有关简单的Swing侦听器代码的一些疑问

Question

I am studying Java Swing and listener use and I have some problems with the following simple code:

package com.mycompany.experiment;

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

import javax.swing.JButton;
import javax.swing.JFrame;

public class ListenerExp1 {

    private class MyButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent evt) {
            // Qui scriviamo il codice che vogliamo eseguire all' azione del
            // click sul pulsante
        }
    }

    JFrame frame = new JFrame("FrameDemo");
    frame.setMinimumSize(new Dimension(800, 400));

    JButton button = new JButton("Cliccami");
    MyButtonListener listener = new MyButtonListener();

    button.addActionListener(listener);

}

In this code I declare a listener class and I try to create a JFrame and a JButton objects.

The problem is that when I try to add some properties to these objects by the lines:

frame.setMinimumSize(new Dimension(800, 400));

and

button.addActionListener(listener);

Eclipse sign me the following error on these 2 lines:

Multiple markers at this line - Syntax error on token(s), misplaced construct(s) - Syntax error on token "setMinimumSize", = expected after this

What is wrong?

Tnx

Andrea

Answer 1

These statements should be in a code block such as a method a constructor

JFrame frame = new JFrame("FrameDemo");
frame.setMinimumSize(new Dimension(800, 400));

JButton button = new JButton("Cliccami");
MyButtonListener listener = new MyButtonListener();

button.addActionListener(listener);

In addition you want to invoke them in the EDT like so

SwingUtilities.invokeLater(new Runnable() {

        @Override
        public void run() {
            JFrame frame = new JFrame("FrameDemo");
            frame.setMinimumSize(new Dimension(800, 400));

            JButton button = new JButton("Cliccami");
            MyButtonListener listener = new MyButtonListener();

            button.addActionListener(listener);
        }
    });

Read: Initial Threads

Answer 2

In java, you can not initialize instance without or outside of method.

put following of your code in a method.

JFrame frame = new JFrame("FrameDemo");
frame.setMinimumSize(new Dimension(800, 400));

JButton button = new JButton("Cliccami");
MyButtonListener listener = new MyButtonListener();

button.addActionListener(listener);