Build a Java Desktop App with Swing ☕

Project Goal

To create a simple desktop window with three components:

• A text field to enter a name.
• A button to submit the name.
• A label to display the greeting.

What You'll Need

• Java Development Kit (JDK) installed on your system.
• A simple text editor or a Java IDE like IntelliJ IDEA, Eclipse, or VS Code.

The main window of any Swing application is called a `JFrame`. Let's create a class for our application that extends `JFrame` to inherit all the properties of a window.

Create a new file named `GreeterApp.java` and add the following basic structure:

import javax.swing.*;

public class GreeterApp extends JFrame {

    // The constructor is where we'll set up our window
    public GreeterApp() {
        // 1. Set the title of the window
        setTitle("Greeter App");

        // 2. Set the size of the window (width, height)
        setSize(400, 150);

        // 3. Tell the program to exit when the window is closed
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        // Create and show the GUI on the Event Dispatch Thread
        SwingUtilities.invokeLater(() -> {
            GreeterApp app = new GreeterApp();
            app.setVisible(true); // 4. Make the window visible
        });
    }
}

An empty window isn't very useful. We need to add our components (also called widgets). We also need a **Layout Manager** to tell Swing how to arrange them.

We will use `FlowLayout`, the simplest layout manager, which places components one after another in a row.

Let's modify our `GreeterApp.java` file.

import javax.swing.*;
import java.awt.*; // Import for FlowLayout
import java.awt.event.*; // Import for event handling (we'll use this in the next step)

public class GreeterApp extends JFrame {

    // Declare our components as instance variables
    private JLabel instructionLabel;
    private JTextField nameField;
    private JButton greetButton;
    private JLabel greetingLabel;

    public GreeterApp() {
        setTitle("Greeter App");
        setSize(400, 150);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // --- LAYOUT ---
        // Set the layout manager for the window
        setLayout(new FlowLayout());

        // --- COMPONENTS ---
        // 1. Create the components
        instructionLabel = new JLabel("Enter your name:");
        nameField = new JTextField(15); // 15 is the approximate width
        greetButton = new JButton("Greet!");
        greetingLabel = new JLabel(""); // Initially empty

        // 2. Add the components to the window's content pane
        add(instructionLabel);
        add(nameField);
        add(greetButton);
        add(greetingLabel);
    }
    
    // main method remains the same
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            GreeterApp app = new GreeterApp();
            app.setVisible(true);
        });
    }
}

If you compile and run the code now, you'll see a window with all the components arranged in a line. However, clicking the button does nothing yet.

GUI applications are **event-driven**. They wait for the user to do something (like click a button) and then react to that event. We need to attach an **`ActionListener`** to our button to define what happens when it's clicked.

We will add this logic inside the `GreeterApp` constructor, right after we create the components.

// ... inside the GreeterApp constructor, after adding components ...

// --- EVENT HANDLING ---
// Add an ActionListener to the button.
// We'll use a lambda expression for concise code.
greetButton.addActionListener(e -> {
    // This is the code that runs when the button is clicked
    String name = nameField.getText(); // Get text from the text field
    
    // Check if the name is not empty
    if (!name.trim().isEmpty()) {
        greetingLabel.setText("Hello, " + name + "!"); // Set the label's text
    } else {
        greetingLabel.setText("Please enter a name.");
    }
});

Here is the final, complete code for `GreeterApp.java`. You can copy this into a file and run it.

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

public class GreeterApp extends JFrame {

    private JLabel instructionLabel;
    private JTextField nameField;
    private JButton greetButton;
    private JLabel greetingLabel;

    public GreeterApp() {
        setTitle("Greeter App");
        setSize(400, 150);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new FlowLayout());

        instructionLabel = new JLabel("Enter your name:");
        nameField = new JTextField(15);
        greetButton = new JButton("Greet!");
        greetingLabel = new JLabel("");

        add(instructionLabel);
        add(nameField);
        add(greetButton);
        add(greetingLabel);

        greetButton.addActionListener(e -> {
            String name = nameField.getText();
            if (!name.trim().isEmpty()) {
                greetingLabel.setText("Hello, " + name + "!");
            } else {
                greetingLabel.setText("Please enter a name.");
            }
        });
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            GreeterApp app = new GreeterApp();
            app.setVisible(true);
        });
    }
}

How to Run the Application

1. Using an IDE (Recommended):

Simply open the `GreeterApp.java` file in IntelliJ, Eclipse, or another Java IDE and click the "Run" button next to the `main` method.

2. Using the Command Line:

Open a terminal, navigate to the folder where you saved `GreeterApp.java`, and run these two commands:

# Compile the Java code into bytecode
javac GreeterApp.java

# Run the compiled program
java GreeterApp

A window should appear on your screen. Type your name and click the button!