The Basics of Making a GUI in Java

This article assumes you know the Java programming language. A Graphical User Interface (GUI) in Java is essentially a hierarchy of Containers and Components. Containers are classes that contain Components and other Containers. A Component is something the user can interact with; a button, a text field, a slider, etc.

Let's start with the basics: creating a window. I'll go over the following code line by line (sorry about the lack of code formatting; AssociatedContent removes space at the beginning of a line):

import javax.swing.*;

public class GUIFun
{
public static void main(String[] args) { new GUIFun(); }

public GUIFun()
{
JFrame f = new JFrame();
f.setSize(600, 600);
f.setTitle("My Window!");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setVisible(true);
}
}

The first line creates a new JFrame object. A JFrame is a Container that represents a window. On the next line, we set the size of our window, in pixels, and after that tell the window what to say in the bar on the top. The next line tells the window that if someone closes it, end the program. If you don't have this line, the program will continue to run after the window has been closed.

If you want the program to continue running, there are other options. One is JFrame.HIDE_ON_CLOSE, which closes the window but doesn't end the program, JFrame.DISPOSE_ON_CLOSE, which closes the window and disposes it so that any memory taken up by the frame is freed, and JFrame.DO_NOTHING_ON_CLOSE, which tells the window to stay open even if the user tries to close it.

After you finish setting all these properties, we can show the window with the setVisible(true) command. Run this code, and you will see a blank window. Let's make our window more interesting by adding a button:

import javax.swing.*;

public class GUIFun
{
public static void main(String[] args) { new GUIFun(); }

public GUIFun()
{
JFrame f = new JFrame();
f.setSize(600, 600);
f.setTitle("My Window!");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JButton helloButton = new JButton("Hello");
f.add(helloButton);
f.setVisible(true);
}
}

Compile and run this code. What do you see? You should see a giant button that fills up the entire window, which does nothing when clicked. This is because a JFrame can only hold one Component/Container at a time, so it is stretched to fill the entire JFrame. The JButton does nothing because we haven't yet told the button to do anything when clicked. We can fix the first problem by introducing another Container: the JPanel. A JPanel is an "invisible" Container, because unlike the JFrame, you cannot actually see the JPanel itself, but you can see all of the Components it contains.

import javax.swing.*;

public class GUIFun
{
public static void main(String[] args) { new GUIFun(); }

public GUIFun()
{
JFrame f = new JFrame();
f.setSize(600, 600);
f.setTitle("My Window!");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JButton helloButton = new JButton("Hello");
JButton goodbyeButton = new JButton("Goodbye");
JPanel panel = new JPanel();
panel.add(helloButton);
panel.add(goodbyeButton);

f.add(panel);
f.setVisible(true);
}
}

You might have noticed that I slipped in another JButton. That's there to show you that the JPanel can hold as many Components that you like.

Now, let's make those buttons actually do something. We can accomplish this with an "ActionListener". An ActionListener is an interface that includes only one method: actionPerformed(ActionEvent event). When you create a class that implements ActionListener, you can add it to a Component, and when that Component is interacted with, the Component calls the actionPerformed method of all the ActionListeners that have been added to it. If you have an ActionListener that handles several different Components, you can give each Component an "action command", which can be used to distinguish between the Components. Here is the final version of our class:

import java.awt.event.*;
import javax.swing.*;

public class GUIFun implements ActionListener
{
private JLabel messageLabel;
private JFrame f;

public static void main(String[] args) { new GUIFun(); }

public GUIFun()
{
f = new JFrame();
f.setSize(600, 600);
f.setTitle("My Window!");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JPanel panel = new JPanel();

messageLabel = new JLabel("Hello");
JButton helloButton = new JButton("Hello");
helloButton.addActionListener(this);
helloButton.setActionCommand("hello");
JButton goodbyeButton = new JButton("Goodbye");
goodbyeButton.addActionListener(this);
goodbyeButton.setActionCommand("goodbye");
JButton quitButton = new JButton("Quit");
quitButton.addActionListener(this);
quitButton.setActionCommand("quit");

panel.add(messageLabel);
panel.add(helloButton);
panel.add(goodbyeButton);
panel.add(quitButton);

f.add(panel);
f.setVisible(true);
}

public void actionPerformed(ActionEvent e)
{
String actionCommand = e.getActionCommand();
if (actionCommand.equals("hello"))
{
messageLabel.setText("Hello");
f.repaint();
}
else if (actionCommand.equals("goodbye"))
{
messageLabel.setText("Goodbye");
f.repaint();
}
else
{
System.exit(0);
}
}
}