java向下转型

Question

java小白，读java核心技术介绍Graphics2D类是Graphics的子类，里面有段代码
public void paintComponent(Graphics g){

Graphics2D g2 = (Graphics2D) g
......

}
这里向下转型不会报错吗？

Answer 1

public class test{
    public static void main(String[] args) {
        Graohics g = new Graphics2D();
        Graphics2D g2;
        if(g instanceof Graphics2D)
            g2 =  (Graphics2D) g;
        else
            System.out.print("error");
    }
}

class Graohics{

}

class Graphics2D extends Graohics{

}

output:(空)

public class test{
    public static void main(String[] args) {
        Graohics g = new Graphics();
        Graphics2D g2 =  (Graphics2D) g;
    }
}

java test
Exception in thread "main" java.lang.ClassCastException: Graohics cannot be cast to Graphics2D
    at test.main(test.java:5)

如果你传入的对象是一个Graphics2D 就不会出现错误。