如何在Java中画一条线？

Question

I want to draw a line in java in eclipse. I made this code but I am getting error in line : paint2d.add(paintComponent());

import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Game extends JPanel {

    public void paintComponent (Graphics2D g) {
        Graphics2D g2 = (Graphics2D) g;
        g2.drawLine(30, 40, 80, 100);
    }

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        frame.setSize(400, 420);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Game paint2d = new Game();
        paint2d.add(paintComponent()); // The method paintComponent(Graphics2D) in the type Game is not applicable for the arguments ()
        frame.setVisible(true);
    }
}

How to fix that error?

Is my code suitable for the purpose of drawing a line?

Thanks.

Answer 1

You're not overriding the method correctly. The argument to paintComponent is of type Graphics , not Graphics2D , but you can cast to Graphics2D . Also you need to add the Game panel to the frame as a content pane:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;

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

public class Game extends JPanel
{

    @Override
    public void paintComponent(Graphics g)
    {
        Graphics2D g2 = (Graphics2D) g;
        g2.setColor(Color.BLACK);
        g2.drawLine(30, 40, 80, 100);
    }

    public static void main(String[] args)
    {
        JFrame frame = new JFrame();
        frame.setSize(400, 420);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Game game = new Game();
        frame.setContentPane(game);

        frame.setVisible(true);
        frame.invalidate();
    }

}

Answer 2

There are two errors in your code/approach:

1. The method paintComponent has a Graphics as parameter, not a Graphics2D .
2. Game is a JPanel and must be added into the JFrame to be shown.

The source code below solves the problems.

import java.awt.Graphics;
import java.awt.Graphics2D;

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

public class Game extends JPanel {

    public void paintComponent (Graphics g) {
        Graphics2D g2 = (Graphics2D) g;
        g2.drawLine(30, 40, 80, 100);
    }

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        frame.setSize(400, 420);

        // Adds Game panel into JFrame
        frame.add(new Game());

        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}