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)

如果你傳入的對(duì)象是一個(gè)Graphics2D 就不會(huì)出現(xiàn)錯(cuò)誤。