Java 是否可以让JLabel根据变量值更改其文本？

我创建了一个JLabel，如果变量计数==-1，
如果变量计数=0，则为“文本B”；如果变量计数=1，则为“文本C”

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

我使用Swing创建了我的界面，您可以在下面看到

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

红色矩形显示JLabel应该在哪里

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

我尝试创建3个JLabel，并在变量计数值条件适用时更改setVisible（布尔值）。这不起作用，因为我遇到以下错误：

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

线程“main”java.lang.NullPointerException中出现异常 位于tempconverterUI.TempConverter.main（TempConverter.java:354） C:\Users\x\AppData\Local\NetBeans\Cache\8.1\executor snippets\run.xml:53:Java返回：1

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

而且JLabel不能放在GUI中的同一位置（不可能重叠）

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

我尝试使用jLabel.setText（）来更改jLabel中显示的文本，只要变量条件适用。我得到了一个与上面类似的错误（如果不是相同的话）

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

我已经阅读了其他一些帖子并进行了进一步的研究，发现有些人建议设置ActionListeners，但我不确定它们是否能与简单的变量一起工作，而不是GUI中的组件

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

我的代码如下：

package tempconverterUI;

import javax.swing.JOptionPane;
import messageBoxes.UserData;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.WString;

public class TempConverter extends javax.swing.JFrame {

public interface someLib extends Library
{
      public int engStart(); 
      public int endStop();
      public int engCount();
      public WString engGetLastError();
      public int engSetAttribute(WString aszAttributeID, WString aszValue);

}

/**
 * Creates new form TempConverter
 */
public TempConverter() {
    initComponents();
}

/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

package-tempconverterUI；
导入javax.swing.JOptionPane；
导入messagebox.UserData；
导入com.sun.jna.Library；
导入com.sun.jna.Native；
导入com.sun.jna.WString；
公共类TempConverter扩展了javax.swing.JFrame{
公共接口someLib扩展库
{
公共int-engStart（）；
公共int endStop（）；
公共int engCount（）；
public WString engGetLastError（）；
public int engSetAttribute（WString aszAttributeID，WString aszValue）；
}
/**
*创建新的表单转换器
*/
公共临时转换器（）{
初始化组件（）；
}
/**
*从构造函数中调用此方法来初始化表单。
*警告：不要修改此代码。此方法的内容始终为
*由表单编辑器重新生成。
*/
@抑制警告（“未选中”）
//                           
私有组件（）{

布局是在这里创建的，后面是温度转换方法和无关组件的功能（我认为这与本例无关）

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

/**
*@param指定命令行参数
*/
公共静态void main（字符串参数[]）{
/**这是创建登录表单的地方*/
UserData.popUp（）；
/**在此之后，将调用库函数，它将返回变量计数值*/
someLib=（someLib）Native.loadLibrary（“someLib”，someLib.class）；
int startResult=lib.engStart（）；
System.out.println（startResult）；
如果（开始结果<0）
{
System.out.println（lib.engGetLastError（））；
}
System.out.println（UserData.getAcInput（））；
int setAtResult=lib.engSetAttribute（新的WString（“代码”），UserData.getaciput（）；
系统输出打印项次（setAtResult）；
如果（setAtResult<0）
{
System.out.println（lib.engGetLastError（））；
}

接下来是一段代码，我应该从中控制要显示的JLabel文本

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    int count = lib.engCount();
    System.out.println(count);
    if (count == -1)
    {
        System.out.println(lib.engGetLastError());

    }
    else if (count == 0)
    {

    }
    else
    {

    }

    new TempConverter().setVisible(true);  
}

// Variables declaration - do not modify                     
private javax.swing.JPanel bottomPanel;
private javax.swing.JButton convertButton;
private static javax.swing.JButton button;
private javax.swing.JTextField from;
private javax.swing.JComboBox<String> fromCombo;
private javax.swing.JLabel fromLabel;
private javax.swing.JLabel title;
private javax.swing.JTextField to;
private javax.swing.JComboBox<String> toCombo;
private javax.swing.JLabel toLabel;
private javax.swing.JPanel topPanel;
// End of variables declaration                   

int count=lib.engCount（）；
系统输出打印项次（计数）；
如果（计数==-1）
{
System.out.println（lib.engGetLastError（））；
}
否则如果（计数==0）
{
}
其他的
{
}
新建TempConverter（）.setVisible（true）；
}
//变量声明-不修改
私有javax.swing.JPanel底部面板；
私有javax.swing.JButton转换按钮；
私有静态javax.swing.JButton按钮；
来自的私有javax.swing.JTextField；
私有javax.swing.jcombox fromCombo；
私有javax.swing.JLabel fromLabel；
私有javax.swing.JLabel标题；
私有javax.swing.JTextField到；
私有javax.swing.JComboBox-toCombo；
私有javax.swing.JLabel-toLabel；
私有javax.swing.JPanel-topPanel；
//变量结束声明

}

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

如果您能提供一个简单的代码示例，这将是非常棒的，因为我对Java（以及编程）是新手。

问题：

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

不要将JLabel设置为可见，而是首先将其添加到GUI中，默认情况下保持可见，然后通过

setText（…）

简单地设置其文本

为包含JLabel公共方法的类指定允许外部类设置标签文本的功能。类似于

public void setLabelText（String text）

，以及在JLabel上的方法调用

setText（text）

像调试任何其他NPE一样调试NullPointerException——查看stacktrace，找到抛出它的行，然后回顾代码，查看该行上的键变量为null的原因

更改JLabel的时间和方式将取决于您要侦听的事件。如果是用户输入，则您将希望响应该输入，无论是添加到JButton或JTextField的ActionListener，还是添加到JRadioButton的itemListener

如果您希望监听变量状态的更改，不管变量如何更改，那么使用PropertyChangeSupport和PropertyChangeListener将其设置为“绑定属性”（）

关于后者的一个例子：

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

import javax.swing.*;
import javax.swing.event.SwingPropertyChangeSupport;

@SuppressWarnings("serial")
public class ShowCount extends JPanel {
    private static final int TIMER_DELAY = 1000;
    private JLabel countLabel = new JLabel("                 ");
    private CountModel model = new CountModel();

    public ShowCount() {
        model.addPropertyChangeListener(CountModel.COUNT, new ModelListener(this));

        setPreferredSize(new Dimension(250, 50));
        add(new JLabel("Count:"));
        add(countLabel);

        Timer timer = new Timer(TIMER_DELAY, new TimerListener(model));
        timer.start();
    }

    public void setCountLabelText(String text) {
        countLabel.setText(text);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }

    private static void createAndShowGui() {
        ShowCount mainPanel = new ShowCount();
        JFrame frame = new JFrame("ShowCount");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }
}

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

问题：

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {

/**This is where the Login form gets created*/              
    UserData.popUp();

/**After this the Library functions are called, which will return the variable count value*/    
    someLib lib = (someLib) Native.loadLibrary("someLib", someLib.class);

    int startResult = lib.engStart(); 
    System.out.println(startResult);
    if (startResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

    System.out.println(UserData.getAcInput());
    int setAtResult = lib.engSetAttribute(new WString("CODE"), UserData.getAcInput());
    System.out.println(setAtResult);
    if (setAtResult < 0)
    {
        System.out.println(lib.engGetLastError());
    }

不要将JLabel设置为可见，而是首先将其添加到GUI中，默认情况下保持可见，然后通过

setText（…）

简单地设置其文本

为包含JLabel公共方法的类指定允许外部类设置标签文本的功能。类似于

public void setLabelText（String text）

，以及在JLabel上的方法调用

setText（text）

像调试任何其他NPE一样调试NullPointerException--查看stacktrace，找到抛出它的行，