某只鸟是一只鸽子

class People {
}

class Teacher extends People {
}

public class Demo3 {
public static void main(String[] args) {
People tom = new Teacher();
}
}
class Bird { }

class Pigeon extends Bird { }

public class Demo4 {
public static void main(String[] args) {
Bird bird = new Pigeon(); // 某只鸽子是一只鸟
Pigeon pigeon = bird; // 某只鸟是一只鸽子
}
}