将其转换为真正的子类并强制系统输入 print(SubElement e)。这有可能吗？

Answer 1

运行的重载方法是在编译时选择的，这就是为什么选择 Element 版本而不是 SubElement 版本的原因。看起来更合乎逻辑的是让 Element 或子类包含应该打印的数据。

class Element {

    public String getName() {
        return "e";
    }
}

class SubElement extends Element {
    public String getName() {
        return "sub";
    }
}

然后在打印方法中：

private static void print(Element e) {
    System.out.println(e.getName());
}

这是否有意义将取决于Element类实际上是什么以及打印的数据代表什么。

Answer 2

Yes. You can use Visitor pattern. However it's suitable for stablished well-defined hierarchies, because the Visitor interface you have to define needs a method for each type.

interface ElementVisitor {
   visit(Element e);
   visit(SubElement se);
}

class ElementerPrinter implements ElementVisitor {
   visit(Element e) { System.out.println("e"); }
   visit(SubElement e) { System.out.println("sub"); }
}

class Element {
  // here's the trick, Element knows that this is Element
  // and childs have to implement it!
  // if the parent-most class is an interface it force you to implement!
  accept(ElementVisitor visitor) { visitor.visit(this); } 
}

class SubElement {
  // here's the trick, Element knows that this is SubElement
  accept(ElementVisitor visitor) { visitor.visit(this); }
}

Answer 3

print() needs to become an instance method of Element, really. You're trying to imitate polymorphism in a hard way otherwise. If you wish to do that, you can't really avoid some series of if statements of mappings from Class to function objects. Why bother?

Answer 4

您是否能够将行为差异推送到元素类中？

Element e = new SubElement();
print(e);
... 

private static void print(Element e) {
    System.out.println(e.getMessageToPrint());
}

// no longer needed
//
//private static void print(SubElement e) {
//    System.out.println("sub");
//}

这样，SubElement可以重写getMessageToPrint()方法。

或者更好：

Element e = new SubElement();
e.print();

Answer 5

我会选择不同的方法。任何一个

1. 使用其他人建议的多态性，扩展 Element 以添加 print() 方法（可以被子类覆盖）或
2. 定义一个辅助接口并结合使用策略和工厂模式：

基类

 public class Element{}

派生类

 public class SubElement extends Element{}

打印元素的帮助界面

public interface PrintHelper{
    void print(Element element);
}

工厂为给定元素获取最佳 PrintHelper

public class PrintHelperFactory{

    private final Map<Class<? extends Element>, PrintHelper> registeredHelpers =
        new HashMap<Class<? extends Element>, PrintHelper>();

    // Register a PrintHelper for a given Element class.
    public void registerHelper(final Class<? extends Element> clazz,
      final PrintHelper helper){
        this.registeredHelpers.put(clazz, helper);
    }

    // Get the most specific PrintHelper for a given Element.
    public PrintHelper getHelperForElement(final Element element){
        Class<? extends Element> clazz = element.getClass();
        while(!Object.class.equals(clazz)){
            if(this.registeredHelpers.containsKey(clazz)){
                return this.registeredHelpers.get(clazz);
            }
            clazz = (Class<? extends Element>) clazz.getSuperclass();
        }
        return null;
    }

}

客户端测试类，作为 Java 应用程序运行

public class Main{

    public static void main(final String[] args){

        final PrintHelperFactory factory = new PrintHelperFactory();
        factory.registerHelper(Element.class, new PrintHelper(){
            @Override
            public void print(final Element element){
                System.out.println("Element");
            }
        });
        factory.registerHelper(SubElement.class, new PrintHelper(){
            @Override
            public void print(final Element element){
                System.out.println("Sub Element");
            }
        });

        // test it with an Element  
        final Element elem = new Element();
        factory.getHelperForElement(elem).print(elem);

        // test it with a sub class
        final Element sub = new SubElement();
        factory.getHelperForElement(sub).print(sub);

    }

}

输出

Element
Sub Element