子类是父类的类型，但父类不是子类的类型。
子类的实例可以声明为父类型，但父类的实例不能声明为子类型。
class Vehicle {}
public class Car extends Vehicle {
public static void main(String args[]){
Vehicle v1 = new Vehicle(); //父类型
Vehicle v2 = new Car(); //子类的实例可以声明为父类型
Car c1 = new Car(); // 子类型
Car c2 = new Vehicle(); //这句会报错，父类型的实例不能声明为子类型
//Car（子类）是Vehicle（父类）类型, Vehicle（父类）不是Car（子类）类型
boolean result1 = c1 instanceof Vehicle; // true
boolean result2 = c1 instanceof Car; // true
boolean result3 = v1 instanceof Vehicle; // true
boolean result4 = v1 instanceof Car; // false
boolean result5 = v2 instanceof Vehicle; // true
boolean result6 = v2 instanceof Car; // true
System.out.println(result1);
System.out.println(result2);
System.out.println(result3);
System.out.println(result4);
System.out.println(result5);
System.out.println(result6);
}
}
从执行结果来看，虽然
