Java 使用get方法时，字符串返回null

我试图在另一个类中设置字符串的值，然后从第三个类中获取它。现在我正在尝试使用set和get方法，但我无法让它工作！当用户启动程序时，会出现一个按钮。当按下按钮时，测试字符串获取值Hello。然后我调用set类并在参数中进行测试。现在在set方法中，它是一个System.out.println，它打印Hello，但是当我试图从第二个类中获取该方法时，它打印null。我不知道为什么！以下是我的课程：

public class Class1 {
    public static void main(String[] args) {
        Class3 c3 = new Class3();

        c3.setFilename("Hello");

        JFrame frame = new JFrame("Action Listener");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());

        JButton button = new JButton("Click me!");
        frame.add(button);

        Class2 c2 = new Class2();

        button.addActionListener(c2);

        frame.setSize(300, 100);
        frame.setVisible(true);
    }
}

这是我的第二个类，它是我想在下一个类中使用的文件名字符串：

public class Class2 implements ActionListener {

    String filename;

    public void actionPerformed(ActionEvent e) {
        Class3 c3 = new Class3();

        filename = c3.getFilename();

        System.out.println(filename); // Prints null
    }
}

下面是第三节课：

public class Class3 {
    String filename;

    public String getFilename() {
        return filename;
    }

    public void setFilename(String filename) {
        this.filename = filename;

        System.out.println(filename); // Prints Hello
    }
}

你的问题包括：

您从未设置过train类的文件名检查它-您在哪里调用fileName=something？ 您正在创建新的列车对象，并希望新对象能够感知对其他列车对象所做的更改。Java不是这样工作的。您需要测试更改的同一对象。 您的程序命名约定既混乱又危险。请记住，类名以大写字母开头。不要给变量起与不相关的类相同的名字，比如你的train类和train button。 您的程序有很多不必要的复杂性，例如使调试和增强变得困难的并行阵列。 您正在交换JPanel的内容，而不是使用更易于使用的CardLayout来进行交换。 例如，可以将JButton字符串名和文件名封装在类或枚举中。使用类的优点是，您可以从文件中读入数据，并从文件数据创建类的对象，从而提供额外的灵活性，但是出于我的示例的目的，为了避免添加额外的文件，我将使用枚举，例如

public enum ButtonFileName {
   MONDAY("Monday", KeyEvent.VK_M, "monday_blues.jpg"), 
   TUESDAY("Tuesday", KeyEvent.VK_T, "tuesday_fun.jpg"), 
   WEDNESDAY("Wednesday", KeyEvent.VK_W, "humpday.jpg"), 
   THURSDAY("Thursday", KeyEvent.VK_H, "almost_friday.jpg");

   private String buttonText;
   private String fileName;
   private int mnemonic;

   private ButtonFileName(String buttonText, int mnemonic, String fileName) {
      this.buttonText = buttonText;
      this.mnemonic = mnemonic;
      this.fileName = fileName;
   }

   public String getButtonText() {
      return buttonText;
   }

   public int getMnemonic() {
      return mnemonic;
   }

   public String getFileName() {
      return fileName;
   }
}

然后我可以给我的主类一个ButtonFileName变量，selectedButtonFileName，然后在按下按钮时填充该变量。我还可以为每个ButtonFileName枚举创建自己的操作，并使用这些操作设置按钮。例如：

import java.awt.CardLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.*;

@SuppressWarnings("serial")
public class Home2 extends JPanel {
   private static final String TRAIN_PANEL = "train panel";
   private static final String BTN_ROW_PANEL = "btn row panel";
   private CardLayout cardLayout = new CardLayout();
   private ButtonFileName selectedButtonFileName = null;
   private JPanel buttonRowPanel = new JPanel();
   private JPanel trainPanel = new JPanel();

   public Home2() {
      buttonRowPanel = createButtonRowPanel();
      trainPanel = createTrainPanel();

      setLayout(cardLayout);
      add(buttonRowPanel, buttonRowPanel.getName());
      add(trainPanel, trainPanel.getName());
   }

   private JPanel createButtonRowPanel() {
      JPanel btnRowPanel = new JPanel(new GridLayout(1, 0, 5, 5));
      btnRowPanel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
      btnRowPanel.setName(BTN_ROW_PANEL);
      for (ButtonFileName btnFileName : ButtonFileName.values()) {
         JButton btn = new JButton(new RowButtonAction(btnFileName, this));
         btnRowPanel.add(btn);
      }
      return btnRowPanel;
   }

   private JPanel createTrainPanel() {
      JPanel trainPanel = new JPanel();
      trainPanel.setName(TRAIN_PANEL);
      JButton trainBtn = new JButton(new TrainAction("Train", KeyEvent.VK_T, this));
      trainPanel.add(trainBtn);
      return trainPanel;
   }

   public ButtonFileName getSelectedButtonFileName() {
      return selectedButtonFileName;
   }

   public void setSelectedButtonFileName(ButtonFileName selectedButtonFileName) {
      this.selectedButtonFileName = selectedButtonFileName;
   }

   public void nextCardLayoutView() {
      cardLayout.next(this);
   }

   private static void createAndShowGui() {
      JFrame frame = new JFrame("Home2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(new Home2());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }

}

又是枚举

enum ButtonFileName {
   MONDAY("Monday", KeyEvent.VK_M, "monday_blues.jpg"), 
   TUESDAY("Tuesday", KeyEvent.VK_T, "tuesday_fun.jpg"), 
   WEDNESDAY("Wednesday", KeyEvent.VK_W, "humpday.jpg"), 
   THURSDAY("Thursday", KeyEvent.VK_H, "almost_friday.jpg");

   private String buttonText;
   private String fileName;
   private int mnemonic;

   private ButtonFileName(String buttonText, int mnemonic, String fileName) {
      this.buttonText = buttonText;
      this.mnemonic = mnemonic;
      this.fileName = fileName;
   }

   public String getButtonText() {
      return buttonText;
   }

   public int getMnemonic() {
      return mnemonic;
   }

   public String getFileName() {
      return fileName;
   }
}

我对火车按钮的行动

import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;

@SuppressWarnings("serial")
public class TrainAction extends AbstractAction {
   private Home2 home;

   public TrainAction(String name, int mnemonic, Home2 home) {
      super(name);
      putValue(MNEMONIC_KEY, mnemonic);
      this.home = home;
   }

   @Override
   public void actionPerformed(ActionEvent e) {
      System.out.println("Selected File Name: "
            + home.getSelectedButtonFileName().getFileName());
      home.nextCardLayoutView();
   }
}

我对行按钮的操作

import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;

@SuppressWarnings("serial")
public class RowButtonAction extends AbstractAction {
   private ButtonFileName btnFileName;
   private Home2 home;

   public RowButtonAction(ButtonFileName btnFileName, Home2 home) {
      super(btnFileName.getButtonText());
      putValue(MNEMONIC_KEY, btnFileName.getMnemonic());
      this.btnFileName = btnFileName;
      this.home = home;
   }

   @Override
   public void actionPerformed(ActionEvent e) {
      home.setSelectedButtonFileName(btnFileName);
      home.nextCardLayoutView();
   }
}

---

至于灵活性，假设我想添加另一个JButton和文件名。将Friday元素添加到枚举中，就完成了。

什么不起作用？您是否尝试过使用IDE的调试器工具调试代码？代码太多，上下文太少。@Mohammad是的，我尝试过用IDE和System.out.println调试代码，当它到达第三个类时返回null。@Bennyz返回null。@WeeRox不仅如此。你应该从你的例子中删除所有与你的问题无关的东西。考虑从零开始和编写新的示例，大多数情况下，该方法比移除现有代码更快和更安全。