java - 令牌上的语法错误，“类”，无效类型

Question

带星号的部分是它显示的错误。谁能向我解释为什么会这样？一开始我已经有一个公共课。如果我去掉括号“()”，后面的代码都会显示错误。谢谢！

    public class ButtonTester {

/**
 * @param args
 */
public static void main(String[] args) {
// TODO Auto-generated method stub
            new adasdad();
}



//Setting up variables
private static final int FRAME_WIDTH = 500;
private static final int FRAME_HEIGHT = 600;

private   JButton button = new JButton ("A");
private   JButton button2 = new JButton ("B");

 **class ExP12_4 () {**
    //Creating a frame
    //Creating a panel
    ...

    ...

    //Display frame
    ...

          }

Answer 1

您必须删除括号并为该类创建一个构造函数：

class ExP12_4 {
    private JFrame frame;
    private JPanel panel;

    private JButton button;
    private JButton button2;

    public ExP12_4 (){
        button = new JButton ("A");
        button2 = new JButton ("B");

        frame = new JFrame ();
        panel = new JPanel ();

        panel.add(button);
        panel.add(button2);
        frame.add(panel); 

        ...
    }
}

查看本教程以获取有关 Java 类的更多信息

Answer 2

类定义中不允许使用括号。这些属于构造函数。您还需要将语句放在代码块中，例如构造函数或init方法：

public class ExP12_4 {

    private static final int FRAME_WIDTH = 400;
    private static final int FRAME_HEIGHT = 300;
    private JButton button;
    private JButton button2;

    public ExP12_4() {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();

        panel.add(button);
        panel.add(button2);
        frame.add(panel);

        // etc.
    }
}

Answer 3

它应该["a"]代替("a").