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java�c�d��强制转换

C鸡鸡 — Wed, 22 Jul 2009 15:44:00 GMT

问题�Q?/span>
class A{
int sum;
}
class B extends A{
double sum;
public static void main(String args[]){
A a=new A();
B b=new B();
b=(A)a;//异常�Q�请教why?
}
}

回答�Q?br />

形象一点说�Q?br /> class 人{
int sum;
}
class 男�h extends 人{
double sum;
public static void main(String args[]){
�?a=new �?);
男�h b=new 男�h();
b=(男�h)a;
}
}
你能�?#8220;�?#8221;强制转换�?#8220;男�h”吗？�Q�很昄��不能
所�?#8220;父类向子�c��{换是强制转换”是指�q�种情况
A a=new B();
B b=(B)a;
此时a实际上是一个B的实例，但被定义为A�Q�这样就无法调用B中定义的�Ҏ(gu��)��Q�只有在强制转换为B的实例后才能调用B中定义的�Ҏ(gu��)��——前提是�Q�a是B的实�?br /> 如果�?br /> A a=new A();
B b=new B();
b=(A)a;
a本��n是A的实例，不具有B的特征，不可能�{换�ؓ(f��)B的实例�?br /> �q�就像你不能
Object obj=new Object();
String str = (String)obj;

C鸡鸡 2009-07-22 23:44 发表评论

C鸡鸡 — Sat, 27 Jun 2009 04:19:00 GMT

import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.*;
import java.awt.font.*;
import java.awt.geom.*;

public class CreateImage
{
    public static void main(String[] args) throws Exception
    {
        int width = 100;
        int height = 100;
        String s = "你好";

        File file = new File("c:/image.jpg");

        Font font = new Font("Serif", Font.BOLD, 10);
        BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = (Graphics2D)bi.getGraphics();
        g2.setBackground(Color.WHITE);
        g2.clearRect(0, 0, width, height);
        g2.setPaint(Color.RED);

        FontRenderContext context = g2.getFontRenderContext();
        Rectangle2D bounds = font.getStringBounds(s, context);
        double x = (width - bounds.getWidth()) / 2;
        double y = (height - bounds.getHeight()) / 2;
        double ascent = -bounds.getY();
        double baseY = y + ascent;

        g2.drawString(s, (int)x, (int)baseY);

        ImageIO.write(bi, "jpg", file);
    }
}

�l�果�Q?br />

C鸡鸡 2009-06-27 12:19 发表评论