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java中這個程序的Main是什么|我如何在main中調用line方法?

[英]What will be the Main of this program in java |How do i call the line method in main?

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
    }

}

您的問題表明您尚不清楚圖形和繪圖在 Swing GUI 中的工作方式。 給你一些建議:

  • 不要讓你的類擴展 java.awt.Frame 因為這沒有意義。 您不是在創建 AWT 框架窗口,也不需要這個。
  • 創建一個擴展 JPanel 的類,並按照Swing 繪畫教程(現已添加鏈接)向您展示的那樣在其paintComponent 方法中進行繪制
  • 您永遠不會直接調用繪圖代碼,而是由 JVM 間接調用。
  • 如果您想從 GUI 本身的外部進行繪制,請繪制到 BufferedImage,然后在 GUI 中顯示該圖像。
  • 不要使用通過在 GUI 組件上調用getGraphics()獲得的 Graphics 對象,因為這樣獲得的對象不會持久存在,這可能會導致您創建不穩定的圖形或更糟——拋出 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();
        });
    }
}

如果您想在程序運行時添加線條,請再次使用在 JPanel 的paintComponent 方法中繪制的BufferedImage。 當需要向GUI添加新行時,分別使用getGraphics()createGraphics()從BufferedImage中提取Graphics或Graphics2D對象(這樣做就可以了),用這個Graphics對象繪制,處理Graphics對象,並重新繪制 GUI。 例如,在下面的代碼中,我通過在 JButton 的 ActionListener 中添加代碼(實際上是它的 AbstractAction,它類似於 ActionListener 但功能更強大)來在按下按鈕時畫一條新線:

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();
        });
    }
}

Line 是一個非靜態方法,從靜態方法(這里是 main)調用它的唯一方法是擁有一個類的實例。

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