将 instanceof 缩小到特定的子类

Question

如何缩小 instanceof 到特定的 child 而不会以 parent class 结束。

例如，如果我应该有一个 parent class WhosebugUsers 那么我有 TenKUsers、HundredKusers 等的直接子 class等等..现在我的问题是我如何使用instanceof来获取Object是否是WhosebugUsers的直接子class之一的实例并且没有结束可以选择我的 object 是 WhosebugUsers 的一个实例 .. 这可能吗？

Answer 1

只需使用 instanceof TenKUsers（等）成语。

它会告诉您您正在查看的对象是否是该基数的子对象 class。

由于您暗示 TenKUsers 是 WhosebugUsers 的子对象，因此 instanceof WhosebugUsers 将也成为该对象的 true，当然。

Answer 2

您可以通过检查 .getClass() 排除超类本身，但 "grandchildren" 也将被匹配...

您可以使用反射，但通常应避免反射（有点 "cheating"）。

public class T {

    static class WhosebugUsers {}

    static class TenKUsers extends WhosebugUsers {}
    static class HundredKusers extends WhosebugUsers {}

    static class EtcEtc extends TenKUsers {}

    public static void main(String[] args) {

        System.out.println("Instance of String: "+checkInstance(new String()));
        System.out.println("Instance of WhosebugUsers (superclass): "+checkInstance(new WhosebugUsers()));
        System.out.println("Instance of TenKUsers (direct subclass): "+checkInstance(new TenKUsers()));
        System.out.println("Instance of HundredKusers (direct subclass): "+checkInstance(new HundredKusers()));
        System.out.println("Instance of EtcEtc (indirect subclass): "+checkInstance(new EtcEtc()));

        System.out.println();
        System.out.println("using reflection:");
        System.out.println("Instance of String: "+checkInstanceUsingReflection(new String()));
        System.out.println("Instance of WhosebugUsers (superclass): "+checkInstanceUsingReflection(new WhosebugUsers()));
        System.out.println("Instance of TenKUsers (direct subclass): "+checkInstanceUsingReflection(new TenKUsers()));
        System.out.println("Instance of HundredKusers (direct subclass): "+checkInstanceUsingReflection(new HundredKusers()));
        System.out.println("Instance of EtcEtc (indirect subclass): "+checkInstanceUsingReflection(new EtcEtc()));

    }

    private static boolean checkInstance(Object o) {
        return (o instanceof WhosebugUsers) && (o.getClass() != WhosebugUsers.class);
    }

    private static boolean checkInstanceUsingReflection(Object o) {
        Class<?> superclass = o.getClass().getSuperclass();
        return superclass == WhosebugUsers.class;
    }
}

哪个returns:

Instance of String: false
Instance of WhosebugUsers (superclass): false
Instance of TenKUsers (direct subclass): true
Instance of HundredKusers (direct subclass): true
Instance of EtcEtc (indirect subclass): true

using reflection:
Instance of String: false
Instance of WhosebugUsers (superclass): false
Instance of TenKUsers (direct subclass): true
Instance of HundredKusers (direct subclass): true
Instance of EtcEtc (indirect subclass): false

也许你应该重新考虑你的类。似乎 TenKUsers 也可能是存储在 WhosebugUsers 中的布尔标志。甚至更好：给你的 WhosebugUsers 一个 EnumSet<Archievments> 并使 Archievments 包含 "TenKUser" 和 "HundredKusers".

Answer 3

除了 Mena 的回答，如果你想使用 instanceof 来检查，你可以让你的超类 Whosebugusers 抽象以确保没有实例可以存在然后检查：

object instanceof Whosebugusers

将 instanceof 缩小到特定的子类

narrow down instanceof to a specific subclass

java

inheritance