GridLayout not filling JPanel

Question

I'm having a problem with GridLayout and the entire JPanel not being filled. I have a N * M Grid, and I'm filling it with N * M Tiles (They extend JPanel). After adding all these tiles there is still space on the bottom and the right side of the JPanel. I thought GridLayout was supposed to fill up the entire JPanel, and make everything in it evenly sized?

Edit: I didn't want to post all the code since there are multiple classes. Thought maybe there was a simple solution.

public class MainFrame extends JFrame {

    private static final long serialVersionUID = 3985493842977428355L;
    private final int FRAME_HEIGHT = 800;
    private final int FRAME_WIDTH = 700;

    private MainPanel mainPanel;

    public MainFrame() {
        super("Test");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(FRAME_HEIGHT, FRAME_WIDTH);
        setLocationRelativeTo(null);    
        mainPanel = new MainPanel();
        getContentPane().add(mainPanel);
        setVisible(true);
    }
}

public class MainPanel extends JPanel {
    /**
     * 
     */
    private static final long serialVersionUID = 3421071253693531472L;

    private RoutePanel routePanel;
    private ControlPanel controlPanel;
    private GridBagConstraints gridBagConstraints;
    private GridBagLayout gridBagLayout;

    public MainPanel() {
        routePanel = new RoutePanel();
        controlPanel = new ControlPanel();
        setBackground(Color.black);
        gridBagLayout = new GridBagLayout();
        setLayout(gridBagLayout);
        gridBagConstraints = new GridBagConstraints();
        createLayout();
    }

    private void createLayout() {
        gridBagConstraints.weightx = 1;
        gridBagConstraints.weighty = 1;
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 0;
        gridBagConstraints.anchor = GridBagConstraints.NORTH;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagLayout.setConstraints(routePanel, gridBagConstraints);
        add(routePanel);

        gridBagConstraints.weightx = 1;
        gridBagConstraints.weighty = .05;
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 1;
        gridBagConstraints.anchor = GridBagConstraints.SOUTH;
        gridBagConstraints.fill = GridBagConstraints.BOTH;
        gridBagLayout.setConstraints(controlPanel, gridBagConstraints);
        add(controlPanel);
    }
}

public class RoutePanel extends JPanel{
    /**
     * 
     */
    private static final long serialVersionUID = 4366703088208967594L;
    private final int GRID_ROWS = 30;
    private final int GRID_COLS = 47;


    public RoutePanel() {
        setLayout(new GridLayout(GRID_ROWS, GRID_COLS, 0, 0));
        for(int i = 0; i < GRID_ROWS * GRID_COLS; i++) {
            add(new Tile());
        }
        setBackground(Color.orange);
    }
}

public ControlPanel() {

    }

public Tile() {
        setBackground(Color.gray);
        Border blackline;
        blackline = BorderFactory.createLineBorder(Color.black);
        setBorder(blackline);
    }

Answer 1

Since you're not posting code, we are forced to guess (and this is not good).

My guess: you may be setting the size or the preferred sizes of some of your components or the GUI, and this is causing gaps at the edges of your GUI.

Solution: don't do this. Let the components size themselves via their preferred sizes and the layout managers when pack() is called on the GUI.

For more helpful help, create and post your sscce. For a question like this, code is really mandatory, and I'm a bit surprised actually that you didn't post yours.

---

Edit 1

For example, note the difference in the GUI's produced:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridLayout;

import javax.swing.*;

public class GapExample {
   private static final int M = 5;
   private static final int N = 6;
   private static final int PREF_W = 700;
   private static final int PREF_H = 500;

   @SuppressWarnings("serial")
   private static void createAndShowGui() {

      // *** attempt 1: set preferredSize of the mainPanel JPanel ***
      JPanel mainPanel = new JPanel(new GridLayout(M, N));
      mainPanel.setBorder(BorderFactory.createLineBorder(Color.red));
      mainPanel.setPreferredSize(new Dimension(PREF_W, PREF_H));

      for (int i = 0; i < M; i++) {
         for (int j = 0; j < N; j++) {
            String text = String.format("Foo [%d, %d]", i, j);
            JLabel label = new JLabel(text, SwingConstants.CENTER);
            label.setBorder(BorderFactory.createLineBorder(Color.blue));
            mainPanel.add(label);
         }
      }
      JOptionPane.showMessageDialog(null, mainPanel,
            "Attempt 1, setPreferredSize of mainPane",
            JOptionPane.PLAIN_MESSAGE);

      // *** attempt 2: override the getPreferredSize of the JLabel cells in the
      // grid ***
      mainPanel = new JPanel(new GridLayout(M, N));
      mainPanel.setBorder(BorderFactory.createLineBorder(Color.red));
      final Dimension labelPrefSize = new Dimension(PREF_W / N, PREF_H / M);
      for (int i = 0; i < M; i++) {
         for (int j = 0; j < N; j++) {
            String text = String.format("Foo [%d, %d]", i, j);
            JLabel label = new JLabel(text, SwingConstants.CENTER) {
               @Override
               public Dimension getPreferredSize() {
                  return labelPrefSize;
               }
            };
            label.setBorder(BorderFactory.createLineBorder(Color.blue));
            mainPanel.add(label);
         }
      }
      JOptionPane
            .showMessageDialog(null, mainPanel,
                  "Attempt 2, override getPreferredSize of the JLabel cells in the grid",
                  JOptionPane.PLAIN_MESSAGE);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

In the second dialog, I have the JLabels return a preferred size and let the containers that hold them set the best size to display the GUI, and you'll with the second attempt, the grid fits much better.

This displays for the first (bad) attempt a gap between the inner and outer components:

And the second (good) attempt shows no such gap: