我的 Java Swing 绘图工具无法运行

Question

所以，我做了我的第一个有用的应用程序 - 一个绘画和绘图工具，但是......它无法运行（启动）。我不知道，问题是在我的电脑还是代码中......这是代码：

    package MaddpawNightpainter;
    import java.awt.BorderLayout;
    import java.awt.Container;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JPanel;
    public class Nightpainter2 {
      JButton clearBtn, blackBtn, blueBtn, greenBtn, redBtn, magentaBtn;
      Nightpainter drawArea;
      ActionListener actionListener = new ActionListener() {
      public void actionPerformed(ActionEvent e) {
          if (e.getSource() == clearBtn) {
            drawArea.clear();
          } else if (e.getSource() == blackBtn) {
            drawArea.black();
          } else if (e.getSource() == blueBtn) {
            drawArea.blue();
          } else if (e.getSource() == greenBtn) {
            drawArea.green();
          } else if (e.getSource() == redBtn) {
            drawArea.red();
          } else if (e.getSource() == magentaBtn) {
            drawArea.magenta();
          }
        }
      };
      public static void main(String[] args) {
        new Nightpainter().show();
      }

  public void show() {
    // create main frame
    JFrame frame = new JFrame("Nightpainter 1.0");
    Container content = frame.getContentPane();
    // set layout on content pane
    content.setLayout(new BorderLayout());
    // create draw area
    drawArea = new Nightpainter();
    // add to content pane
    content.add(drawArea, BorderLayout.CENTER);
    // create controls to apply colors and call clear feature
    JPanel controls = new JPanel();
    clearBtn = new JButton("Clear");
    clearBtn.addActionListener(actionListener);
    blackBtn = new JButton("Black");
    blackBtn.addActionListener(actionListener);
    blueBtn = new JButton("Blue");
    blueBtn.addActionListener(actionListener);
    greenBtn = new JButton("Green");
    greenBtn.addActionListener(actionListener);
    redBtn = new JButton("Red");
    redBtn.addActionListener(actionListener);
    magentaBtn = new JButton("Magenta");
    magentaBtn.addActionListener(actionListener);
    // add to panel
    controls.add(greenBtn);
    controls.add(blueBtn);
    controls.add(blackBtn);
    controls.add(redBtn);
    controls.add(magentaBtn);
    controls.add(clearBtn);
    // add to content pane
    content.add(controls, BorderLayout.NORTH);
    frame.setSize(500, 600);
    // can close frame
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    // show the swing paint result
    frame.setVisible(true);
    // :)
  }
}

另一个文件：

package MaddpawNightpainter;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import javax.swing.JComponent;
/*
Author: Bogdan Ganev
Title: Maddpaw Nightpainter
Description: Nightpainter is a drawing tool, part of Maddpaw - Multifunctional App for Designers,
Developers And Writers
Copyright: Copyright (C) 2016 Bogdan Ganev. All rights reserved. Maddpaw, Multifunctional App for Designers,
Developers And Writers is a trademark of Bogdan Ganev. Java TM is a trademark of Oracle Corporation (R) 
*/
public class Nightpainter extends JComponent {
  // Image in which we're going to draw
  private Image image;
  // Graphics2D object ==> used to draw on
  private Graphics2D g2;
  // Mouse coordinates
  private int currentX, currentY, oldX, oldY;
  public Nightpainter() {
    setDoubleBuffered(false);
    addMouseListener(new MouseAdapter() {
      public void mousePressed(MouseEvent e) {
        // save coord x,y when mouse is pressed
        oldX = e.getX();
        oldY = e.getY();
      }
    });
    addMouseMotionListener(new MouseMotionAdapter() {
      public void mouseDragged(MouseEvent e) {
        // coord x,y when drag mouse
        currentX = e.getX();
        currentY = e.getY();
        if (g2 != null) {
          // draw line if g2 context not null
          g2.drawLine(oldX, oldY, currentX, currentY);
          // refresh draw area to repaint
          repaint();
          // store current coords x,y as olds x,y
          oldX = currentX;
          oldY = currentY;
        }
      }
    });
  }
  protected void paintComponent(Graphics g) {
    if (image == null) {
      // image to draw null ==> we create
      image = createImage(getSize().width, getSize().height);
      g2 = (Graphics2D) image.getGraphics();
      // enable antialiasing
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      // clear draw area
      clear();
    }
    g.drawImage(image, 0, 0, null);
  }
  // now we create exposed methods
  public void clear() {
    g2.setPaint(Color.white);
    // draw white on entire draw area to clear
    g2.fillRect(0, 0, getSize().width, getSize().height);
    g2.setPaint(Color.black);
    repaint();
  }
  public void red() {
    // apply red color on g2 context
    g2.setPaint(Color.red);
  }
  public void black() {
    g2.setPaint(Color.black);
  }
  public void magenta() {
    g2.setPaint(Color.magenta);
  }
  public void green() {
    g2.setPaint(Color.green);
  }
  public void blue() {
    g2.setPaint(Color.blue);
  }
}

那么问题出在我的电脑还是代码中？顺便说一下，它是 10,3 KB...

Answer 1

将 NightPainter2 类中的代码更改为：

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Nightpainter2 {
  static JButton clearBtn, blackBtn, blueBtn, greenBtn, redBtn, magentaBtn;
  static Nightpainter drawArea;
  static ActionListener actionListener = new ActionListener() {
  public void actionPerformed(ActionEvent e) {
      if (e.getSource() == clearBtn) {
        drawArea.clear();
      } else if (e.getSource() == blackBtn) {
        drawArea.black();
      } else if (e.getSource() == blueBtn) {
        drawArea.blue();
      } else if (e.getSource() == greenBtn) {
        drawArea.green();
      } else if (e.getSource() == redBtn) {
        drawArea.red();
      } else if (e.getSource() == magentaBtn) {
        drawArea.magenta();
      }
    }
  };
  public static void main(String[] args) {
    // create main frame
        JFrame frame = new JFrame("Nightpainter 1.0");
        // set layout on content pane
        frame.setLayout(new BorderLayout());
        // create draw area
        drawArea = new Nightpainter();
        // add to content pane
        frame.add(drawArea, BorderLayout.CENTER);
        // create controls to apply colors and call clear feature
        JPanel controls = new JPanel();
        clearBtn = new JButton("Clear");
        clearBtn.addActionListener(actionListener);
        blackBtn = new JButton("Black");
        blackBtn.addActionListener(actionListener);
        blueBtn = new JButton("Blue");
        blueBtn.addActionListener(actionListener);
        greenBtn = new JButton("Green");
        greenBtn.addActionListener(actionListener);
        redBtn = new JButton("Red");
        redBtn.addActionListener(actionListener);
        magentaBtn = new JButton("Magenta");
        magentaBtn.addActionListener(actionListener);
        // add to panel
        controls.add(greenBtn);
        controls.add(blueBtn);
        controls.add(blackBtn);
        controls.add(redBtn);
        controls.add(magentaBtn);
        controls.add(clearBtn);
        // add to content pane
        frame.add(controls, BorderLayout.NORTH);
        frame.setSize(500, 600);
        // can close frame
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // show the swing paint result
        frame.setVisible(true);
        // :)
  }

我不知道它为什么有效，我只是在新的 NightPainter2.show（） 中看到一条通过"show"的行;所以尝试了这个。没想到它起作用，老实说:)

您可能希望添加frame.setLocationRelativeTo（null）;使其在屏幕中央打开。