在Java 中 类型转换

public class Fruit {
    private String color;
}
public class Banana extends Fruit {
    private String color;
    public Banana(String color) {
        this.color = color;
    }
    public String getColor() {
        return color;
    }
    public static void main(String[] args) {
        Fruit fruit = new Banana("yellow");
        if (fruit instanceof Banana) {
            Banana banana = (Banana) fruit;
            System.out.println(banana.getColor());
        }
    }
}

• 比较麻烦

fun main() {
    val fruit: Fruit = Banana("green")

    if (fruit is Banana) {
        println(fruit.color)
    }
    //fruit.color// 没有用 is 判断不能直接调用
}

• kotlin is 判断后，类型自动转换

强制转换作用范围

    var x: String? = "hello"
    if (x != null) {
        println(x.length) //类型 String (Smart cast to kotlin.String)
    }
    val length = x.length //类型 String?

• 在if 判断非空内，kotlin 智能帮我们转换了非空类型。

不支持智能转换的情况

var y: String? = "welcome"

fun main() {
    if (y != null) {
        println(y.length) //IDE报错提示
    }
}

• Smart cast to ‘String’ is impossible, because ‘y’ is a mutable property that could have been changed by this time
• 为什么？因为y是可变的，有可能其他地方如线程修改了它
• 可使用 val 定义 y

类型安全转换 as?

当as 转换失败返回 null 而不是抛异常

val fruit: Fruit = Banana("green")  
println((fruit as? Person)?.title)