Working Example

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Main extends JFrame implements ActionListener {
	private JLabel labelQuestion;
	private JLabel labelWeight;
	private JTextField fieldWeight;
	private JButton buttonTellMe;
	public Main() {
		super("Water Calculator");
		initComponents();
		setSize(Toolkit.getDefaultToolkit().getScreenSize());
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}
	private void initComponents() {
		labelQuestion = new JLabel("How much water should a person drink?");
		labelWeight = new JLabel("My weight (kg):");
		fieldWeight = new JTextField(5);
		buttonTellMe = new JButton("Tell Me");
		setLayout(new FlowLayout());
		add(labelQuestion);
		add(labelWeight);
		add(fieldWeight);
		add(buttonTellMe);
		buttonTellMe.addActionListener(this);
	}
	public void actionPerformed(ActionEvent event) {
		String message = "Buddy, you should drink %.1f L of water a day!";
		float weight = Float.parseFloat(fieldWeight.getText());
		float waterAmount = calculateWaterAmount(weight);
		message = String.format(message, waterAmount);
		JOptionPane.showMessageDialog(this, message);
	}
	private float calculateWaterAmount(float weight) {
		return (weight / 10f) * 0.4f;
	}
	public static void main(String[] args) {
		new Main().setVisible(true);
	}
}