操作初始化中的差异

Question

我试图创建程序，其中Action类是一个类的内部类，但其对象的初始化发生在另一个类中，但我总是遇到异常。下面我介绍了我的问题的简化模型：

有一个类构建GUI并包含内部操作类：

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.InputEvent;
import java.awt.event.KeyEvent;
public class SomeClass {
    JTextArea area;
    public static void main(String[] args){
        SomeClass outerAction = new SomeClass();
        outerAction.go();
    }
    public void go(){
    JFrame frame = new JFrame();
    area = new JTextArea(5,10);
    JToolBar toolBar = new JToolBar(SwingConstants.HORIZONTAL);
    CreateAction createAction = new CreateAction();
    JButton button1 = new JButton(innerGetAction());
    JButton button2 = new JButton(createAction.otherGetAction());
    toolBar.add(button1);
    toolBar.add(button2);
    frame.getContentPane().add(BorderLayout.PAGE_START,toolBar);
    frame.getContentPane().add(BorderLayout.CENTER,area);
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
}
public Action innerGetAction(){
    Action action = new SomeAction("DO","Do sth",KeyStroke.getKeyStroke(KeyEvent.VK_N, InputEvent.CTRL_MASK));
    return action;
}
    public class SomeAction extends AbstractAction{
        public SomeAction(String name, String desc, KeyStroke key){
            super(name);
            putValue(SHORT_DESCRIPTION,desc);
            putValue(ACCELERATOR_KEY,key);
        }
        public void actionPerformed(ActionEvent e) {
            area.setText("Done");
        }
    }
}

有两个JButton具有相同的Action，但第一个的首字母缩写出现在SomeClass中，另一个出现在单独的CreateActionClass:中

import javax.swing.*;
import java.awt.event.InputEvent;
import java.awt.event.KeyEvent;
public class CreateAction {
    public static Action otherGetAction(){
        Action action = new SomeClass().new SomeAction("DO","Do sth",KeyStroke.getKeyStroke(KeyEvent.VK_N, InputEvent.CTRL_MASK));
        return action;
    }
}

一个按钮工作正常，另一个按钮则异常。我不明白为什么。

你能告诉我吗：

1. 为什么在类CreateAction中初始化的Action没有SomeClass中的acces to字段？如果getAction是SomeClass的方法，一切都可以，它可以修改JTextArea，但当它是CreateAction的方法时，会出现java.lang.NullPointerException。这有什么区别
2. 在不同的类中初始化Action对象，而不是在它的Action类和构造函数中初始化，这有意义吗

原谅我可怜的榜样。

Stacktrace:

线程"AWT-EventQueue-0"java.lang.NullPointerException中出现异常在SomeClass$SomeAction.actionPerformed（SomeClass.java:40）在javax.swing.AbstractButton.fireActionPerformed（AbstractButton.java：2022）在javax.swing.AbstractButton$Handler.actionPerformed（AbstractButton.java:2346）在javax.swing.DefaultButtonModel.fireActionPerformed（DefaultButtonModel.java:402）位于javax.swing.DefaultButtonModel.setPressed（DefaultButtonModel.java:259）位于javax.swing.plaf.basic.BasicButtonListener.mouseRelease（BasicButton Listener.java:252）位于java.awt.AWTEventMulticast.mouseRelease（AWTEventmulticast.java:289）位于java.awt.Component.processMouseEvent（Component.java:6525）位于javax.swing.JComponent.processMouseEvent（JComponent.java：3321）位于java.awt.Component.processEvent（Component.java:6290）位于java.awt.Container.processEvent（Container.java:2234）位于java.awt.Component.dispatchEventImpl（Component.java:4881）位于java.awt.Container.dispatchEventImpl（Container.java:2292）位于java.awt.Component.dispatchEvent（Component.java:4703）位于java.awt.LightweightDispatcher.retargetMouseEvent（Container.java:4898）位于java.awt.LightweightDispatcher.processMouseEvent（Container.java:4533）位于java.awt.LightweightDispatcher.dispatchEvent（Container.java:4462）位于java.awt.Container.dispatchEventImpl（Container.java:2278）位于java.awt.Window.dispatchEventImpl（Window.java:2739）位于java.awt.Component.dispatchEvent（Component.java:4703）位于java.awt.EventQueue.dispatchEventImpl（EventQueue.java:746）位于java.awt.EventQueue.access$400（EventQueue.java:97）位于java.awt.EventQueue$3.run（EventQueue.java:697）位于java.awt.EventQueue$3.run（EventQueue.java:691）位于java.security.AccessController.doPrivileged（本机方法）位于java.security.ProtectionDomain$1.doIntersectionPrivilege（ProtectionDomain.java:75）位于java.security.ProtectionDomain$1.doIntersectionPrivilege（ProtectionDomain.java:86）位于java.awt.EventQueue$4.run（EventQueue.java:719）位于java.awt.EventQueue$4.run（EventQueue.java:717）位于java.security.AccessController.doPrivileged（本机方法）位于java.security.ProtectionDomain$1.doIntersectionPrivilege（ProtectionDomain.java:75）位于java.awt.EventQueue.dispatchEvent（EventQueue.java:716）在java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:201）位于java.awt.EventDispatchThread.pumpEventsForFilter（EventDispatchThread.java:116）位于java.awt.EventDispatchThread.pumpEventsForHierarchy（EventDispatchThread.java:105）位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:101）位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:93）在java.awt.EventDispatchThread.run（EventDispatchThread.java:82）

Answer 1

您得到一个NullPointerException，因为您使用了错误的SomeClass在此处创建操作：

Action action = new SomeClass().new SomeAction("DO", "Do sth",
        KeyStroke.getKeyStroke(KeyEvent.VK_N, InputEvent.CTRL_MASK));

此new SomeClass()不是显示的SomeClass！

相反，考虑传入可视化对象。

public static Action otherGetAction(SomeClass someClass) {
  return someClass.new SomeAction("DO", "Do sth",
        KeyStroke.getKeyStroke(KeyEvent.VK_N, InputEvent.CTRL_MASK));
}

---

如果要在GUI之外设置Action，请考虑将其从GUI中分离出来。例如：

import java.awt.event.ActionEvent;
import javax.swing.*;
public class Test2 extends JPanel implements Fooable {
   private JTextField field = new JTextField(10);
   private JTextArea textarea = new JTextArea(20, 30);
   private JButton button = new JButton();
   public Test2() {
      textarea.setFocusable(false);
      JScrollPane scrollPane = new JScrollPane(textarea);
      scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
      add(field);
      add(scrollPane);
      add(button);
   }
   public void setActions(Action action) {
      button.setAction(action);
      field.setAction(action);
   }
   @Override
   public String getFieldText() {
      field.selectAll();
      return field.getText();
   }
   @Override
   public void appendText(String text) {
      textarea.append(text + "n");
   }

   private static void createAndShowGui() {
      Test2 mainPanel = new Test2();
      Action myAction = new MyAction("Press Me", KeyEvent.VK_P, mainPanel);
      mainPanel.setActions(myAction);
      JFrame frame = new JFrame("Test2");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}
interface Fooable {
   String getFieldText();
   void appendText(String text);
}
class MyAction extends AbstractAction {
   private Fooable fooable;
   public MyAction(String name, int mnemonic, Fooable fooable) {
      super(name);
      putValue(MNEMONIC_KEY, mnemonic);
      this.fooable = fooable;
   }
   @Override
   public void actionPerformed(ActionEvent e) {
      if (fooable != null) {
         fooable.appendText(fooable.getFieldText());
      }
   }
}