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多态允许不同类的对象对同一消息做出不同的响应。在 Java 中，多态主要通过继承和接口实现。

场景一：方法参数多态

// 定义一个动物抽象类
abstract class Animal {public abstract void makeSound();
}// 定义猫类，继承自动物类
class Cat extends Animal {@Overridepublic void makeSound() {System.out.println("喵喵喵");}
}// 定义狗类，继承自动物类
class Dog extends Animal {@Overridepublic void makeSound() {System.out.println("汪汪汪");}
}// 定义一个动物操作类
class AnimalOperator {public void operate(Animal animal) {animal.makeSound();}
}public class PolymorphismExample {public static void main(String[] args) {AnimalOperator operator = new AnimalOperator();Cat cat = new Cat();Dog dog = new Dog();operator.operate(cat); // 传入猫对象operator.operate(dog); // 传入狗对象}
}

场景二：返回值多态

// 定义一个形状抽象类
abstract class Shape {public abstract double area();
}// 定义圆形类，继承自形状类
class Circle extends Shape {private double radius;public Circle(double radius) {this.radius = radius;}@Overridepublic double area() {return Math.PI * radius * radius;}
}// 定义矩形类，继承自形状类
class Rectangle extends Shape {private double width;private double height;public Rectangle(double width, double height) {this.width = width;this.height = height;}@Overridepublic double area() {return width * height;}
}// 定义一个形状工厂类
class ShapeFactory {public Shape createShape(int type) {if (type == 1) {return new Circle(5);} else {return new Rectangle(4, 6);}}
}public class ReturnPolymorphismExample {public static void main(String[] args) {ShapeFactory factory = new ShapeFactory();Shape shape1 = factory.createShape(1);Shape shape2 = factory.createShape(2);System.out.println("Shape 1 area: " + shape1.area());System.out.println("Shape 2 area: " + shape2.area());}
}

向上转型和向下转型示例

// 定义一个父类
class Parent {public void print() {System.out.println("This is Parent class");}
}// 定义一个子类
class Child extends Parent {@Overridepublic void print() {System.out.println("This is Child class");}public void childMethod() {System.out.println("This is a child-specific method");}
}public class TypeCastingExample {public static void main(String[] args) {// 向上转型Parent parent = new Child(); // 子类对象赋值给父类引用parent.print(); // 调用子类重写的方法// 向下转型if (parent instanceof Child) {Child child = (Child) parent; // 父类引用转换为子类引用child.print();child.childMethod(); // 调用子类特有的方法}}
}

instanceof 关键字使用示例

instanceof 关键字用于检查一个对象是否是某个类或接口的实例。

// 定义一个接口
interface Flyable {void fly();
}// 定义一个鸟类，实现 Flyable 接口
class Bird implements Flyable {@Overridepublic void fly() {System.out.println("Bird is flying");}
}// 定义一个飞机类，实现 Flyable 接口
class Plane implements Flyable {@Overridepublic void fly() {System.out.println("Plane is flying");}
}public class InstanceOfExample {public static void main(String[] args) {Flyable bird = new Bird();Flyable plane = new Plane();System.out.println(bird instanceof Bird); // 输出 trueSystem.out.println(bird instanceof Flyable); // 输出 trueSystem.out.println(plane instanceof Bird); // 输出 false}
}