Question

package drawinglinebymethods;

import java.awt.*;    
import javax.swing.JFrame;

public class DrawingLineByMethods extends Frame {

    public JFrame f=new JFrame();

    void fra_size()
    {
        f.setSize(450, 300);
    }

    void fra_visible()
    {
        f.setVisible(true);
    }

    void fra_title()
    {
        f.setTitle(" java frame");
    }

    void exit()
    {
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public void line(Graphics g) {
        g.drawLine(10, 10, 20, 300);
    }

    public static void main(String[] args) {
        DrawingLineByMethods obj = new DrawingLineByMethods();
        obj.fra_size();
        obj.fra_title();
        obj.fra_visible();
        obj.fra_exit();
        obj.line(g);  // <-- error here
    }

}

Answer 1

您的问题表明您还不清楚Swing GUI中的图形和绘图是如何工作的。一些建议：

不要让你的类扩展java.awt.Frame，因为这没有意义。您没有创建AWT框架窗口，也不需要这样做。
创建一个扩展JPanel并在其paintComponent方法中绘制的类，因为Swing painting tutorials（现已添加的链接）显示给您。
您永远不会直接调用绘图代码，而是由JVM间接调用。
如果您想从GUI本身外部绘制，则绘制到BufferedImage，然后在GUI中显示。
不要使用通过在GUI组件上调用getGraphics()获得的Graphics对象，因为这样获得的对象不会持久存在，这可能会导致您创建不稳定的图形或更糟糕 - 抛出NullPointerException。

例如：

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import javax.swing.*;

@SuppressWarnings("serial")
public class LineDraw extends JPanel {
    private static final int PREF_W = 450;
    private static final int PREF_H = 300;

    public LineDraw() {

    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        // use rendering hints to draw smooth lines
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        // do drawing here
        g.drawLine(10, 10, 20, 300);
    }

    private static void createAndShowGui() {
        LineDraw mainPanel = new LineDraw();

        JFrame frame = new JFrame("Line Draw");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            createAndShowGui();
        });
    }
}

如果要在程序运行时添加行，请再次使用在JPanel的paintComponent方法中绘制的BufferedImage。当您需要向GUI添加新行时，分别使用getGraphics()或createGraphics()从BufferedImage中提取Graphics或Graphics2D对象（可以执行此操作），使用此Graphics绘制object，处理Graphics对象，并重新绘制GUI。例如，在下面的代码中，我通过在JButton的ActionListener中添加代码来按下按钮时绘制一个新行（实际上它的AbstractAction类似于ActionListener但功能更强大）：

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.image.BufferedImage;

import javax.swing.*;

@SuppressWarnings("serial")
public class LineDraw extends JPanel {
    private static final int PREF_W = 450;
    private static final int PREF_H = 300;
    private BufferedImage img;
    private int yDistance = 20;
    private int deltaY = 10;

    public LineDraw() {
        img = new BufferedImage(PREF_W, PREF_H, BufferedImage.TYPE_INT_ARGB);
        add(new JButton(new DrawLineAction("Draw Line", KeyEvent.VK_D)));
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        // draw the buffered image here
        if (img != null) {
            g.drawImage(img, 0, 0, this);
        }

        // use rendering hints to draw smooth lines
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        // do drawing here
        g.drawLine(10, 10, 20, 300);
    }

    public void drawNewLines() {
        Graphics2D g2 = img.createGraphics();
        g2.setColor(Color.BLACK);
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        yDistance += deltaY;
        g2.drawLine(10, 10, yDistance, PREF_H);

        g2.dispose();
        repaint();
    }

    private class DrawLineAction extends AbstractAction {
        public DrawLineAction(String name, int mnemonic) {
            super(name); // give button its text
            putValue(MNEMONIC_KEY, mnemonic); // alt-hot key for button
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            drawNewLines();
        }
    }

    private static void createAndShowGui() {
        LineDraw mainPanel = new LineDraw();

        JFrame frame = new JFrame("Line Draw");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            createAndShowGui();
        });
    }
}

Answer 2

Line是一个非静态方法，从静态方法（此处为main）调用它的唯一方法是拥有该类的实例。

java中这个程序的主要内容是什么？如何在main中调用line方法？

2 个答案: