如何在Java中使用instanceof判断对象类型

instanceof

true

false

// 解决方案
public class Animal { }
public class Dog extends Animal { }

public class Main {
    public static void main(String[] args) {
        Animal animal = new Animal();
        Dog dog = new Dog();

        System.out.println(animal instanceof Animal); // true
        System.out.println(dog instanceof Dog);       // true
        System.out.println(dog instanceof Animal);    // true，因为 Dog 是 Animal 的子类
        System.out.println(animal instanceof Dog);    // false，Animal 不是 Dog 的子类

        // 避免空指针异常
        Animal nullAnimal = null;
        System.out.println(nullAnimal instanceof Animal); // false，避免空指针异常

        // 使用 instanceof 进行类型转换前的检查
        if (animal instanceof Dog) {
            Dog myDog = (Dog) animal; // 类型转换
            // ...
        } else {
            System.out.println("animal 不是 Dog 类型的实例");
        }

        // 接口的判断
        interface Swimmable { }
        class Fish implements Swimmable { }

        Fish fish = new Fish();
        System.out.println(fish instanceof Swimmable); // true

    }
}

instanceof

List<Animal>

Dog

Cat

Animal

instanceof

import java.util.ArrayList;
import java.util.List;

public class PolymorphismExample {

    static class Animal {
        public void makeSound() {
            System.out.println("Generic animal sound");
        }
    }

    static class Dog extends Animal {
        @Override
        public void makeSound() {
            System.out.println("Woof!");
        }

        public void fetch() {
            System.out.println("Dog is fetching the ball");
        }
    }

    static class Cat extends Animal {
        @Override
        public void makeSound() {
            System.out.println("Meow!");
        }

        public void scratch() {
            System.out.println("Cat is scratching");
        }
    }

    public static void main(String[] args) {
        List<Animal> animals = new ArrayList<>();
        animals.add(new Dog());
        animals.add(new Cat());
        animals.add(new Animal());

        for (Animal animal : animals) {
            animal.makeSound(); // 多态调用，根据实际类型执行相应的方法

            if (animal instanceof Dog) {
                Dog dog = (Dog) animal;
                dog.fetch(); // Dog 特有的方法
            } else if (animal instanceof Cat) {
                Cat cat = (Cat) animal;
                cat.scratch(); // Cat 特有的方法
            }
        }
    }
}

instanceof

if-else

switch

instanceof

// 使用多态避免 instanceof
abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing a circle");
    }
}

class Square extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing a square");
    }
}

public class ShapeExample {
    public static void main(String[] args) {
        List<Shape> shapes = new ArrayList<>();
        shapes.add(new Circle());
        shapes.add(new Square());

        for (Shape shape : shapes) {
            shape.draw(); // 无需 instanceof，直接调用 draw 方法
        }
    }
}

instanceof

getClass()

instanceof

getClass()

public class ClassVsInstanceof {
    static class Animal {}
    static class Dog extends Animal {}

    public static void main(String[] args) {
        Animal animal = new Animal();
        Dog dog = new Dog();
        Animal animalDog = new Dog(); // 多态

        System.out.println(animal instanceof Animal); // true
        System.out.println(dog instanceof Animal);    // true
        System.out.println(animalDog instanceof Animal); // true
        System.out.println(animal instanceof Dog);    // false

        System.out.println(animal.getClass() == Animal.class); // true
        System.out.println(dog.getClass() == Dog.class);       // true
        System.out.println(animalDog.getClass() == Dog.class); // true，注意这里是 Dog
        System.out.println(animalDog.getClass() == Animal.class); // false

    }
}

getClass()

instanceof