文本字段、标签和按钮 [英] Text Fields, Labels and Buttons

问题描述

我在理解 GUI 以及为什么我的程序无法正常运行时遇到了一些困难.是因为我必须扩展到 JFrame 类吗?这是一个代码:

I am having some difficulty understanding GUIs and why my program won't run properly. Is it because I have to extend to a JFrame class? Here is a code:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListner;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Lab_10
{
    public static void main (String[] args)
    {
        final double EARTHQUAKE_RATE= 8.0;

        final int FRAME_WIDTH= 300;
        final int FRAME_HEIGHT= 200;
        frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
        frame.setTitle("Richter Scale");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true); 

        JFrame frame = new JFrame();
        JLabel label = new JLabel();
        JPanel panel = new JPanel();
        panel.add(button);
        panel.add(label);
        panel.add(rictherfield);
        panel.add(rictherlabel);
        add(panel);

        JLabel rictherlabel = new JLabel ("Ricther: ");

        final int FIELD_WIDTH = 10;
        JTextField rictherField = new JTextField(FIELD_WIDTH);
        richterField.setText("" + EARTHQUAKE_RATE);

        JButton button = new JButton("Enter");

        ActionListner listner = new AddLabelListener();
        button.addActionListner(listner);

        class AddLabelListener implements ActionListener
        {
            public void actionPerformed(ActionEvent event)
            {
                label.setText("Most structures fall");
            }
        }
    }
}

我收到很多错误，指出程序找不到我的 ActionListeners 等的符号.

I am getting a lot of errors stating that the program cannot find the symbols for my ActionListeners etc.

错误是:

  _10.java:2: error: cannot find symbol
    import java.awt.event.ActionListner;
                         ^
      symbol:   class ActionListner
      location: package java.awt.event
    Lab_10.java:17: error: cannot find symbol
    frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
    ^
      symbol:   variable frame
      location: class Lab_10
    Lab_10.java:18: error: cannot find symbol
    frame.setTitle("Richter Scale");
    ^
      symbol:   variable frame
      location: class Lab_10
    Lab_10.java:19: error: cannot find symbol
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    ^
      symbol:   variable frame
      location: class Lab_10
    Lab_10.java:20: error: cannot find symbol
    frame.setVisible(true); 
    ^
      symbol:   variable frame
      location: class Lab_10
    Lab_10.java:30: error: cannot find symbol
    panel.add(button);
              ^
      symbol:   variable button
      location: class Lab_10
    Lab_10.java:32: error: cannot find symbol
    panel.add(rictherfield);
              ^
      symbol:   variable rictherfield
      location: class Lab_10
    Lab_10.java:33: error: cannot find symbol
    panel.add(rictherlabel);
              ^
      symbol:   variable rictherlabel
      location: class Lab_10
    Lab_10.java:34: error: cannot find symbol
    add(panel);
    ^
      symbol:   method add(JPanel)
      location: class Lab_10
    Lab_10.java:40: error: cannot find symbol
    richterField.setText("" + EARTHQUAKE_RATE);
    ^
      symbol:   variable richterField
      location: class Lab_10
    Lab_10.java:45: error: cannot find symbol
    ActionListner listner = new AddLabelListener();
    ^
      symbol:   class ActionListner
      location: class Lab_10
    Lab_10.java:45: error: cannot find symbol
    ActionListner listner = new AddLabelListener();
                                ^
      symbol:   class AddLabelListener
      location: class Lab_10
    Lab_10.java:49: error: cannot find symbol
    class AddLabelListener implements ActionListener
                                      ^
      symbol:   class ActionListener
      location: class Lab_10
    Lab_10.java:53: error: local variable label is accessed from within inner class; needs to be declared final
    label.setText("Most structures fall");

推荐答案

Wow... 好吧，首先，ActionListener 被错误地拼写为ActionListner".仔细查看这些单词的拼写.简单的印刷错误会产生语法错误.

Wow... OK, first, ActionListener is spelled incorrectly as "ActionListner." Look very closely at the spelling of those words. Simple typographical errors generate syntax errors.

您的其余问题归结为这个非常简单的警告:订单很重要.您的订单应如下所示:

The rest of your problems boil down to this very simple caveat: order matters. Your order should be as follows:

1) 声明和创建对象；2) 声明并创建所有依赖对象；3)配置对象；4) 操作对象.

1) declare and create objects; 2) declare and create all dependent objects; 3) configure objects; 4) manipulate objects.

这意味着您的代码:

frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Richter Scale");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

JFrame frame = new JFrame(); 

不起作用，因为您试图弄乱尚未创建的框架.首先创建它，如下所示:

Won't work because you're trying to mess with a frame that hasn't yet been created. Create it first, like so:

JFrame frame = new JFrame();

frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Richter Scale");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

同样适用于您的标签和按钮.

Same holds true for your label and your button.

JLabel rictherlabel = new JLabel ("Ricther: ");

需要先来

panel.add(label);

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