Java：如何在不使用父构造函数的情况下从子对象实例化父对象？

Question

Here's the example:

public class Car{
  private float param;
  public Car(float rawParam) {
    // process rawParam before assign it to this.param
    this.param = rawParam * 2;
  }
}

public class Truck extends Car{
  public Truck(Car car) {
    // How do I instantiate Car?
  }
}

There's no default constructor, so I can't just create some arbitrary parent object then clone the passed car by reflection. The only constructor provided needs some raw params which are unknown to the parent object itself. How can I instantiate Car and Truck in this case? Thanks!

Answer 1

If what you want his instance a Truck based on the value of another instance of Car , so copy a Car into a Truck , using your constructor, this COULD look like this :

public Truck(Car car) {
    super(car.getParam());
}

Assuming getParam exists and return the value you want!!

Note that if car is null, you will received a RuntimeException (a NullPointerException ), and you can't check that since Super need to be the first statement.

Well, you can but in some ways, like using a ternary operation :

public Truck(Car car) {
    super(car != null ? car.getParam() : -1);
}

There are other solution but this is not the point here.