Question

我想用Java旋转图片来执行此页面中的代码：

http://beginwithjava.blogspot.com/2009/02/rotating-image-with-java.html

到目前为止，我所做的是打开一个新的JFrame，在其中我放了一个带有jLabel的jPanel;这个jLabel使用icon属性将我的图像加载到标签中。之后，我从标签中选择了单击鼠标事件，并输入以下代码：

 private void jLabel1MouseClicked(java.awt.event.MouseEvent evt) {                                     
        // TODO add your handling code here:
        Graphics2D g2d=(Graphics2D)g; // Create a Java2D version of g.
        g2d.translate(170, 0); // Translate the center of our coordinates.
        g2d.rotate(1);  // Rotate the image by 1 radian.
        g2d.drawImage(image, 0, 0, 200, 200, this);
    }

它编译时没有错误，但是当我点击标签时会出现以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javaapplication74.NewJFrame.jLabel1MouseClicked(NewJFrame.java:94)
    at javaapplication74.NewJFrame.access$000(NewJFrame.java:17)
    at javaapplication74.NewJFrame$1.mouseClicked(NewJFrame.java:50)
    at java.awt.Component.processMouseEvent(Component.java:6508)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6270)

有什么方法可以解决这个问题吗？

第94行的错误指向：

g2d.translate(170, 0);

我添加了以下代码，但图片无法旋转：

package javaapplication74;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.Toolkit;
import javax.swing.ImageIcon;

/**
 *
 * @author 
 */
public class NewJFrame extends javax.swing.JFrame {

    /**
     * Creates new form NewJFrame
     */


    // Create a constructor method
    Image image;
    Graphics g;
    int angle;
    public NewJFrame() {
        initComponents();
        image = Toolkit.getDefaultToolkit().getImage("download.jpg");

    }
    public void paintComponent(Graphics g){
        Graphics2D g2d;
         g2d=(Graphics2D)g; // Create a Java2D version of g.
         g2d.translate(60, 0); // Translate the center of our coordinates.
         g2d.rotate(angle);  // Rotate the image by 1 radian.
         g2d.drawImage(image, 0, 0, 200, 200, this);
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        jLabel1 = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jLabel1.setIcon(new javax.swing.ImageIcon("C:\\Users\\Documents\\NetBeansProjects\\JavaApplication74\\src\\javaapplication74\\download.jpg")); // NOI18N
        jLabel1.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                jLabel1MouseClicked(evt);
            }
        });

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(19, 19, 19)
                .addComponent(jLabel1)
                .addContainerGap(20, Short.MAX_VALUE))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(26, 26, 26)
                .addComponent(jLabel1)
                .addContainerGap(27, Short.MAX_VALUE))
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(77, 77, 77)
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(261, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(59, 59, 59)
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(141, Short.MAX_VALUE))
        );

        pack();
    }// </editor-fold>                        

    private void jLabel1MouseClicked(java.awt.event.MouseEvent evt) {                                     
        // TODO add your handling code here:
        angle=20;
        jLabel1.repaint();
    }                                    

    /**
     * @param args the command line arguments
     */

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
            }
        });

    }
    // Variables declaration - do not modify                     
    private javax.swing.JLabel jLabel1;
    private javax.swing.JPanel jPanel1;
    // End of variables declaration                   
}

图像如下： enter image description here 什么可能是错的？

由于

Answer 1

Swing中的绘画应该在paintComponent方法的上下文中完成，如您的示例所示。

您应该做的是设置一个变量作为mouseClicked方法中的当前旋转角度并调用repaint

然后，在您的paintComponent方法中，您会执行类似......

的操作

Graphics2D g2d=(Graphics2D)g; // Create a Java2D version of g.
g2d.translate(170, 0); // Translate the center of our coordinates.
// You'll need to define angleOfRotatiomInRadians
// as an instance variable like image
g2d.rotate(angleOfRotatiomInRadians);  // Rotate the image by 1 radian.
g2d.drawImage(image, 0, 0, 200, 200, this);

根据对原始问题的更改进行了更新

所以基于你的示例代码...

错误＃1：

public class NewJFrame extends javax.swing.JFrame

...监守

错误＃2：

public void paintComponent(Graphics g){

如果您使用了@Override注释，编译器会告诉您，您试图覆盖父类层次结构中不存在的方法，这可能是您第一次报警铃

这意味着，您的paintComponent方法永远不会被调用，因为顶级容器（例如JFrame）实际上没有paintComponent方法。出于多种原因，您还应避免从顶层容器延伸或直接绘制。

除了双缓冲问题之外，这会让您看到一个容器，难以重用UI。相反，您应该尝试在JPanel作为基础创建UI，这样您就可以决定组件的使用时间和位置，使其更加实用。

有关在Swing中绘画的更多详情，请查看Performing Custom Painting和Painting in AWT and Swing

错误＃3：

jLabel1.repaint();

不会做太多，因为它包含对未经修改的图像参考的引用，所以重新绘制它无论如何都不会做太多，事实上，因为你正在尝试绘制图像本身，使用标签只是浪费时间

错误＃4：

当你真的不确定自己在做什么时，依靠表单编辑器。表单编辑器是一个强大的工具，但您需要先了解UI API的工作原理才能真正使用它。

我还鼓励开发人员在那里开始使用UI代码，这将使您更好地了解可以实现的目标以及何时何时不使用编辑器。

自定义绘画不是一个简单的主题，特别是当您不具备底层API的丰富经验时。花一些时间熟悉Swing API中可用的内容，它是如何工作的，以及何时应该潜入并做自己的事情。

作为一个例子......

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class RotateImage {

    public static void main(String[] args) {
        new RotateImage();
    }

    public RotateImage() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new RotatePane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class RotatePane extends JPanel {

        private double imageRotationAngle = 0;
        private BufferedImage img;

        public RotatePane() {
            try {
                img = ImageIO.read(new File("Java.png"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            addMouseListener(new MouseAdapter() {

                @Override
                public void mousePressed(MouseEvent e) {
                    imageRotationAngle += Math.toRadians(1);
                    repaint();
                }

            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            int x = (getWidth() - img.getWidth()) / 2;
            int y = (getHeight()- img.getHeight()) / 2;
            g2d.translate(x, y);
            g2d.rotate(imageRotationAngle, img.getWidth() / 2, img.getHeight() / 2);
            g2d.drawImage(img, 0, 0, this);
            g2d.dispose();
        }
    }

}

图像不会在netbeans中旋转

1 个答案: