为什么不出现任何一条线？

Question

我正在编写一个程序，我可以绘制一些行，并且我正在测试Canvas和awt.Frame如何合作。这是代码：

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.ArrayList;

public class Drawing extends Canvas{

    Point2D pt1, pt2;
    private ArrayList<Shape> linesList;
    private Shape shape = null;
    public Drawing(){
        linesList = new ArrayList<Shape>();

        Point p1 = new Point();p1.x = 200;p1.y = 200;
        Point p2 = new Point();p2.x = 300;p2.y = 300;

        pt1 = (Point2D) p1;
        pt2 = (Point2D) p2;
        //Line k = new Line(p1,p2,Color.BLUE);
        shape = new Line2D.Double ( pt1, pt2 );
        linesList.add(shape);
    }

    public void paint(Graphics g){
        Graphics2D g2d = (Graphics2D) g;

        //g2d.drawLine(200, 200, 200, 300);
        //Shape k = null;
        //k = new Shape();
        //g2d.setPaint(Color.BLACK );
        g2d.setColor(Color.GREEN);
        for ( Shape line : linesList )
        {   
            g2d.draw(line);          
        }   
    }

    public static void main(String[] args){
        Frame f = new Frame("Rysuje");

        f.setResizable(false);
        f.setSize(600, 600);
        f.setVisible(true);
        Drawing myFrame = new Drawing();

        f.add(myFrame);
    }
}

我的问题是：g2d.draw(line);方法中的paint()为什么不划线？事实上它并没有画任何东西。但是当我写g2d.drawLine(200, 200, 200, 300);时，它有效......

Answer 1

使用此来源..

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.ArrayList;

public class Drawing extends Canvas{

    Point2D pt1, pt2;
    private ArrayList<Shape> linesList;
    private Shape shape = null;
    public Drawing(){
        linesList = new ArrayList<Shape>();

        Point p1 = new Point();p1.x = 200;p1.y = 200;
        Point p2 = new Point();p2.x = 300;p2.y = 300;

        pt1 = (Point2D) p1;
        pt2 = (Point2D) p2;
        //Line k = new Line(p1,p2,Color.BLUE);
        shape = new Line2D.Double ( pt1, pt2 );
        linesList.add(shape);
    }

    public void paint(Graphics g){
        Graphics2D g2d = (Graphics2D) g;

        g2d.setColor(Color.RED);
        g2d.drawLine(20, 20, 100, 100);
        //Shape k = null;
        //k = new Shape();
        //g2d.setPaint(Color.BLACK );
        g2d.setColor(Color.GREEN);
        for ( Shape line : linesList )
        {
            g2d.draw(line);
        }
        g2d.setColor(Color.BLUE);
    }

    public static void main(String[] args){
        Frame f = new Frame("Rysuje");

        f.setResizable(false);
        f.setSize(400, 400);
        f.setVisible(true);
        Drawing myFrame = new Drawing();

        f.add(myFrame);
    }
}

..我明白这一点：

现在我觉得绿色线出现了，所以我对你期望看到的不同之处感到有些困惑。你看到了同样的事吗？