Java 除非我将所有内容都放在一个类中，否则JMenuBar不会出现

我想用菜单栏做一个框架，但我不能让它显示出来。我尝试使用

pack（）

，因为在类似的问题中有人建议使用它，但这使我的框架不存在，最后我只使用了上面的窗口栏

如果我把所有的东西都放在一个类中，它就会起作用；所以我假设我的问题是关于Java结构的一些基本问题

这是我的JMenuBar课程：

package jpaintnet;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.event.MenuEvent;
import javax.swing.event.MenuListener;

public class JMenuBar extends JFrame implements MenuListener, ActionListener {

    JMenuBar mb;
    JMenu addImage, addObject, save;
    JMenuItem circle, rectangle, triangle, line;

    public JMenuBar() {

        mb = new JMenuBar();

        addObject = new JMenu("Add Object");
        mb.add(addObject);

        addImage = new JMenu("Add a new image");
        mb.add(addImage);

        save = new JMenu("save");
        mb.add(save);


        circle = new JMenuItem("Create a circle");
        circle.addActionListener(this);
        addObject.add(circle);

        rectangle = new JMenuItem("Create a rectangle");
        rectangle.addActionListener(this);
        addObject.add(rectangle);

        triangle = new JMenuItem("Create a triangle");
        triangle.addActionListener(this);
        addObject.add(triangle);

        line = new JMenuItem("Create a line");
        line.addActionListener(this);
        addObject.add(line);
    }



    @Override
    public void actionPerformed(ActionEvent e) {
       //here goes everything!
    }

    @Override
    public void menuSelected(MenuEvent e) {
    }

    @Override
    public void menuDeselected(MenuEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public void menuCanceled(MenuEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
}

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.event.MenuEvent;
import javax.swing.event.MenuListener;

/*
 * 1. Renamed your class "JMenuBar" to "MyMenuBar" - do not use Java class names. Its bad practice
 * 2. Extend from JMenuBar class to create Menu Bar. Whereas JFrame requires to create a frame
 */
public class MyMenuBar extends JMenuBar implements MenuListener, ActionListener { 
  JMenu addImage, addObject, save;
  JMenuItem circle, rectangle, triangle, line;

  public MyMenuBar() {


      addObject = new JMenu("Add Object");
      add(addObject);

      addImage = new JMenu("Add a new image");
      add(addImage);

      save = new JMenu("save");
      add(save);


      circle = new JMenuItem("Create a circle");
      circle.addActionListener(this);
      addObject.add(circle);

      rectangle = new JMenuItem("Create a rectangle");
      rectangle.addActionListener(this);
      addObject.add(rectangle);

      triangle = new JMenuItem("Create a triangle");
      triangle.addActionListener(this);
      addObject.add(triangle);

      line = new JMenuItem("Create a line");
      line.addActionListener(this);
      addObject.add(line);
  }



  @Override
  public void actionPerformed(ActionEvent e) {
     //here goes everything!
  }

  @Override
  public void menuSelected(MenuEvent e) {
  }

  @Override
  public void menuDeselected(MenuEvent e) {
      throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
  }

  @Override
  public void menuCanceled(MenuEvent e) {
      throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
  }
}

我的JFrame类：

package jpaintnet;

import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.JFrame;
import java.awt.HeadlessException;
import javax.swing.JMenuBar;
import javax.swing.JPanel;

public class Frame extends JFrame
{

    public Frame(String title) throws HeadlessException {

        JMenuBar mb = new JMenuBar();
        JPanel panel = new JPanel();

        this.setBounds(100,100,640,480);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setBackground(Color.white);

        this.getContentPane().add(BorderLayout.SOUTH, panel);
        this.getContentPane().add(BorderLayout.NORTH, mb);

        this.setVisible(true);
    }
}

    import java.awt.BorderLayout;
import java.awt.Color;

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

public class MyFrame
  extends
  JFrame {

  public MyFrame( String title ) {
    MyMenuBar mb = new MyMenuBar();
    JPanel panel = new JPanel();

    this.setBounds( 100, 100, 640, 480 );
    this.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    this.setBackground( Color.white );

    this.getContentPane().add( BorderLayout.SOUTH, panel );
    // this.getContentPane().add( BorderLayout.NORTH, mb ); //Do not use this
    // method to set menu bar
    this.setJMenuBar( mb ); // this is ideal method to set menu bar

    this.setTitle( title ); // Missing as you have supplied title but missed to set it to frame
    this.setVisible( true );
  }

  // added main method here from your Main class
  public static void main( String[] args ) {

    MyFrame f = new MyFrame( "JPaintNet" );
    f.setVisible( true );
  }
}

最后，我的主要观点是：

package jpaintnet;

public class Main {

    public static void main(String[] args) {

        Frame f = new Frame("JPaintNet");
        f.setVisible(true);
    }
}

我已经对其中一个

setVisible（）

方法进行了注释，但为了以防万一，我最后再次将其放在这里

---

提前谢谢你。

你犯了几个错误

在

框架中

类删除以下导入

import javax.swing.JMenuBar;

因为一旦导入了

swing.JMenuBar

jvm就会忽略您创建的

JMenuBar

您的

JMenuBar

类应该从

javax.swing.JMenuBar

扩展，而不是从

JFrame

扩展

public class JMenuBar extends javax.swing.JMenuBar implements MenuListener, ActionListener

去掉

mb=newjmenubar（）JMenuBar

构造函数中的code>语句。因为当您创建一个

新JMenuBar（）

时，jvm要做的第一件事就是调用它的构造函数。在构造函数中创建另一个

new JMenuBar（）

意味着您不断创建大量

JMenuBar

s，这最终将导致

StackOverFlow

错误

删除

JMenuBar

类中的变量

mb

，并替换为

this

this.add(addObject);
this.add(addImage);
this.add(save);

---

那你的窗户就可以开了

---

更新：

---

作为一个例子，请记住要使类名唯一，特别是在使用java开发的库时。这里还有一个附加提示，在创建数据库时避免使用java关键字作为表名或列名。如果将来要使用hibernate之类的框架，这将为您省去很多麻烦。

@Andrew，已经更新了下面提供的代码，供您参考。参考内联注释：

这是您的MyMenuBar即JMenuBar类：

package jpaintnet;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.event.MenuEvent;
import javax.swing.event.MenuListener;

public class JMenuBar extends JFrame implements MenuListener, ActionListener {

    JMenuBar mb;
    JMenu addImage, addObject, save;
    JMenuItem circle, rectangle, triangle, line;

    public JMenuBar() {

        mb = new JMenuBar();

        addObject = new JMenu("Add Object");
        mb.add(addObject);

        addImage = new JMenu("Add a new image");
        mb.add(addImage);

        save = new JMenu("save");
        mb.add(save);


        circle = new JMenuItem("Create a circle");
        circle.addActionListener(this);
        addObject.add(circle);

        rectangle = new JMenuItem("Create a rectangle");
        rectangle.addActionListener(this);
        addObject.add(rectangle);

        triangle = new JMenuItem("Create a triangle");
        triangle.addActionListener(this);
        addObject.add(triangle);

        line = new JMenuItem("Create a line");
        line.addActionListener(this);
        addObject.add(line);
    }



    @Override
    public void actionPerformed(ActionEvent e) {
       //here goes everything!
    }

    @Override
    public void menuSelected(MenuEvent e) {
    }

    @Override
    public void menuDeselected(MenuEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public void menuCanceled(MenuEvent e) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
}

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.event.MenuEvent;
import javax.swing.event.MenuListener;

/*
 * 1. Renamed your class "JMenuBar" to "MyMenuBar" - do not use Java class names. Its bad practice
 * 2. Extend from JMenuBar class to create Menu Bar. Whereas JFrame requires to create a frame
 */
public class MyMenuBar extends JMenuBar implements MenuListener, ActionListener { 
  JMenu addImage, addObject, save;
  JMenuItem circle, rectangle, triangle, line;

  public MyMenuBar() {


      addObject = new JMenu("Add Object");
      add(addObject);

      addImage = new JMenu("Add a new image");
      add(addImage);

      save = new JMenu("save");
      add(save);


      circle = new JMenuItem("Create a circle");
      circle.addActionListener(this);
      addObject.add(circle);

      rectangle = new JMenuItem("Create a rectangle");
      rectangle.addActionListener(this);
      addObject.add(rectangle);

      triangle = new JMenuItem("Create a triangle");
      triangle.addActionListener(this);
      addObject.add(triangle);

      line = new JMenuItem("Create a line");
      line.addActionListener(this);
      addObject.add(line);
  }



  @Override
  public void actionPerformed(ActionEvent e) {
     //here goes everything!
  }

  @Override
  public void menuSelected(MenuEvent e) {
  }

  @Override
  public void menuDeselected(MenuEvent e) {
      throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
  }

  @Override
  public void menuCanceled(MenuEvent e) {
      throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
  }
}

这是您的MyFrame，即Frame类：

package jpaintnet;

import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.JFrame;
import java.awt.HeadlessException;
import javax.swing.JMenuBar;
import javax.swing.JPanel;

public class Frame extends JFrame
{

    public Frame(String title) throws HeadlessException {

        JMenuBar mb = new JMenuBar();
        JPanel panel = new JPanel();

        this.setBounds(100,100,640,480);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setBackground(Color.white);

        this.getContentPane().add(BorderLayout.SOUTH, panel);
        this.getContentPane().add(BorderLayout.NORTH, mb);

        this.setVisible(true);
    }
}

    import java.awt.BorderLayout;
import java.awt.Color;

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

public class MyFrame
  extends
  JFrame {

  public MyFrame( String title ) {
    MyMenuBar mb = new MyMenuBar();
    JPanel panel = new JPanel();

    this.setBounds( 100, 100, 640, 480 );
    this.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    this.setBackground( Color.white );

    this.getContentPane().add( BorderLayout.SOUTH, panel );
    // this.getContentPane().add( BorderLayout.NORTH, mb ); //Do not use this
    // method to set menu bar
    this.setJMenuBar( mb ); // this is ideal method to set menu bar

    this.setTitle( title ); // Missing as you have supplied title but missed to set it to frame
    this.setVisible( true );
  }

  // added main method here from your Main class
  public static void main( String[] args ) {

    MyFrame f = new MyFrame( "JPaintNet" );
    f.setVisible( true );
  }
}

将创建如下窗口：

---

我想这正是您想要的。

您正在使用import javax.swing.JMenuBar并创建自己的类JMenuBar。永远不要使用重复的类名。如果您想要一个定制的JMenuBar，您应该扩展它并在那里添加额外的特性。此外，首先，JMenuBar不需要一个新类来添加项；所有这些都可以在框架类中的JMenuBar对象上调用。

公共类JMenuBar扩展了JFrame

@faris，非常感谢！我再也不会那样做了。你真的帮了我：）@ClaudiaProvoste请接受对你解决问题最有帮助的答案。它有助于未来的读者。如果答案没有帮助，请在下面留下评论。因此海报可以相应地更新它们。如果可能的话，不要重复使用现有的类名，这只是让人困惑（是的，我知道OP做了，所以让他们停止）