为什么在尝试将实例变量打印为引用变量时出现异常？

Question

我正在学习对象，我正在尝试使用引用变量作为实例变量，但即使我已经给出了值，我也会得到“java.lang.NullPointerException”。

我做错了什么？

这是我的代码：

public static void main(String[] args) {

    Car car = new Car();
    car.brand = "Nissan";
    car.modelYear = 1998;
    car.owner.name = "Marshall";
    System.out.println("The car brand is "+car.brand +
            "car model is "+ car.modelYear+
            "cars owners name is "+ car.owner.name);
    }
}

class Car {
    Person owner;
    String  brand;
    int modelYear;
    int horsePower;
}

class Person {
    String name;
}

Answer 1

因为car.owner没有初始化

car.owner = 新人();

为什么在尝试将实例变量打印为引用变量时出现异常？

Why do I get Exception when I'm trying to print Instance variabel as Refrence variabel?

java

printing

object

instance-variables