Question

在frame3中，我单击按钮时调用frame4，并将节省的数据值从第3帧传递到第4帧。当我使用对话框从构造函数打印值时，它将打印准确的值。但是为什么当我尝试在保存时执行添加操作时却得到空指针异常？

// giving accurate value
JOptionPane.showMessageDialog(null, "from frame4 test1: "+savings);  

BigDecimal bd= new BigDecimal(2);

savings_new=savings;
savings_new=savings_new.add(bd);
// giving null pointer exception
JOptionPane.showMessageDialog(null, "from frm4 test3: "+savings_new);

`Frame4`

import java.awt.EventQueue;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.math.BigDecimal;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JSeparator;


public class Frame4 {

private JFrame frame;
private JLabel eid;
private String id_str3;
static int id_int3;
static BigDecimal savings,savings_new;

/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                Frame4 window = new Frame4(0, null);
                window.frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}
public Frame4(int id,BigDecimal save) {
    initialize();
    this.id_int3=id;
    this.savings=save;
}

/**
 * Initialize the contents of the frame.
 */
private void initialize() {
    frame = new JFrame();
    frame.setBounds(200,100,800,550);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().setLayout(null);

    JOptionPane.showMessageDialog(null, "from frame4 test1: "+savings); // giving accurate value
    BigDecimal bd= new BigDecimal(2);

    savings_new=savings;
    savings_new=savings_new.add(bd);
    JOptionPane.showMessageDialog(null, "from frm4 test3: "+savings_new);




    id_str3=Integer.toString(id_int3);
     eid = new JLabel("");
        eid.setFont(new Font("Tahoma", Font.PLAIN, 14));
        eid.setBounds(143, 83, 46, 13);
        frame.getContentPane().add(eid);                                                    //fetching id from frame1
        eid.setText(id_str3);

        JLabel lblEmployeeId = new JLabel("Employee ID:");
        lblEmployeeId.setFont(new Font("Tahoma", Font.PLAIN, 14));
        lblEmployeeId.setBounds(43, 79, 83, 23);
        frame.getContentPane().add(lblEmployeeId);

        JLabel lblDob = new JLabel("Name:");
        lblDob.setFont(new Font("Tahoma", Font.PLAIN, 14));
        lblDob.setBounds(214, 83, 46, 14);
        frame.getContentPane().add(lblDob);

        JLabel name = new JLabel("");
        name.setFont(new Font("Tahoma", Font.PLAIN, 13));
        name.setBounds(280, 83, 88, 14);
        frame.getContentPane().add(name);

        JLabel lblDob_1 = new JLabel("DOB:");
        lblDob_1.setFont(new Font("Tahoma", Font.PLAIN, 14));
        lblDob_1.setBounds(378, 83, 46, 14);
        frame.getContentPane().add(lblDob_1);

        JLabel dob = new JLabel("");
        dob.setFont(new Font("Tahoma", Font.PLAIN, 13));
        dob.setBounds(434, 82, 74, 15);
        frame.getContentPane().add(dob);

        JLabel lblDivision = new JLabel("Division:");
        lblDivision.setFont(new Font("Tahoma", Font.PLAIN, 14));
        lblDivision.setBounds(535, 83, 66, 14);
        frame.getContentPane().add(lblDivision);

        JLabel div = new JLabel("");
        div.setFont(new Font("Tahoma", Font.PLAIN, 13));
        div.setBounds(611, 82, 77, 15);
        frame.getContentPane().add(div);

        JSeparator separator = new JSeparator();
        separator.setBounds(43, 128, 665, 15);
        frame.getContentPane().add(separator);

        JButton btnLogout = new JButton("LOGOUT");
        btnLogout.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                frame = new JFrame("Exit");
                if(JOptionPane.showConfirmDialog(frame, "Confirm if you want to exit","Login system",
                        JOptionPane.YES_NO_OPTION)==JOptionPane.YES_NO_OPTION) {
                    System.exit(0);
            }
            }}
        );
        btnLogout.setFont(new Font("Tahoma", Font.PLAIN, 14));
        btnLogout.setBounds(588, 28, 89, 23);
        frame.getContentPane().add(btnLogout);

        JLabel label = new JLabel("");
        label.setBounds(74, 238, 46, 14);
        frame.getContentPane().add(label);

    //  label.setText(savings.toString());

        /** 
         *  Header fetching details start :
         */

        Connection conn= null;
        try {
            Class.forName ("org.h2.Driver");
            conn=DriverManager.getConnection("jdbc:h2:tcp://localhost/~/test;ACCESS_MODE_DATA=rws","sa","");
        } catch (ClassNotFoundException e) {

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


        try {
            String fetch_query= "SELECT * FROM employee_master WHERE id=?";

             PreparedStatement statement = conn.prepareStatement(fetch_query);
             statement.setString(1,id_str3);
            ResultSet rs= statement.executeQuery();
            if(rs.next()) {
                name.setText(rs.getString("name"));
                dob.setText(rs.getString("dob"));
                div.setText(rs.getString("division"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

        /**
         *  :Header fetching close
         */

/*************************************/


}
}

控制台显示为：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Frame4.initialize(Frame4.java:62)
    at Frame4.<init>(Frame4.java:43)
    at Frame3$5.actionPerformed(Frame3.java:479)

Answer 1

也许代码frame.getContentPane()为空。

来自构造函数的参数上的空指针异常

`Frame4`

1 个答案: