Java swing簡單地址簿事件錯誤

Question

我正在嘗試製作一個簡單的地址簿GUI，它最終能夠將所有字段（如姓名編號）添加到數據庫中。目前，我想補充的方法來改變，當按下一個按鈕會發生什麼，但我不斷收到這個錯誤...

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 

我在想，如果有人可以幫助我瞭解爲什麼我不斷收到這個錯誤甚至無論如何改善這個程序的結構。謝謝！

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

你遮蔽你的實例變量...

你已經宣佈...

JTextField jtf_firstName, jtf_lastName, jtf_phoneNum; 

作爲實例字段，但在layComponents，你重新聲明他們

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

這意味着實例字段是null。