How to set size of buttons same in Java Swing?

Question

I am new in swing and trying to set the same size for buttons. However, I didn't find the exact solution on the internet. Please note that I must use setPreferredSize(). Dimension is not the correct solution for now. I want to get the size of calcButton and use in setPreferredSize().

Here is the picture:

and the code:

import javax.swing.*;

/**
   The KiloConverter class displays a JFrame that
   lets the user enter a distance in kilometers. When 
   the Calculate button is clicked, a dialog box is 
   displayed with the distance converted to miles.
*/

public class KiloConverter extends JFrame
{
   private JPanel panel;             // To reference a panel
   private JLabel messageLabel;      // To reference a label
   private JTextField kiloTextField; // To reference a text field
   private JButton calcButton;       // To reference a button
   private JButton alertButton;
   private final int WINDOW_WIDTH = 310;  // Window width
   private final int WINDOW_HEIGHT = 100; // Window height

   /**
      Constructor
   */

   public KiloConverter()
   {
      // Set the window title.
      setTitle("Kilometer Converter");

      // Set the size of the window.
      setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

      // Specify what happens when the close button is clicked.
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      // Build the panel and add it to the frame.
      buildPanel();

      // Add the panel to the frame's content pane.
      add(panel);

      // Display the window.
      setVisible(true);
   }

   /**
      The buildPanel method adds a label, text field, and
      and a button to a panel.
   */

   private void buildPanel()
   {
      // Create a label to display instructions.
      messageLabel = new JLabel("Enter a distance " +
                                "in kilometers");

      // Create a text field 10 characters wide.
      kiloTextField = new JTextField(10);

      // Create a button with the caption "Calculate".
      calcButton = new JButton("Calculate");
      alertButton = new JButton("Alert");
      // --------ERROR HERE----------
      alertButton.setPreferredSize(new getPreferredSize(calcButton));


      // Create a JPanel object and let the panel
      // field reference it.
      panel = new JPanel();

      // Add the label, text field, and button
      // components to the panel.
      panel.add(messageLabel);
      panel.add(kiloTextField);
      panel.add(calcButton);
      panel.add(alertButton);
   }

   /**
      main method
   */

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

Answer 1

Yours is an XY Problem where you ask, "how do I do 'X'?" when the better solution is not to do 'X' at all, but rather to do 'Y'.

Here you ask how to set the preferred size of a button in order to make the button sizes equal, when the better and in fact canonically correct solution is not to do this, but rather to use a better layout manager, one that sizes the buttons the same for you -- a GridLayout.

For example, create a new JPanel to hold the buttons, say called buttonPanel, give it a GridLayout, say new GridLayout(1, 0, 3, 0), then add the buttons to it, and then add this JPanel to the main JPanel.

e.g.,

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

@SuppressWarnings("serial")
public class LayoutEg extends JPanel {
    private static final int KILO_FIELD_COLS = 15;
    private static final int GAP = 3;
    private JTextField kiloTextField = new JTextField(KILO_FIELD_COLS);
    private JButton calcButton = new JButton("Calculate");
    private JButton alertButton = new JButton("Alert");

    public LayoutEg() {
        // add ActionListeners etc.... 

        JPanel enterPanel = new JPanel();
        enterPanel.add(new JLabel("Enter a distance in kilometers:"));
        enterPanel.add(kiloTextField);

        JPanel buttonPanel = new JPanel(new GridLayout(1, 0, GAP, 0));
        buttonPanel.add(calcButton);
        buttonPanel.add(alertButton);

        setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
        setLayout(new GridLayout(0, 1));
        add(enterPanel);
        add(buttonPanel);
    }

    private static void createAndShowGui() {
        LayoutEg mainPanel = new LayoutEg();

        JFrame frame = new JFrame("Kilometer Converter");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

---

Explanation of the code:

Constant for the number of columns in the kiloTextField

private static final int KILO_FIELD_COLS = 15;

Constant for the gap between your JButtons and for the gap around the main JPanel (the empty border)

private static final int GAP = 3;

JTextField that is 15 columns wide

private JTextField kiloTextField = new JTextField(KILO_FIELD_COLS);

Top JPanel that holds a JLabel and a JTextField. It uses FlowLayout by default:

JPanel enterPanel = new JPanel();
enterPanel.add(new JLabel("Enter a distance in kilometers:"));
enterPanel.add(kiloTextField);

Here is the meat of the issue, create a JPanel that uses a GridLayout(1, 0, 3, 0), that is a grid layout with 1 row, with a variable number of columns (the 0 second parameter) that has a 3 horizontal gap and no vertical gap. Then add our buttons to it:

JPanel buttonPanel = new JPanel(new GridLayout(1, 0, GAP, 0));
buttonPanel.add(calcButton);
buttonPanel.add(alertButton);

Put a border around this main JPanel that is 3 pixels wide

setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));

Give the main JPanel a GridLayout that has variable number of rows (0) and 1 column, and add both the enterPanel and the buttonPanel to it:

setLayout(new GridLayout(0, 1));
add(enterPanel);
add(buttonPanel);

---

More explanation in comments, especially the key method, .pack():

// create the main JPanel
LayoutEg mainPanel = new LayoutEg();

// create a JFrame to put it in, although better to put into a JDialog
JFrame frame = new JFrame("Kilometer Converter");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

// add the LayoutEg JPanel into the JFrame
frame.getContentPane().add(mainPanel);

// **** key method*** that tells the layout managers to work
frame.pack();

// center and display
frame.setLocationRelativeTo(null);
frame.setVisible(true);