Question

为了理解向下转换，我做了以下代码。

class Vehicle{
    protected int tyres=0;
    protected String name="default";
    Vehicle(){

    }
    Vehicle(String aname){
        name=aname;
    }
    //abstract void setTyres(int number);
    public void print(){
        System.out.println(name+":"+tyres);
    }
}

class TwoWheeler extends Vehicle{
    TwoWheeler(){
        tyres=2;
    }
    public void print(){
        System.out.println(name+":"+tyres);
    }

}

class FourWheeler extends Vehicle{
    FourWheeler(){
        tyres=4;
    }
    public void print(){
        System.out.println(name+":"+tyres);
    }

}

public class vehicles {
    public static void main(String[] args) {
        try{
        Vehicle v= new Vehicle("Dummy");
        v.print();
        v= new TwoWheeler();
        v.print();
        }
        catch(Exception e){
            System.out.println(e.toString());
        }
    }
}

现在输出是假：0 默认值：2

虽然我期待Dummy：0 Dummy：2 看起来第二次调用父构造函数？请解释这里发生的事情。另外，如何在不调用父项的情况下进行向下转换？

Answer 1

第二次调用父构造函数，因为您正在创建两个对象。

    Vehicle v = new Vehicle("Dummy"); // first object created here
    v.print();
    v = new TwoWheeler(); // second object created here

您不能将基类实例的类型（例如您创建的第一个对象）更改为子类实例。

您可以做的是定义一个接受TwoWheeler参数的Vehicle构造函数，并使用传递的Vehicle的属性初始化新实例。这称为复制构造函数。

TwoWheeler(Vehicle source) {
    super (source.getName());
    tyres=2;
}

您的main将如下所示：

    Vehicle v = new Vehicle("Dummy");
    v.print();
    v = new TwoWheeler(v);
    v.print();

Answer 2

Vehicle v = new Vehicle("Dummy"); 
//First object create with following properties,
name = "Dummy"
tyres = 0

v = new TwoWheeler(); 
//Second object created with following properties,

 /*   Flow goes like,
    TwoWheeler() constructor called,
    flow again go up to super and assign name with "default",
    while flow come back to TwoWheeler() constructor ,
    tyres = 2 assign.
 */

最后，根据您显示的内容输出。进入

在Java中再次关注父类构造函数？

2 个答案: