Java 中使用的“instanceof"运算符是什么?

问题描述

instanceof 运算符的用途是什么?我见过类似的东西

What is the instanceof operator used for? I've seen stuff like

if (source instanceof Button) {
    //...
} else {
    //...
}

但对我来说没有任何意义.我已经做了我的研究，但只是举了例子，没有任何解释.

But none of it made sense to me. I've done my research, but came up only with examples without any explanations.

推荐答案

instanceof 关键字是一个二元运算符，用于测试一个对象(instance) 是给定类型的子类型.

instanceof keyword is a binary operator used to test if an object (instance) is a subtype of a given Type.

想象一下:

interface Domestic {}
class Animal {}
class Dog extends Animal implements Domestic {}
class Cat extends Animal implements Domestic {}

想象一个 dog object，用 Object dog = new Dog() 创建，然后:

Imagine a dog object, created with Object dog = new Dog(), then:

dog instanceof Domestic // true - Dog implements Domestic
dog instanceof Animal   // true - Dog extends Animal
dog instanceof Dog      // true - Dog is Dog
dog instanceof Object   // true - Object is the parent type of all objects

然而，使用Objectanimal = new Animal();,

animal instanceof Dog // false

因为Animal 是Dog 的超类型并且可能不那么精致".

because Animal is a supertype of Dog and possibly less "refined".

还有，

dog instanceof Cat // does not even compile!

这是因为Dog既不是Cat的子类型也不是超类型，也没有实现.

This is because Dog is neither a subtype nor a supertype of Cat, and it also does not implement it.

请注意，上面用于 dog 的变量是 Object 类型.这是为了表明 instanceof 是一个 运行时 操作，并将我们带到一个/用例:在运行时根据对象类型做出不同的反应.

Note that the variable used for dog above is of type Object. This is to show instanceof is a runtime operation and brings us to a/the use case: to react differently based upon an objects type at runtime.

注意事项:expressionThatIsNull instanceof T 对于所有类型 T 都是假的.

Things to note: expressionThatIsNull instanceof T is false for all Types T.

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