Creating a draw rectangle (filled with black color) function in Java for a grid

Question

I have created a grid in my program. Below is the code used to create the grid.

import java.awt.Graphics;

import javax.swing.JComponent;
import javax.swing.JFrame;

class Grid extends JComponent {
    public void paint(Graphics g) {
        g.drawRect (10, 10, 800, 500);    

        for (int i = 10; i <= 800; i+= 10)
            g.drawLine (i, 10, i, 510);

        for (int i = 10; i <= 500; i+= 10)
            g.drawLine (10, i, 810, i);
    }
}

public class CoreControl {

    public static void main(String[] a) {
        JFrame window = new JFrame();
        window.setSize(840,560);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.getContentPane().add(new Grid());
        window.setVisible(true);
    }

}

What I want to do is to create a function which will draw a rectangle (filled with black color) based on the coordinates that I give it. Basically I want to populate certain cells of the grid with black color and my idea is to draw black filled rectangles on the cell coordinates. How do I make this function?

I tried making another class called drawRectangle and called it in the main function like so window.getContentPane().add(new drawRectangle()); however that did not work (only drawRectangle shows up and not the grid).

I also want to be able to use this function repeatedly to keep creating rectangles.

How do I do create this function?

Also if you know a better way that I should structure this program please let me know (I am new to Java so I am open to any suggestions).

Solution

1. Don't use paint, use paintComponent and don't forget to call super.paintComponent
2. JComponent may not be the best choice, JPanel is probably a better choice
3. What's wrong with Graphics#fillRect(int, int, int, int)?

You might to take a look at Performing Custom Painting and 2D Graphics for more details.

I'd advice against trying to have a second component to performing the filling. Simply provide a method in you grid class that supplies the cell's x/y position (in grid terms) and fill the cell within the paintComponent method

Updated with example

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Point;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class CoreControl {

    public static class Grid extends JPanel {

        private List<Point> fillCells;

        public Grid() {
            fillCells = new ArrayList<>(25);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            for (Point fillCell : fillCells) {
                int cellX = 10 + (fillCell.x * 10);
                int cellY = 10 + (fillCell.y * 10);
                g.setColor(Color.RED);
                g.fillRect(cellX, cellY, 10, 10);
            }
            g.setColor(Color.BLACK);
            g.drawRect(10, 10, 800, 500);

            for (int i = 10; i <= 800; i += 10) {
                g.drawLine(i, 10, i, 510);
            }

            for (int i = 10; i <= 500; i += 10) {
                g.drawLine(10, i, 810, i);
            }
        }

        public void fillCell(int x, int y) {
            fillCells.add(new Point(x, y));
            repaint();
        }

    }

    public static void main(String[] a) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                Grid grid = new Grid();
                JFrame window = new JFrame();
                window.setSize(840, 560);
                window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                window.add(grid);
                window.setVisible(true);
                grid.fillCell(0, 0);
                grid.fillCell(79, 0);
                grid.fillCell(0, 49);
                grid.fillCell(79, 49);
                grid.fillCell(39, 24);
            }
        });
    }
}

Comments

[Veda Sadhak]

This might sound stupid but w/e i'm new to Java. How do I use Graphics.fillRect(int,int.int,int)? Right now this is how to would like to use it: function drawRectangle(int x1, int y1, int x2, int y2) { Graphics.fillRect(int,int.int,int) } is this possible? Does the program automatically add the rectangle to the frame?

[MadProgrammer]

No. fillRect is a method of Graphics, you need a reference to the Graphics context. All painting should be done within the context of a paint method (preferably paintComponent). In your Grid class I would provide method called something like fillGrid(int x, int y) which tells you what cell to fill. In your paintComponent method, I would then determine the rectangle bounds of this cell and fill using fillRect (ps I updated the answer ;))

[Veda Sadhak]

Ok so just to test out the fillRect I have added public void paint(Graphics g, int x, int y) { g.fillRect(x, y, 10, 10); } in the Grid Class. How do I use this function?

[Veda Sadhak]

Awesome. Thanks, I really appreciate it.

[Veda Sadhak]

Your answer already solved my question. However, just for learning purposes could you explain the line for (Point fillCell : fillCells) { i'm not familiar with the syntax. Also could you explain private List<Point> fillCells;.

[MadProgrammer]

Well,basically, my example allows you to set 0 or more cells to be filled. In order to track what cells are filled, I use a List, which is simply a sequential list of objetcs (in this case of Point objects). Take a look at Colleations for more details. The for loop is a shortcutted version of a for-next loop know as a for-each loop. Take a look at this for more info