How to paint inside JPanel

问题

I have a JPanel inside a Jframe.I want to draw a line inside JPanel, using paint(Graphics g) method. But it is not working. Please Someone help me on this issue. Here is the code. Thank you all in advance.

import java.awt.Color;

import java.awt.Graphics;

import javax.swing.JFrame;

import javax.swing.JPanel;

import javax.swing.SwingUtilities;

public class JavaGraph {

JPanel myPanel;

public JPanel createPanel()
{

    myPanel=new JPanel();
    myPanel.setLayout(null);
    //myPanel.setBackground(Color.black);
    return myPanel;

}
public static void  display()
{
    JFrame frame=new JFrame();
    JavaGraph j=new JavaGraph();
    frame.add(j.createPanel());
    frame.setVisible(true);
    frame.setSize(400,400);
    }

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

}

回答1:

you would

override getPreferredSize for JPanel, otherwise JPanel with Java2D only to returns zero Dimension
override paintComponent(), more see in Oracle tutorial
read Oracle tutorial Inintial Thread

you wouldn't

myPanel.setLayout(null); use Null Layout
frame.setSize(400,400); for JFrame, JPanel etc, because JComponents (override getPreferredSize) are designated to returns proper Dimension to its contianer, JFrame.pack(before JFrame.setVisible) to calculate, determine proper size for container(s) in pixels
invoke any code to set, change or intialize Swing GUI, after frame.setVisible(true); is called

来源：https://stackoverflow.com/questions/19251941/how-to-paint-inside-jpanel

标签

java

swing

jpanel

paint