Question

我希望能够将子类更改为超类，然后根据需要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？代码是否需要重组？

Answer 1

在此代码段中，您将GenericEvent作为静态类型（需要event的规范）和SpecificEvent作为动态类型（实际实现）：

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

如果您假设event是SpecificEvent，则转换为目标类型：

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

在大多数情况下，您首先要检查以获取动态类型：

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

上面的instanceof还会检查SpecificEvent的子类。如果您想明确检查event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;
}

来回转换超类和子类

1 个答案: