当我在类别按钮之后单击 select 按钮时，为什么不打印 String 而是打印 null？

Question

package simpleclass;

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileNotFoundException;
import java.io.IOException;
import javax.swing.JButton;
import javax.swing.JFrame;

public class SimpleClass extends BaseClass {
    JButton b1 = new JButton("Category");
    JButton b2 = new JButton("Select");
    JFrame f = new JFrame();

    SimpleClass() {
        f.add(b1);
        f.add(b2);
        f.setLayout(new GridLayout(1, 2));
        f.setSize(200, 200);
        f.setLocationRelativeTo(null);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);

        b1.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                try {
                    new SubClass();
                } catch (IOException ex) {
                }
            }
        });
        b2.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println(astring);
            }
        });
    }

    public static void main(String[] args) {
        new SimpleClass();
    }
}

class BaseClass {
    String astring;

    public void setString(String astring) {
        this.astring = astring;
    }
}

class SubClass extends BaseClass {
    SubClass() throws FileNotFoundException {
        super.setString("WHYnull");
    }
}

在此代码中有一个基 class BaseClass 和两个子 class SimpleClass 和 SubClass。有两个按钮。当按下按钮 "Category" 时，字符串 "astring" 被设置为 "WHYnull"。现在我的问题是，当我之后按下 "Select" 按钮时，为什么会打印 "null"？变量"astring"不是独占吗？

Answer 1

您说：

When the button "Category" is pressed, then the String "astring" is being set to "WHYnull".

是的，这是正确的，但仅适用于您在第一个 ActionListoner 中创建（和丢弃）的 new SubClass()。对原程序的字段aString没有影响，当前的this.

我的建议：这不是应该用继承来解决的问题。将继承完全排除在这幅图之外。

我愿意：

  b1.addActionListener(new ActionListener() {

     @Override
     public void actionPerformed(ActionEvent e) {
        // try {
        // new SubClass();
        // } catch (IOException ex) {
        //
        // }

        setString("Fubar");
     }
  });

另请注意，您的代码不应包含空的 catch 块，因为那 Java 相当于闭着眼睛开车。是的，令人兴奋，但也非常危险。

您询问了使用多个类的 GUI，一个常见的例子是模型-视图-控制或 MVC 类型的程序设计，其中程序的逻辑驻留在模型中，GUI 驻留在视图中，并且两者在控件中的交互。例如，以您的小程序为例，可以这样做：

import java.awt.event.ActionEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class SimpleMCV {

   private static void createAndShowGui() {
      final SimpleView simpleView = new SimpleView();
      final SimpleModel simpleModel = new SimpleModel("");
      MainControl mainControl = new MainControl();
      mainControl.setUp(simpleView, simpleModel);

      JFrame frame = new JFrame("Simple GUI");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(simpleView);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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

@SuppressWarnings("serial")
class SimpleView extends JPanel {
   private JTextField textField = new JTextField(10);

   public SimpleView() {
      textField.setFocusable(false);
      add(textField);
   }

   public void setTextFieldText(String text) {
      textField.setText(text);
   }

   public void addAction(Action action) {
      add(new JButton(action));
   }
}

class MainControl {
   public void setUp(final SimpleView simpleView, final SimpleModel simpleModel) {
      simpleView.addAction(new SimpleControl(simpleModel, "Fubar"));
      simpleView.addAction(new SimpleControl(simpleModel, "Snafu"));
      simpleView.addAction(new SimpleControl(simpleModel, "Bohica"));

      simpleModel.addChangeListener(new ChangeListener() {

         @Override
         public void stateChanged(ChangeEvent e) {
            simpleView.setTextFieldText(simpleModel.getSomeText());
         }
      });
   }
}

@SuppressWarnings("serial")
class SimpleControl extends AbstractAction {
   private SimpleModel simpleModel;

   public SimpleControl(SimpleModel simpleModel, String name) {
      super(name);
      int mnemonic = (int) name.charAt(0);
      putValue(MNEMONIC_KEY, mnemonic);
      this.simpleModel = simpleModel;
   }

   @Override
   public void actionPerformed(ActionEvent e) {
      simpleModel.setSomeText(e.getActionCommand());
   }

}

class SimpleModel {
   private List<ChangeListener> listenerList = new ArrayList<>();
   private String someText;

   public SimpleModel(String someText) {
      this.someText = someText;
   }

   public String getSomeText() {
      return someText;
   }

   // change text and notify all listeners
   public void setSomeText(String someText) {
      this.someText = someText;
      for (ChangeListener changeListener : listenerList) {
         changeListener.stateChanged(new ChangeEvent(this));
      }
   }

   // allow outside classes to add a ChangeListener
   public void addChangeListener(ChangeListener cl) {
      listenerList.add(cl);
   }
}

当我在类别按钮之后单击 select 按钮时，为什么不打印 String 而是打印 null？

When I click select button after category button, why isn't the String being printed and null being printed instead?

java

user-interface

class

hierarchy