java - 如何使addActionListener和add工作？

Question

我浏览了很多与 addActionListener 有关的帖子，但没有一个真正帮助我解决这个问题。我只是想让这个程序在修改了一段时间后运行，但我仍然不知道如何让我的“addActionListener”和“add”工作，所以任何帮助将不胜感激

我遇到的问题大约在中间，然后在底部附近，都有箭头指向它们。我是这个网站的新手，所以我不知道如何对代码进行编号(如果可以的话):( 对于给您带来的不便，我们深表歉意，并感谢您的帮助!

    import java.awt.BorderLayout;
    import java.awt.GridLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JPanel;
    import javax.swing.JTextArea;

    public class Phone extends JFrame
    {
       private JButton keyJButton[];
       private JPanel keyJPanel;
       private JPanel lcdJPanel;
       private JTextArea lcdJTextArea;
       private String lcdOutput = "";
       private int count;

       // constructor sets up GUI
       public Phone()
       {
          super( "Phone" );

          lcdJTextArea = new JTextArea( 4, 15 );
          lcdJTextArea.setEditable( false );
          lcdJPanel.add( lcdJTextArea );

           JButton keyJButton[] = new JButton[15];

          // initialize all digit key Buttons
          for ( int i = 3; i <= 11; i++ )
             keyJButton[ i ] = new JButton( String.valueOf( i - 2 ) );

          // initialize all non-digit key Buttons
          keyJButton[ 0 ] = new JButton( "Send" );
          keyJButton[ 1 ] = new JButton( "clr" );
          keyJButton[ 2 ] = new JButton( "End" );
          keyJButton[ 12 ] = new JButton( "*" );
          keyJButton[ 13 ] = new JButton( "0" );
          keyJButton[ 14 ] = new JButton( "#" );

          keyJButton[0].addActionListener;//<------------------THIS IS WHERE I'M HAVING                  //                                                                 PROBLEMS

                public void actionPerformed( ActionEvent e ) 
                {
                   lcdOutput = "Calling...\n\n" + lcdOutput;
                   lcdJTextArea.setText( lcdOutput );
                } // end method actionPerformed
              // end new ActionListener
        // end addActionListener call

          keyJButton[ 1 ].addActionListener(

             new ActionListener()
             {
                public void actionPerformed( ActionEvent e ) 
                {
                   if ( lcdOutput.length() == 0 || 
                      lcdOutput.substring( 0, 1 ).equals( "C" ) )
                      return;
                   else
                   {
                      lcdOutput = lcdOutput.substring( 0, ( lcdOutput.length() - 1 ) );
                      lcdJTextArea.setText( lcdOutput );
                   } // end else
                } // end method actionPerformed
             } // end object ActionLstener
          ); // end addActionListener call

          keyJButton[ 2 ].addActionListener(

             new ActionListener()
             {
                public void actionPerformed( ActionEvent e ) 
                {            
                   lcdJTextArea.setText( " " );
                   lcdOutput = "";
                } // end method actionPerformed
             } // end new ActionListener
          ); // end ActionListener call

          for ( int i = 3; i <= 14; i++ )
          {
             keyJButton[ i ].addActionListener(

                new ActionListener()
                {
                   public void actionPerformed( ActionEvent e ) 
                   {
                      lcdOutput += e.getActionCommand();

                      if ( lcdOutput.substring( 0, 1 ).equals( "C" ) )
                          return;

                      lcdJTextArea.append( e.getActionCommand() );
                   } // end method actionPerformed
                } // end new ActionListener
             ); // end addActionListener call
          } // end for loop

          // set keyJPanel layout to grid layout
          keyJPanel = new JPanel();
          keyJPanel.setLayout( new GridLayout( 5, 3 ) );

          // add buttons to keyJPanel
          for ( int i = 0; i <= 14; i++ )
             keyJPanel.add( keyJButton[ i ] );

          // add components to container
          add( lcdOutput, BorderLayout.NORTH );//<---------------THIS AS WELL
       } // end Phone constructor
    } // end class Phone

Answer 1

尝试将 ActionListener 添加到 keyJButton[0] 时，您没有获得正确的语法。看起来你对其他人来说是正确的，但是这一行:

keyJButton[0].addActionListener;

应替换为:

keyJButton[0].addActionListener(
             new ActionListener()
             {

不要忘记关闭任何附加的方括号和大括号。

您也无法将 lcdOutput 添加到框架中，因为它是一个字符串，而不是某种组件。也许您可以构建一个标签并添加它？

add( new JLabel(lcdOutput), BorderLayout.NORTH );

编辑:刚刚意识到第 27 行会抛出 NPE。在添加任何内容之前，您需要初始化 lcdJPanel。