我有一个空指针异常，我似乎找不到解决方法。我已经在网上搜索了几个小时，但似乎找不到我的问题。
问题是.getText（）给我空值。现在，我找不到解决方法。下面的代码：
import javax.swing.JFrame;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Random;
public class MainPart extends JFrame
{
private String currentuser;
public JPanel north, center, south;
private JButton testone, buy, sell, ButtonSetBalance, ButtonAddBalance;
public JTextField nums, share, price, thebalance;
public JLabel result, mybalance, myshowbalance;
public double balance;
public MainPart()
{
}
public MainPart(String username)
{
currentuser = username;
GridBagConstraints c = new GridBagConstraints();
north = new JPanel(new GridBagLayout());
north.setBackground(Color.cyan);
north.setSize(300,300);
getContentPane().add(north, BorderLayout.SOUTH);
testone = new JButton("Start/Stop");
c.gridx = 0;
c.gridy = 0;
north.add(testone, c);
nums = new JTextField(10);
c.gridx = 1;
c.gridy = 0;
north.add(nums, c);
result = new JLabel("result");
c.gridx = 2;
c.gridy = 0;
north.add(result, c);
ButtonSetBalance = new JButton("Set Balance");
c.gridx = 0;
c.gridy = 1;
north.add(ButtonSetBalance, c);
ButtonAddBalance = new JButton("Add Balance");
c.gridx = 1;
c.gridy = 1;
north.add(ButtonAddBalance, c);
thebalance = new JTextField(10);
c.gridx = 2;
c.gridy = 1;
north.add(thebalance,c);
mybalance = new JLabel("Balance: ");
c.gridx = 0;
c.gridy = 2;
north.add(mybalance, c);
myshowbalance = new JLabel(Double.toString(balance));
c.gridx = 1;
c.gridy = 2;
north.add(myshowbalance,c);
System.out.println("The balance: " + thebalance.getText());
newevent e = new newevent();
testone.addActionListener(e);
ButtonAddBalance.addActionListener(e);
ButtonSetBalance.addActionListener(e);
}
public class newevent extends MainPart implements ActionListener
{
public boolean engine = false;
public Thread t;
MainPart action;
public void actionPerformed(ActionEvent evt)
{
if(evt.getSource() == testone)
{
//t = new Thread(new StockNums(true));
//t.start();
if(engine == false)
{
t = new Thread(new StockNums(true));
t.start();
System.out.println("Start");
engine = true;
}
else
{
t.interrupt();
System.out.println("Stop");
engine = false;
}
}
else if(evt.getSource() == ButtonSetBalance)
{
System.out.println("Inside set theres: ");
try
{
action = new MainPart();
action.setBalance(Double.parseDouble(thebalance.getText()));
}
catch(Exception e){System.out.println(e);}
}
else if(evt.getSource() == ButtonAddBalance)
{
try
{
action = new MainPart();
action.addBalance(Double.parseDouble(thebalance.getText()));
}
catch(Exception e){ System.out.println(e);}
}
}
}
public void setBalance(double bal)
{
balance = bal;
System.out.println("Balance = £" + balance);
}
public void addBalance(double bal)
{
balance += bal;
System.out.println("Added " + bal + " to balance. New balance = £");
}
public void setLabelText(String i)
{
result.setText(i);
}
}
该问题特别是在侦听ButtonAddBalance和ButtonSetBalance的事件中。它给出了NullPointerException，但我找不到修复的方法。应该从标签“ thebalance”中获取文本并将其解析为Double，然后根据具体情况将其添加到balance或将其设置为balance全局double变量。谢谢！
错误在这里：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at MainPart$newevent.actionPerformed(MainPart.java:115)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6373)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6138)
at java.awt.Container.processEvent(Container.java:2085)
at java.awt.Component.dispatchEventImpl(Component.java:4735)
at java.awt.Container.dispatchEventImpl(Container.java:2143)
at java.awt.Component.dispatchEvent(Component.java:4565)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4621)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4282)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4212)
at java.awt.Container.dispatchEventImpl(Container.java:2129)
at java.awt.Window.dispatchEventImpl(Window.java:2478)
at java.awt.Component.dispatchEvent(Component.java:4565)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:679)
at java.awt.EventQueue.access$000(EventQueue.java:85)
at java.awt.EventQueue$1.run(EventQueue.java:638)
at java.awt.EventQueue$1.run(EventQueue.java:636)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:652)
at java.awt.EventQueue$2.run(EventQueue.java:650)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:649)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
和StockNums类在这里（尽管我认为此错误不需要它？）：
import java.util.*;
public class StockNums extends MainPart implements Runnable
{
int num;
boolean active;
Random r = new Random();
MainPart n = new MainPart();
public StockNums(boolean get)
{
System.out.println("Inside StockNums");
active = get;
}
public void run()
{
try
{
while(active == true)
{
//result.setText(Double.toString(random(Integer.parseInt(nums.getText()))));
System.out.println(Double.toString(random(1000)));
Thread.sleep(1000); // One second refresh
}
}
catch(Exception e){}
}
public double random(int num)
{
Random number = new Random();
return number.nextInt(num);
}
//result.setText(Double.toString(random(Integer.parseInt(nums.getText()))));
}
最佳答案
问题出在这里：newevent扩展了MainPart（据我所知，这是不必要的）。这意味着thebalance内的newevent不是封闭的thebalance的MainPart字段，而是thebalance本身的newevent字段，该字段从未初始化。
如注释中所述，此代码还有其他各种问题，但这就是导致您的NullPointerException的原因。删除extends MainPart，您应该可以继续。