java - 怎样打字这个类？

我有一个名为BasketBallPlayer的超类

我有一个名为ProBasketBallPlayer的子类

如果我创建一个对象

BasketBallPlayer bp1;
bp1=new BasketBallPlayer("Tim Duncan", "Center", "Spurs", 83, 220, 4, 5, 8);


public class BasketBallPlayer {
    protected String name;
    protected String position;
    protected String team;
    protected int height;
    protected int weight;
    protected int agility;
    protected int speed;
    protected int ballHandling;

    public BasketBallPlayer() {
        this.name = "unknown";
        this.position = "unknown";
        this.team = "unknown";
        this.height = 0;
        this.weight = 0;
        this.agility = 0;
        this.speed = 0;
        this.ballHandling = 0;
    }

    public BasketBallPlayer( String name, String position, String team)
    {
        this.name = name;
        this.position = position;
        this.team = team;
        this.height = 0;
        this.weight = 0;
        this.agility = 0;
        this.speed = 0;
        this.ballHandling = 0;
    }

    public BasketBallPlayer (String name, String position, String team, int height, int weight,
                int agility, int speed, int ballHandling)
    {
        this.name = name;
        this.position = position;
        this.team = team;
        this.height = height;
        this.weight = weight;
        this.agility = agility;
        this.speed = speed;
        this.ballHandling = ballHandling;
    }

如何在不获取ClassCastException的情况下将其转换为ProBasketballPlayer

这是ProBasketballPlayer构造函数

public class ProBasketballPlayer extends BasketBallPlayer {
    protected int yearsInLeague;
    protected String role;

    public ProBasketballPlayer()
    {
        super();
        yearsInLeague = 0;
        role = "bench";
    }

    public ProBasketballPlayer( String name, String position, String team )
    {
        super(name, position, team);
        this.name = name;
        this.position = position;
        this.team = team;
        yearsInLeague = 0;
        role = "bench";
    }

    public ProBasketballPlayer(String name, String position, String team, int height, int weight,
            int agility, int speed, int ballHandling, int yearsInLeague, String role)
    {
        super(name, position, team, height, weight, agility, speed, ballHandling);
        this.yearsInLeague = yearsInLeague;
        this.role = role;
    }

最佳答案

你不能。强制转换不会改变对象的任何内容，它只是告诉编译器可以将对象解释为一个类，该类位于类层次结构的最上层，而不能向下。一旦实例化一个对象，就是这样-该对象无疑是您实例化的类的成员，并且是不可撤销的成员。它是一个BasketballPlayer，永远不会是ProBasketballPlayer。如果您想要专业版，请实例化一个-您可以将Pro用作普通的篮球运动员，反之则不行。

举个例子：

class Foo
{
    int a;
}
class Bar extends Foo
{
    int b;
}

Foo obj = new Foo();
obj.a = 0; // our obj has an "a" field because it is a Foo.
obj.b = 0; // but no "b" field, because it is not a Bar.

// It therefore makes no sense to do this:
((Bar)obj).b = 0; // because that's the same as trying to do "obj.b"

// in either case, "obj" is not a "Bar", and cannot have a "b" field. "obj" will always be a "Foo", never a "Bar".

//
// If however we make a Bar:
Bar obj2 = new Bar();

// we can access both fields, since Bar has both of them
obj2.a = 0;
obj2.b = 0;

// and, since Bar is a SUPERSET of Foo (all Bar are Foo, not all Foo are Bar),
// we can even use obj2 as a Foo, and pass it to methods which accept a Foo.