Panel fills the window in Java swing

Question

I've added two 10x10 grids in a window but I cannot get them to appear in a smaller size. They just fill the entire window. How do I place both of them neatly in the middle in a smaller size so that there's room for some labels and buttons? I've pasted the code below for reference.

public static void main(String[] args) {
    window = new JFrame();
    window.setTitle("Battleship.exe");
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setPreferredSize(new Dimension(600, 600));
    
    P1_container = new JPanel(new GridLayout(10,10));
    P1_container.setBorder(BorderFactory.createLineBorder(Color.black, 5));
    
    compContainer = new JPanel(new GridLayout(10,10));
    compContainer.setBorder(BorderFactory.createLineBorder(Color.black, 5));
    
    grid = new JPanel[10][10];
    for (int i =0; i< 10; i++) {
        for (int j =0; j< 10; j++) {
            grid[i][j] = new JPanel();
            grid[i][j].setBackground(Color.white);
            grid[i][j].setBorder(BorderFactory.createLineBorder(Color.blue, 2));
            grid[i][j].setPreferredSize(new Dimension(25,25));
            P1_container.add(grid[i][j]);
        }
    }
    
    enemyGrid = new JPanel[10][10];
    for (int i =0; i< 10; i++) {
        for (int j =0; j< 10; j++) {
            enemyGrid[i][j] = new JPanel();
            enemyGrid[i][j].setBackground(Color.white);
            enemyGrid[i][j].setBorder(BorderFactory.createLineBorder(Color.red, 2));
            enemyGrid[i][j].setPreferredSize(new Dimension(25, 25));
            compContainer.add(enemyGrid[i][j]);
        }
    }
    
    GridLayout layout = new GridLayout(1, 2);
    layout.setHgap(150);
    mainPanel = new JPanel(layout); 
    
    mainPanel.add(P1_container);
    mainPanel.add(compContainer);
    
    window.add(mainPanel);
    window.pack();
    window.setVisible(true);
}

Answer 1

Your code had 24 compile errors. Once I fixed the compile errors, I modified one line and commented out one line to get this GUI.

Here's the complete runnable code.

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

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class BattleshipGUI {
    
    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.setTitle("Battleship.exe");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//      window.setPreferredSize(new Dimension(600, 600));
        
        JPanel P1_container = new JPanel(new GridLayout(10,10));
        P1_container.setBorder(BorderFactory.createLineBorder(Color.black, 5));
        
        JPanel compContainer = new JPanel(new GridLayout(10,10));
        compContainer.setBorder(BorderFactory.createLineBorder(Color.black, 5));
        
        JPanel[][] grid = new JPanel[10][10];
        for (int i =0; i< 10; i++) {
            for (int j =0; j< 10; j++) {
                grid[i][j] = new JPanel();
                grid[i][j].setBackground(Color.white);
                grid[i][j].setBorder(BorderFactory.createLineBorder(Color.blue, 2));
                grid[i][j].setPreferredSize(new Dimension(25,25));
                P1_container.add(grid[i][j]);
            }
        }
        
        JPanel[][] enemyGrid = new JPanel[10][10];
        for (int i =0; i< 10; i++) {
            for (int j =0; j< 10; j++) {
                enemyGrid[i][j] = new JPanel();
                enemyGrid[i][j].setBackground(Color.white);
                enemyGrid[i][j].setBorder(BorderFactory.createLineBorder(Color.red, 2));
                enemyGrid[i][j].setPreferredSize(new Dimension(25, 25));
                compContainer.add(enemyGrid[i][j]);
            }
        }
        
        GridLayout layout = new GridLayout(0, 2);
        layout.setHgap(150);
        JPanel mainPanel = new JPanel(layout); 
        
        mainPanel.add(P1_container);
        mainPanel.add(compContainer);
        
        window.add(mainPanel);
        window.pack();
        window.setVisible(true);
    }

}

Answer 2

The key to using Swing layout managers is to nest your containers.

Nest mainPanel into another JPanel, one that uses a layout that helps you achieve your goal, say a BorderLayout with mainPanel placed into the BorderLayout.CENTER position. Then add the other gui components to this same outer JPanel at other BorderLayout locations.

If you don't want the mainPanel to expand, then use a different outer layout that does not expand, such as FlowLayout.