来回转换超类和子类

Question

我希望能够将子类更改为超类，然后在需要时返回其子类，以访问所有方法和字段并根据需要对其进行修改。

public class MainClass {
    public static main(String[] args) {
        SpecificEvent completeEvent = new SpecificEvent();
        GenericEvent event = completeEvent;
        event.fire();
        // without creating a new SpecificEvent how can i change str, without using the completeEvent reference, so that event.fire() has a different result?
    }
}

public abstract class GenericEvent {
    public abstract void fire();
}

public class SpecificEvent extends GenericEvent {
    public String str = "fired";
    @Override
    public void fire() {
        System.out.println(str);
    }
}

这可能吗？ 是否需要重组代码？

Answer 1

在此代码段中，您将GenericEvent作为静态类型（需要指定什么event的规范），并将SpecificEvent作为动态类型（实际的实现）：

//no cast needed, because SpecificEvent IS an GenericEvent
GenericEvent event = new SpecificEvent();

如果您假设event是SpecificEvent ，则强制转换为目标类型：

 //unsafe cast, exception is thrown if event is not a SpecificEvent SpecificEvent specEvent = (SpecificEvent) event; 

在大多数情况下，您将首先检查动态类型：

 if(event instanceof SpecificEvent) { //safe cast SpecificEvent specEvent = (SpecificEvent) event; } 

上面的instanceof还会检查SpecificEvent子类。 如果您想显式检查event是否是SpecificEvent （而不是SpecificEvent的子类 ！），请比较动态类型的类对象：

 if(event.getClass() == SpecificEvent.class) { //safe cast SpecificEvent specEvent = (SpecificEvent) event; }