如何对此类进行类型转换？

Question

我有一个叫做 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;
    }

如何在不出现 ClassCastException 的情况下将其类型转换为 ProBasketballPlayer

这是 ProBasketballPlayer 的构造函数

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;
    }

Answer 1

你不能。转换不会改变对象的任何内容，它只是告诉编译器它可以将对象解释为在类层次结构中进一步向上的类，但不能向下。一旦您实例化了一个对象，就是这样 - 该对象绝对是您实例化的类的成员，并且不可撤销。它是BasketballPlayer，永远不会是ProBasketballPlayer。如果您想要 Pro，请实例化一个 - 您可以将 Pro 用作普通篮球运动员，但反之则不行。

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.

Answer 2

如何在不获取的情况下将其类型转换为 ProBasketballPlayer ClassCastException

你不能投错方向。想想下面的两个陈述......

1. ISA BasketballPlayer 还是 ProBasketballPlayer？
2. 都是 BasketBallPlayers ProBasketBall 球员吗？

没有。我打篮球，但我不是职业篮球运动员。

每个Bird 都是Duck。

Bird b = new Bird();
Duck daffy = (Bird)b;
daffy.quack(); // Error here, not all birds quack?

Answer 3

您不能将对象强制转换为子类。对象只能从其构造函数的类型“向下”转换。每个Cat 都是Animal，但不是每个Animal 都是Cat。投射只有一种方式。

您可以做的是在ProBasketballPlayer 中创建一个接受BasketBallPlayer 并在相关字段中复制的构造函数。确保初始化 ProBasketballPlayer 可能还有的任何其他字段。

public ProBasketballPlayer(BasketBallPlayer bbp, int yearsInLeague, String role) {
    super(bbp.getName(), bbp.getPosition(), bbp.getTeam(), bbp.getHeight(), bbp.getWeight(), bbp.getAgility(), bbp.getSpeed(), bbp.getBallHandling());
    this.yearsInLeague = yearsInLeague;
    this.role = role;
}

public public ProBasketballPlayer(BasketBallPlayer bbp) {
    this(bbp, 0, "bench");
}

如果ProBasketballPlayer 与BasketBallPlayer 在同一个包中，那么您可以直接访问字段（例如bbp.name）而不是使用getter。