Why won't main method print anything?

I've been trying to figure out what's wrong with my code. I have to build a GUI and there are no errors. The program builds successfully, but no GUI pops up. So in the main method, I commented out the GUI programming and added a simple System.out.println("hello"); but it does the same thing, i.e., it builds successfully, but does not print anything. Can someone please tell me what's wrong? Thanks!

System.out.println("hello");

/*
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* and open the template in the editor.
*/
package gui;

import java.awt.*;
import javax.swing.*;
import java.awt.Event.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class GUI extends JFrame

GridLayout g = new GridLayout(5, 2);
private JLabel baseIn = new JLabel("Base Input");
private JLabel heightIn = new JLabel("Height Input");
private JTextField base = new JTextField();
private JTextField height = new JTextField();
private JTextField area = new JTextField();
private JButton calc = new JButton("Calculate Area");

public GUI()
super("Triangle Area Calculator");
setSize(500, 300);
setLayout(g);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
add(baseIn);
add(heightIn);
add(base);
add(height);
add(area);
add(calc);
area.setEditable(false);
calc.addActionListener((ActionListener) this);

pack();
setVisible(true);


public void actionPerformed(ActionEvent e)
try
double bInput = Integer.valueOf(base.getText());
double hInput = Integer.valueOf(height.getText());
double aOutput = 0.5*bInput*hInput;
area.setText("Area of your triangle is: " + aOutput);
catch (NumberFormatException n)
System.out.println(n.getMessage());




public static void main(String args)
/*JFrame frame = new JFrame();
GUI one = new GUI();

frame.getContentPane().add(one);
frame.pack();
frame.setVisible(true);*/

System.out.println("hello world");






gui.GUI

4 Answers
4

First, going back to the basic code...

public class GUI extends JFrame
//...

public static void main(String args)
JFrame frame = new JFrame();
GUI one = new GUI();

frame.getContentPane().add(one);
frame.pack();
frame.setVisible(true);





Will fail, because you can't add a window based component to a window. As a general rule of thumb, you should avoid overriding JFrame (and other top level containers) directly and favour something less complex, like JPanel

JFrame

JPanel

public class GUI extends JPanel
//...
public static void main(String args)
EventQueue.invokeLater(new Runnable()
@Override
public void run()
JFrame frame = new JFrame();
frame.add(new GUI());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);

);





Next...

calc.addActionListener((ActionListener) this);


The fact that you need to perform a cast in order to get the code to work is a clear sign that something else is wrong and this is likely to cause a runtime error and crash your program. Perhaps you should start by having a read of How to write a Action Listener and How to Use Buttons, Check Boxes, and Radio Buttons to get a better understanding of how the API works

This is further supported by making use of the @Override annotation, which should be used when ever you "think" you're implementing or overriding existing functionality...

@Override

@Override
public void actionPerformed(ActionEvent e)
//...



This would then fail to compile, as you're not implementing any existing functionality. This functionality is described by the ActionListener interface which you are not implementing.

ActionListener

While you could implement this interface directly, I prefer to avoid doing so, as it exposes functionality that other classes shouldn't have access to and you run the risk of building a "god" method, which is never a good idea.

Instead, I prefer to make use of Java's Anonymous Classes, which provides a much better means for isolating functionality to single use case, for example...

calc.addActionListener(new ActionListener()
@Override
public void actionPerformed(ActionEvent e)
try
double bInput = Integer.valueOf(base.getText());
double hInput = Integer.valueOf(height.getText());
double aOutput = 0.5 * bInput * hInput;
area.setText("Area of your triangle is: " + aOutput);
catch (NumberFormatException n)
System.out.println(n.getMessage());


);


Runnable Example

import java.awt.EventQueue;
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.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class GUI extends JPanel

GridLayout g = new GridLayout(5, 2);
private JLabel baseIn = new JLabel("Base Input");
private JLabel heightIn = new JLabel("Height Input");
private JTextField base = new JTextField();
private JTextField height = new JTextField();
private JTextField area = new JTextField();
private JButton calc = new JButton("Calculate Area");

public GUI()
setLayout(g);
add(baseIn);
add(heightIn);
add(base);
add(height);
add(area);
add(calc);
area.setEditable(false);
calc.addActionListener(new ActionListener()
@Override
public void actionPerformed(ActionEvent e)
try
double bInput = Integer.valueOf(base.getText());
double hInput = Integer.valueOf(height.getText());
double aOutput = 0.5 * bInput * hInput;
area.setText("Area of your triangle is: " + aOutput);
catch (NumberFormatException n)
System.out.println(n.getMessage());


);


public static void main(String args)
EventQueue.invokeLater(new Runnable()
@Override
public void run()
JFrame frame = new JFrame();
frame.add(new GUI());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);

);





Netbeans properties...

Now, if all that still fails to net you a result, you need to make sure that your GUI class is configured as the "Main class".

GUI

Start by right clicking the Netbeans project node and select "Properties" (it's at the bottom).

From the "Projects Properties", select "Run" from the "Build" options down the left side.

Make sure that your GUI class is marked as the "Main Class", use "Browse" to find it if it's not

GUI

Try this

calc.addActionListener(new OptionButtonHandler());


I added one optionButtonHandler class which implements ActionListener. I checked on my IDE and I was able to get the area of the triangle.

private class OptionButtonHandler implements ActionListener
public void actionPerformed(ActionEvent e)
try
double bInput = Integer.valueOf(base.getText());
double hInput = Integer.valueOf(height.getText());
double aOutput = 0.5 * bInput * hInput;
area.setText("Area of your triangle is: " + aOutput);
catch (NumberFormatException n)
System.out.println(n.getMessage());





In your case, GUI is not an ActionListener, so that will fail.

Can you right click on the file with the main method in Netbeans, you should see the run option there, select it. This would allow you set your main method. After this, subsequent clicks on the Green play Button should work.

You do not need to cast your Frame class to an ActionListener. Instead, make it implement the ActionListener interface and your code for the button action should work. But in future, its better to add logic to detect what component triggered the action.

I don't know, but how can you write this :

calc.addActionListener((ActionListener) this);


Your object (this) is a JFrame, you should add 'implements ActionListener' first, or create a separate implementation ...

Next error, you do :

GUI one = new GUI();
frame.getContentPane().add(one);


GUI extends JFrame, its a JFrame, you can't add a JFrame in another one !

I tested with add of 'implements ActionListener' and it runs, but some errors remains ;)
Copy/paste needs wisdom, hum ^^

EDIT

this code is not perfect (and very ugly) but it works for your example :

package com.mead.helmet.core;

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.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.WindowConstants;

public class GUI extends JPanel implements ActionListener

public static void main(final String args)

JFrame frame = new JFrame("Triangle Area Calculator");

frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
GUI one = new GUI();

frame.getContentPane().add(one);
frame.pack();
frame.setVisible(true);

System.out.println("hello world");



GridLayout g = new GridLayout(5, 2);
private final JLabel baseIn = new JLabel("Base Input");
private final JLabel heightIn = new JLabel("Height Input");
private final JTextField base = new JTextField();
private final JTextField height = new JTextField();
private final JTextField area = new JTextField();

private final JButton calc = new JButton("Calculate Area");

public GUI()
super();
setSize(500, 300);
setLayout(g);
add(baseIn);
add(heightIn);
add(base);
add(height);
add(area);
add(calc);
area.setEditable(false);
calc.addActionListener((ActionListener) this);

setVisible(true);


/**
*
* @param event
*/

@Override
public void actionPerformed(final ActionEvent e)
try
double bInput = Integer.valueOf(base.getText());
double hInput = Integer.valueOf(height.getText());
double aOutput = 0.5 * bInput * hInput;
area.setText("Area of your triangle is: " + aOutput);
catch (NumberFormatException n)
System.out.println(n.getMessage());






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