how to set the size of JPanel in java

I have a JPanel in my java code and I want to set its size, I have used JPanel.setSize(500,500); and JPanel.setSize(new Dimension(500,500)); but both are not working. Please tell how I can set the size of JPanel?

Most Swing layout managers respect a component's preferredSize and not its size. You could call setPreferredSize(new Dimension(500, 500)), but this can be overridden later in your code, and can lead to an inflexible GUI. Better to override the JPanel's getPreferredSize() method and return a calculated Dimension that works best in all situations.

For example:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import javax.swing.*;

public class PrefSizePanel extends JPanel {

   private static final int PREF_W = 500;
   private static final int PREF_H = PREF_W;

   public Dimension getPreferredSize() {
      // update as per Marco:
      if (super.isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   // just for fun
   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(new GradientPaint(0, 0, Color.red, 20, 20, Color.blue, true));
      g2.fillOval(0, 0, 2 * getWidth(), 2 * getHeight());
   }

   private static void createAndShowGUI() {
      PrefSizePanel paintEg = new PrefSizePanel();

      JFrame frame = new JFrame("PrefSizePanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(paintEg);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

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

Which displays as

来源：https://stackoverflow.com/questions/25229505/how-to-set-the-size-of-jpanel-in-java

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java

swing

jpanel

preferredsize