Question

我想创建一个空的.ser文件来存储对象的数组列表（如果尚未创建），以便以后在程序中使用。我将添加如果不存在，请在弄清楚当前问题后再进行检查。我有一个nullpointer异常在我的文件=新文件（等）行上。以下是截图的更好解释。

编辑：添加了源代码

/*
* To change this license header, choose License Headers in Project 
Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package dark.rusty;

import java.awt.Color;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;

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

/**
 * Creates new form Gui
 */
public Gui() {
    initComponents();
    setLocationRelativeTo(null);
    setTitle("CMS - Dark&Rusty");
    this.getContentPane().setBackground(new Color(190, 190, 190));

    MenuItem a = new MenuItem("temp", 0, 0);
    ArrayList<MenuItem> tempAList = new ArrayList<MenuItem>();
    createFileIFN();

}

private void createFileIFN() {
    try {

        File myFile = new File((this.getClass().getResource("/text/menu.ser").toString());

        if (myFile.createNewFile()) {
            System.out.println("File is created!");
        } else {
            System.out.println("File already exists.");
        }

    } catch (IOException e) {
        e.printStackTrace();
    }
}

private ArrayList<MenuItem> getArrList(File tempFile) {
    ArrayList<MenuItem> items = new ArrayList<MenuItem>();
    MenuItem tempItem;
    try {
        FileInputStream fileIn = new FileInputStream(tempFile);
        ObjectInputStream in = new ObjectInputStream(fileIn);
        tempItem = (MenuItem) in.readObject();
        in.close();
        fileIn.close();
    } catch (IOException i) {
        i.printStackTrace();
    } catch (ClassNotFoundException c) {
        System.out.println("Employee class not found");
        c.printStackTrace();
    }
    return items;
}

private void writeItemtoFile(MenuItem a, File tempFile) {
    try {

        FileOutputStream fos = new FileOutputStream(tempFile);
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        // write object to file
        oos.writeObject(a);
        // closing resources
        oos.close();
        fos.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

/**
 * 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();
    jButton7 = new javax.swing.JButton();
    jButton8 = new javax.swing.JButton();
    jPanel2 = new javax.swing.JPanel();
    jScrollPane2 = new javax.swing.JScrollPane();
    jList2 = new javax.swing.JList<>();
    jButton6 = new javax.swing.JButton();
    jPanel3 = new javax.swing.JPanel();
    jLabel1 = new javax.swing.JLabel();
    jLabel2 = new javax.swing.JLabel();
    jLabel3 = new javax.swing.JLabel();
    jLabel4 = new javax.swing.JLabel();
    jLabel5 = new javax.swing.JLabel();
    jLabel6 = new javax.swing.JLabel();
    jButton9 = new javax.swing.JButton();
    jButton10 = new javax.swing.JButton();
    jScrollPane3 = new javax.swing.JScrollPane();
    jList3 = new javax.swing.JList<>();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
    setBackground(new java.awt.Color(190, 190, 190));
    setResizable(false);
    setSize(new java.awt.Dimension(800, 600));

    jPanel1.setBackground(new java.awt.Color(190, 190, 190));
    jPanel1.setPreferredSize(new java.awt.Dimension(800, 50));

    jButton7.setIcon(new javax.swing.ImageIcon(getClass().getResource("/images/if_Sidebar_Left_Interface_Menu_Window_1654369.png"))); // NOI18N
    jButton7.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
    jButton7.setMargin(new java.awt.Insets(2, 20, 2, 20));
    jButton7.setPreferredSize(new java.awt.Dimension(32, 32));
    jPanel1.add(jButton7);

    jButton8.setIcon(new javax.swing.ImageIcon(getClass().getResource("/images/if_TaskStroke_1737368.png"))); // NOI18N
    jButton8.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
    jButton8.setMargin(new java.awt.Insets(2, 20, 2, 20));
    jButton8.setPreferredSize(new java.awt.Dimension(32, 32));
    jPanel1.add(jButton8);

    jPanel2.setBackground(new java.awt.Color(190, 190, 190));

    javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
    jPanel2.setLayout(jPanel2Layout);
    jPanel2Layout.setHorizontalGroup(
        jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 800, Short.MAX_VALUE)
    );
    jPanel2Layout.setVerticalGroup(
        jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 100, Short.MAX_VALUE)
    );

    jScrollPane2.setPreferredSize(new java.awt.Dimension(300, 120));

    jList2.setBackground(new java.awt.Color(190, 190, 190));
    jList2.setBorder(javax.swing.BorderFactory.createMatteBorder(2, 2, 2, 2, new java.awt.Color(240, 240, 240)));
    jList2.setFont(new java.awt.Font("Rockwell", 1, 14)); // NOI18N
    jList2.setModel(new javax.swing.AbstractListModel<String>() {
        String[] strings = { "Item 1", "Item 2", "Item 3", "Item 4", "Item 5" };
        public int getSize() { return strings.length; }
        public String getElementAt(int i) { return strings[i]; }
    });
    jList2.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);
    jList2.setPreferredSize(new java.awt.Dimension(290, 80));
    jScrollPane2.setViewportView(jList2);

    jButton6.setIcon(new javax.swing.ImageIcon(getClass().getResource("/images/if_Tick_1737369 (1).png"))); // NOI18N
    jButton6.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
    jButton6.setPreferredSize(new java.awt.Dimension(32, 32));
    jButton6.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            jButton6ActionPerformed(evt);
        }
    });

    jPanel3.setBackground(new java.awt.Color(190, 190, 190));
    jPanel3.setPreferredSize(new java.awt.Dimension(290, 136));

    jLabel1.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel1.setForeground(new java.awt.Color(51, 102, 0));
    jLabel1.setText("Kalan:");
    jLabel1.setPreferredSize(new java.awt.Dimension(120, 30));

    jLabel2.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel2.setForeground(new java.awt.Color(204, 0, 51));
    jLabel2.setText("Odenen:");
    jLabel2.setPreferredSize(new java.awt.Dimension(120, 30));

    jLabel3.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel3.setForeground(new java.awt.Color(0, 153, 255));
    jLabel3.setText("0");
    jLabel3.setToolTipText("");
    jLabel3.setPreferredSize(new java.awt.Dimension(120, 30));

    jLabel4.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel4.setForeground(new java.awt.Color(51, 102, 0));
    jLabel4.setText("5.00");
    jLabel4.setPreferredSize(new java.awt.Dimension(120, 30));

    jLabel5.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel5.setForeground(new java.awt.Color(204, 0, 51));
    jLabel5.setText("13.00");
    jLabel5.setPreferredSize(new java.awt.Dimension(120, 30));

    jLabel6.setFont(new java.awt.Font("Poiret One", 1, 24)); // NOI18N
    jLabel6.setForeground(new java.awt.Color(0, 153, 255));
    jLabel6.setText("İptal:");
    jLabel6.setPreferredSize(new java.awt.Dimension(120, 30));

    javax.swing.GroupLayout jPanel3Layout = new javax.swing.GroupLayout(jPanel3);
    jPanel3.setLayout(jPanel3Layout);
    jPanel3Layout.setHorizontalGroup(
        jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(jPanel3Layout.createSequentialGroup()
            .addContainerGap()
            .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(jPanel3Layout.createSequentialGroup()
                    .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 40, Short.MAX_VALUE)
                    .addComponent(jLabel4, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGroup(jPanel3Layout.createSequentialGroup()
                    .addComponent(jLabel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jLabel5, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel3Layout.createSequentialGroup()
                    .addComponent(jLabel6, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jLabel3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)))
            .addContainerGap())
    );
    jPanel3Layout.setVerticalGroup(
        jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(jPanel3Layout.createSequentialGroup()
            .addContainerGap()
            .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(jLabel4, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
            .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(jLabel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(jLabel5, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 13, Short.MAX_VALUE)
            .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(jLabel3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(jLabel6, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addContainerGap())
    );

    jButton9.setIcon(new javax.swing.ImageIcon(getClass().getResource("/images/if_Dollar_1737376(1).png"))); // NOI18N
    jButton9.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
    jButton9.setPreferredSize(new java.awt.Dimension(32, 32));

    jButton10.setIcon(new javax.swing.ImageIcon(getClass().getResource("/images/if_Wrong_1737371.png"))); // NOI18N
    jButton10.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
    jButton10.setPreferredSize(new java.awt.Dimension(32, 32));

    jScrollPane3.setPreferredSize(new java.awt.Dimension(300, 120));

    jList3.setBackground(new java.awt.Color(190, 190, 190));
    jList3.setBorder(javax.swing.BorderFactory.createMatteBorder(2, 2, 2, 2, new java.awt.Color(240, 240, 240)));
    jList3.setFont(new java.awt.Font("Rockwell", 1, 14)); // NOI18N
    jList3.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);
    jList3.setPreferredSize(new java.awt.Dimension(290, 80));
    jScrollPane3.setViewportView(jList3);

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        .addGroup(layout.createSequentialGroup()
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
                .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGroup(layout.createSequentialGroup()
                    .addGap(6, 6, 6)
                    .addComponent(jScrollPane2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                        .addGroup(layout.createSequentialGroup()
                            .addComponent(jButton6, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                            .addComponent(jButton9, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGroup(layout.createSequentialGroup()
                            .addGap(0, 0, Short.MAX_VALUE)
                            .addComponent(jButton10, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)))
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                    .addComponent(jScrollPane3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addGap(10, 10, 10)))
            .addGap(0, 0, Short.MAX_VALUE))
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
            .addComponent(jPanel3, javax.swing.GroupLayout.PREFERRED_SIZE, 300, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addContainerGap())
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(jButton6, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jScrollPane2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jButton9, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(jButton10, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)))
                .addComponent(jScrollPane3, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(jPanel3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 182, Short.MAX_VALUE)
            .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE))
    );

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

private void jButton6ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    // TODO add your handling code here:
}                                        

/**
 * @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(Gui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(Gui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(Gui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(Gui.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 Gui().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
private javax.swing.JButton jButton10;
private javax.swing.JButton jButton6;
private javax.swing.JButton jButton7;
private javax.swing.JButton jButton8;
private javax.swing.JButton jButton9;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JLabel jLabel4;
private javax.swing.JLabel jLabel5;
private javax.swing.JLabel jLabel6;
private javax.swing.JList<String> jList2;
private javax.swing.JList<String> jList3;
private javax.swing.JPanel jPanel1;
private javax.swing.JPanel jPanel2;
private javax.swing.JPanel jPanel3;
private javax.swing.JScrollPane jScrollPane2;
private javax.swing.JScrollPane jScrollPane3;
// End of variables declaration                   
}

Answer 1

this.getClass().getResource("/text/menu.ser")

如果null不存在，

将返回"/text/menu.ser"。因此，

this.getClass().getResource("/text/menu.ser").toString()

将产生一个NullPointerException。

此外，getResource返回一个URL实例，并且URL的字符串表示形式（例如file:/home/user3524633/text/menu.ser）与本地文件系统路径（即您要与File构造函数一起使用的内容。

为什么首先要使用getResource？您是否要确定相对于应用程序类的路径？

Java在资源文件夹上创建一个空的.ser文件（如果尚不存在）

1 个答案: