Java 按钮处理程序

Question

我在处理按钮事件时遇到问题。我正在创建一个程序，让用户选择他们想要的比萨饼类型，程序计算比萨饼的价格。 I have my layout setup, however when a medium pizza is selected the button is not processing the proper response.谁能给我一些建议？在过去的一个小时里，我查看了我的代码，但我似乎看不到我正在犯的错误。这是我的代码...

public class Prog9Frame extends JFrame implements ActionListener
{
private JLabel title;
private JLabel size;
private JLabel toppings;
private JComboBox crust;
private JRadioButton mediumRadio;
private JRadioButton largeRadio;
private JRadioButton xLargeRadio;
private JCheckBox pepperoniBox;
private JCheckBox sausageBox;
private JCheckBox mushroomsBox;
private JCheckBox onionsBox;
private JLabel total;
private JTextField totalField;
private JButton submit;

public Prog9Frame()
{
    super ("Pizzazz Pizza");
    setLayout( new BorderLayout( 5,5 ) );

    //north region
    title = new JLabel ( "Pizzazz Pizza", JLabel.CENTER );
    add ( title, BorderLayout.NORTH);

    //west region
    JPanel westPanel = new JPanel();
    westPanel.setLayout( new BoxLayout( westPanel, BoxLayout.Y_AXIS ) );

    westPanel.add(Box.createRigidArea( new Dimension( 25,1 )) );
    size = new JLabel ( "Size" );
    westPanel.add( Box.createVerticalStrut((20)) );
    westPanel.add( size );
    mediumRadio = new JRadioButton( "Medium" );
    westPanel.add(Box.createVerticalStrut(20) );
    westPanel.add( mediumRadio );
    largeRadio = new JRadioButton( "Large ");
    westPanel.add(Box.createVerticalStrut(20));
    westPanel.add(largeRadio);
    xLargeRadio = new JRadioButton( "X-Large ");
    westPanel.add(Box.createVerticalStrut(20));
    westPanel.add(xLargeRadio);
    add(westPanel, BorderLayout.WEST);

    //center region
    JPanel centerPanel = new JPanel();
    centerPanel.setLayout( new BoxLayout(centerPanel, BoxLayout.Y_AXIS ));

    toppings = new JLabel ( "Toppings" );
    centerPanel.add(Box.createVerticalStrut(( 20 )) );
    centerPanel.add( toppings );
    pepperoniBox = new JCheckBox( "Pepperoni" );
    centerPanel.add(Box.createVerticalStrut(( 20 )) );
    centerPanel.add( pepperoniBox);
    sausageBox = new JCheckBox( "Sausage" );
    centerPanel.add(Box.createVerticalStrut(( 20 )) );
    centerPanel.add( sausageBox);
    mushroomsBox = new JCheckBox( "Mushrooms" );
    centerPanel.add(Box.createVerticalStrut(( 20 )) );
    centerPanel.add( mushroomsBox);
    onionsBox = new JCheckBox( "Onions" );
    centerPanel.add(Box.createVerticalStrut(( 20 )) );
    centerPanel.add( onionsBox);
    add( centerPanel, BorderLayout.CENTER);

    //east region
    JPanel eastPanel = new JPanel();
    eastPanel.setLayout(new BoxLayout(eastPanel, BoxLayout.Y_AXIS));
    eastPanel.add(Box.createHorizontalStrut(20));
    eastPanel.add(Box.createVerticalStrut(50));
    String[] crustStrings = { "Thin", "Regular", "Deep Dish" };
    JComboBox crust = new JComboBox(crustStrings);
    eastPanel.add(crust);
    eastPanel.add(Box.createVerticalStrut(200));
    add( eastPanel, BorderLayout.EAST);

    //south region
    JPanel southPanel = new JPanel();
    southPanel.setLayout(new FlowLayout( FlowLayout.CENTER) );

    JTextField totalField = new JTextField(10);
    southPanel.add(totalField);
    JButton submit = new JButton ("Submit");
    submit.addActionListener( this );
    southPanel.add( submit );
    add( southPanel, BorderLayout.SOUTH);


}
//handle button events
public void actionPerformed( ActionEvent event )
{
if (mediumRadio.isSelected())
    {
        double pizzaMed = 7.95;
        totalField.setText(new DecimalFormat("###00.00").format(pizzaMed));
        }
    }

}

Answer 1

通过在类的构造函数中重新声明它来隐藏 totalField 变量。这将导致您重新声明变量，具有有效对象引用的变量仅在构造函数中可见，仅在声明它的范围内可见，并且它将使类字段未构造，因此为空。如果你尝试使用它，你会得到一个 NullPointerException 或 NPE。

解决方案不是在构造函数中重新声明变量，而是在其中初始化类字段（或在需要时在类中声明）。

所以而不是：

public class Foo {
   private JTextField bar;

   public Foo() {
     JTextField bar = new JTextField(10);  // re-declaring bar here
   }
}

做：

public class Foo {
   private JTextField bar;

   public Foo() {
     bar = new JTextField(10);  // ** see the difference? **
   }
}

Answer 2

更新您的南部地区代码。您正在重新声明隐藏在 actionPerformed 方法。这就是它给出 java.lang.NullPointerException 的方式。

JTextField totalField = new JTextField(10);

改成

this.totalField = new JTextField(10);