Java JButton ActionListener的错误

我在为JButton添加ActionListener时遇到问题，请帮助解决此问题

错误：

Exception in thread "main" java.lang.NullPointerException
at searchDB.searchDB(searchDB.java:11)
at Ramses.main(Ramses.java:35)

主类：Ramses.java

import javax.swing.JFrame;
import ...........;

public class Ramses {

public static void main(String[] args) {
    // TODO Auto-generated method stub

    Gui GuiObject = new Gui();
    GuiObject.Gui();

    searchDB searchObject = new searchDB();
    searchObject.searchDB(); //error here

}
}

声明JButton的类：Gui.java

import javax.swing.*;
import........;

public class Gui {
public static JButton btnUpdate;
public void Gui() {
JButton btnSearch = new JButton("Search");
    btnSearch.setBounds(463, 112, 91, 23);
    btnSearch.setVisible(true);
    pnUpper.add(btnSearch);

 }
}

实现JButton ActionListener的类：searchDB.java

import javax.swing.*;

public class searchDB{

public void searchDB(){

    HandlerClass handler = new HandlerClass();
    Gui.btnSearch.addActionListener(handler); //error in this line

}
private class HandlerClass implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent event) {
        // TODO Auto-generated method stub
        String cstmrSearch = (String) Gui.cbCustomer.getSelectedItem();
        System.out.println("Selected Customer is :"+cstmrSearch);
    }


}
}

---

你知道这段代码有什么问题吗？

堆栈跟踪是问题的良好指示器

从

Gui的构造函数中删除
void
关键字

public Gui() {

允许分配组件变量。此外，删除构造函数中的
JButton
类型，以确保变量不是：

旁白：
静态
字段被认为是糟糕的设计，请改用实例变量
如果您打算将构造函数添加到
类Gui
和
类searchDB
，则不应将void添加为返回类型

添加void将使其成为一种正常的方法。构造函数将没有返回类型。您还可以在构造函数中包含初始化代码，并且在创建类的对象时将进行初始化。无需进行任何显式调用

Class Gui {

public Gui(){
 //initialization code goes here
}

}

Class searchDB{

public searchDB(){
 //initialization code goes here
}
}

我会设计一些不同的课程。我同意@Reimeus的观点，您应该避免误用静态字段，并且应该避免使用“伪”构造函数，即返回类型为void的构造函数，因为构造函数应该没有返回类型

具体来说，我要改变这一点：

public class Ramses {
  public static void main(String[] args) {
    Gui GuiObject = new Gui();
    GuiObject.Gui();
    searchDB searchObject = new searchDB();
    searchObject.searchDB(); //error here
  }
}

为此：

public class Ramses {
  public static void main(String[] args) {
    Gui gui = new Gui();  // variables should begin with lower-case letter
    // GuiObject.Gui();  // not needed
    SearchDB searchObject = new SearchDB(gui); // pass GUI object in
     // searchObject.searchDB(); // don't need this
  }
}

public class Gui {
  private JButton btnUpdate; // make private and non-static
  public Gui() {
    // JButton btnSearch = new JButton("Search");
    //  btnSearch.setBounds(463, 112, 91, 23); // don't use absolute positioning
    //  btnSearch.setVisible(true);  // not needed

    btnUpdate = new btnUpdate("Search");
    pnUpper.add(btnSearch);
 }

 public AbstractButton getBtnUpdate() {
    return btnUpdate;
 }
}

从以下位置更改Gui：

public class Gui {
public static JButton btnUpdate;
public void Gui() {
JButton btnSearch = new JButton("Search");
    btnSearch.setBounds(463, 112, 91, 23);
    btnSearch.setVisible(true);
    pnUpper.add(btnSearch);

 }
}

为此：

public class Ramses {
  public static void main(String[] args) {
    Gui gui = new Gui();  // variables should begin with lower-case letter
    // GuiObject.Gui();  // not needed
    SearchDB searchObject = new SearchDB(gui); // pass GUI object in
     // searchObject.searchDB(); // don't need this
  }
}

public class Gui {
  private JButton btnUpdate; // make private and non-static
  public Gui() {
    // JButton btnSearch = new JButton("Search");
    //  btnSearch.setBounds(463, 112, 91, 23); // don't use absolute positioning
    //  btnSearch.setVisible(true);  // not needed

    btnUpdate = new btnUpdate("Search");
    pnUpper.add(btnSearch);
 }

 public AbstractButton getBtnUpdate() {
    return btnUpdate;
 }
}

并将searchDB更改为：

public class searchDB{

public void searchDB(){

    HandlerClass handler = new HandlerClass();
    Gui.btnSearch.addActionListener(handler); //error in this line

}
private class HandlerClass implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent event) {
        // TODO Auto-generated method stub
        String cstmrSearch = (String) Gui.cbCustomer.getSelectedItem();
        System.out.println("Selected Customer is :"+cstmrSearch);
    }


}
}

致：

（请参见代码中的注释以获取解释）
谢谢大家的回答，这真的很有帮助。