我一直收到运行时错误，说我在线程“AWT-EventQueue-0”中有一个异常java.lang.NullPointerException它说的是[第65行]，但对我来说它看起来像一个基本的if语句。我可以给一些背景知识。该帧是从一系列帧中调用的，然后一旦选择了状态，就将其发送到包含状态A-C城市的新帧。仅当Alabama不是所选状态时，此当前帧才有效。 （我还没有为其他状态创建其他帧，因此为什么它们被注释掉了）我可以将代码放到它应该调用的另一个帧中，如果有人需要它但我认为这是很多代码要开始用。 在此先感谢大家的帮助。
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.*;
import java.io.*;
import java.util.*;
public class WetBulbByLocationFrameStates3 extends JFrame
{
public JLabel stateLabel;
public JPanel statePanel = new JPanel();
public JComboBox stateBox;
public String [] unitedStates = {"Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", "Delaware", "District of Columbia", "Florida", "Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "South Pacific Islands", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "West Virginia", "Wisconsin", "Wyoming"};
public String [] canadianProvinces = {"Alberta", "British Columbia", "Manitoba", "New Brunswick", "Newfoundland and Labrador", "Northwest Territories", "Nova Scotia", "Nunavut", "Ontario", "Prince Edward Island", "Quebec", "Saskatchewan", "Yukon Territory"};
public JButton nextButton = new JButton("Next");
public JPanel buttonPanel = new JPanel();
public String state;
public String countryGIVEN;
public WetBulbByLocationFrameStates3(String country)
{
Toolkit toolkit = Toolkit.getDefaultToolkit();
Dimension dimensions = toolkit.getScreenSize();
int x = (dimensions.width - 315)/2;
int y = (dimensions.height - 250)/2;
setBounds(x, y, 315, 250);
setVisible(true);
setResizable(false);
setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
countryGIVEN = country;
nextButton.addActionListener(new ButtonListener3());
if (country.equals("United States of America"))
{
stateBox = new JComboBox(unitedStates);
setTitle("Select State");
stateLabel = new JLabel("State");
}
else if (country.equals("Canada"))
{
stateBox = new JComboBox(canadianProvinces);
setTitle("Select Province");
stateLabel = new JLabel("Province");
}
stateBox.addActionListener(new ComboBoxListener2());
statePanel.add(stateLabel);
statePanel.add(stateBox);
add(statePanel, BorderLayout.CENTER);
buttonPanel.add(nextButton);
add(buttonPanel, BorderLayout.SOUTH);
}
private class ButtonListener3 implements ActionListener
{
@Override
public void actionPerformed(ActionEvent e)
{
if (countryGIVEN.equals("United States of America"))
{
if (state.equals("Alabama"))
{
JFrame wbfs1 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("Alaska"))
{
JFrame wbfs2 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("Arizona"))
{
JFrame wbfs3 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("Arkansas"))
{
JFrame wbfs4 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("California"))
{
JFrame wbfs5 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("Colorado"))
{
JFrame wbfs6 = new WetBulbByLocationFrameStateGroupA_C(state);
}
else if (state.equals("Connecticut"))
{
JFrame wbfs7 = new WetBulbByLocationFrameStateGroupA_C(state);
}
}
}
}
private class ComboBoxListener2 implements ActionListener
{
@Override
public void actionPerformed(ActionEvent e)
{
state = (String)stateBox.getSelectedItem();
}
}
}
PS选择Alabama时出现错误。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at WetBulbByLocationFrameStates3$ButtonListener3.actionPerformed(WetBulbByLocationFrameStates3.java:65)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
答案 0 :(得分：0)
第65行就是这个：
if (state.equals("Alabama"))
如果您查看以前的代码，则没有任何内容已初始化state，因此其值为null。
答案 1 :(得分：0)
当您尝试在其上调用state时，equals为空。