Java swing简单通讯录事件错误

Question

我正在尝试制作一个简单的地址簿 GUI，最终能够将姓名等所有字段添加到数据库中。目前，我正在尝试添加方法来更改按下按钮时会发生的情况，但我不断收到此错误......

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at AddressBook.actionPerformed(AddressBook.java:122)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6525)
.
.
. theres some more but you get the point

我想知道是否有人可以帮助我找出为什么我不断收到此错误，甚至无论如何都可以改进该程序的结构。谢谢!

import javax.swing.*;
import javax.swing.border.Border;
import javax.swing.border.EtchedBorder;
import java.awt.*;
import java.awt.event.*;
public class AddressBook implements ActionListener{

    JFrame frame;
    JPanel Master, centerInfo, buttonInfo;
    GridBagConstraints constraints;
    GridBagConstraints topconstraint;
    JLabel jlb_firstName, jlb_lastName, jlb_phoneNum, jlb_searchInfo;
    JTextField jtf_firstName, jtf_lastName, jtf_phoneNum;
    JTextArea jta_searchInfo;
    JButton jb_add, jb_search, jb_share;
    Container pane;

    String firstName, lastName, phoneNum;

    //address book designed in GridBagLayout via swing
    public static void main(String[] args)
    {
        AddressBook a = new AddressBook();
        System.out.println("hello world");
        a.start_gui();
    }


    public void start_gui()
    {   
        JFrame frame = new JFrame("Address Book");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Container pane = new Container();
        pane = frame.getContentPane();
        layComponents(pane);
        //frame.setSize(400, 400);
        frame.setSize(400,400);
        frame.setVisible(true);

    }

    private void layComponents(Container contentPane)
    {
        //PANELS,CONTENTPANE,CONSTRAINTS
        JPanel Master = new JPanel();
        JPanel centerInfo = new JPanel(new GridBagLayout());
        JPanel buttonInfo = new JPanel(new FlowLayout(FlowLayout.LEADING,10,5));
        Master.add(centerInfo, BorderLayout.CENTER);
        Master.add(buttonInfo, BorderLayout.SOUTH);
        contentPane.add(Master);                        //set style of layout for GUI
        GridBagConstraints constraints = new GridBagConstraints();      //layout constraints
        GridBagConstraints topconstraint = new GridBagConstraints();    //constraint against top of GUI

        constraints.insets = new Insets(5,5,10,10);
        //components: firstName, lastName, phoneNum, info, Add, Search(need to add oracle sql db)

        //LABELS
        JLabel jlb_firstName = new JLabel("First Name");
        topconstraint.gridx =1;
        topconstraint.gridy =0;
        topconstraint.insets = new Insets(20,5,10,10);                  //Insets(top,left,right,down)
        centerInfo.add(jlb_firstName, topconstraint);
        JLabel jlb_lastName = new JLabel("Last Name");
        constraints.gridx =1;
        constraints.gridy =2;
        centerInfo.add(jlb_lastName,constraints);
        JLabel jlb_phoneNum = new JLabel("Phone Number");
        constraints.gridx =1;
        constraints.gridy =4;
        centerInfo.add(jlb_phoneNum,constraints);
        JLabel jlb_searchInfo = new JLabel("Search Info");
        constraints.gridx =1;
        constraints.gridy =6;
        centerInfo.add(jlb_searchInfo, constraints);

        //TEXTFIELDS
        JTextField jtf_firstName = new JTextField("Enter New First Name");      //to clear textfield when clicked need actionlisteners
        topconstraint.gridx =3;
        topconstraint.gridy =0;
        centerInfo.add(jtf_firstName,topconstraint);
        JTextField jtf_lastName = new JTextField("Enter New Last Name");
        constraints.gridx =3;
        constraints.gridy =2;
        centerInfo.add(jtf_lastName, constraints);
        JTextField jtf_phoneNum = new JTextField("Enter New Phone Number");
        constraints.gridx =3;
        constraints.gridy =4;
        centerInfo.add(jtf_phoneNum, constraints);

        //TEXTAREA
        Border loweredetched;
        loweredetched = BorderFactory.createEtchedBorder(EtchedBorder.LOWERED);
        JTextArea jta_searchInfo = new JTextArea(10,20);
        jta_searchInfo.append("click me");
        jta_searchInfo.selectAll();
        jta_searchInfo.requestFocus();
        jta_searchInfo.setBorder(loweredetched);
        constraints.gridx =3;
        constraints.gridy =6;
        centerInfo.add(jta_searchInfo, constraints);

        //buttons will use flowlayout
        //BUTTONS
        JButton jb_add = new JButton("Add");
        buttonInfo.add(jb_add);
        JButton jb_search = new JButton("Search");
        buttonInfo.add(jb_search);
        JButton jb_share = new JButton("Share");
        buttonInfo.add(jb_share);   //share info with ppl. need a new share gui

        //Listen for any component actions performed
        jb_add.addActionListener(this);
        //jb_search.addActionListener(this);
        //jb_share.addActionListener(this); 
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        firstName = jtf_firstName.getText();
        System.out.println(firstName);
    }


}

Answer 1

您已经隐藏了实例变量...

您已声明...

JTextField jtf_firstName, jtf_lastName, jtf_phoneNum;

作为实例字段，但在 layComponents 中，您已重新声明它们

JTextField jtf_firstName = new JTextField("Enter New First Name");  

这意味着实例字段为null。