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JButton border set with setBackground()?

So I have some code to change the background color of a button, but when I use the code, it sets the background color and border color.
Is there a way to not make this happen?
Thanks!
Code:
public void highlight(ArrayList<JButton> buttons){
for (JButton j : buttons) {
j.setBorder(new JButton().getBorder());
j.setBackground(Color.GREEN);
j.setForeground(Color.WHITE);
j.setOpaque(true);
j.setBorderPainted(false);
j.setFocusPainted(false);
j.setBorderPainted(false);
}
}

Okay, so this is a hacked version and is based on the idea that you want to maintain the current "look and feel" of the button, but want to use a different fill color
This simply applies a "highlight" color over the button...
import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsConfiguration;
import java.awt.GraphicsEnvironment;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.RenderingHints;
import java.awt.Transparency;
import java.awt.image.BufferedImage;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import org.kaizen.core.ui.ImageUtilities;
public class Main {
public static void main(String[] args) {
new Main();
}
public Main() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
public TestPane() {
setLayout(new GridBagLayout());
HighlightButton btn = new HighlightButton("Help");
btn.setMargin(new Insets(20, 20, 20, 20));
btn.setHighlight(new Color(255, 0, 0, 64));
add(btn);
btn = new HighlightButton("Help");
btn.setMargin(new Insets(20, 20, 20, 20));
btn.setHighlight(new Color(0, 255, 0, 64));
add(btn);
btn = new HighlightButton("Help");
btn.setMargin(new Insets(20, 20, 20, 20));
btn.setHighlight(new Color(0, 0, 255, 64));
add(btn);
btn = new HighlightButton("Help");
btn.setMargin(new Insets(20, 20, 20, 20));
add(btn);
}
}
public class HighlightButton extends JButton {
private Color highlight;
public HighlightButton() {
setOpaque(false);
}
public HighlightButton(String text) {
super(text);
setOpaque(false);
}
public void setHighlight(Color color) {
if (color != highlight) {
Color old = highlight;
this.highlight = color;
firePropertyChange("highlight", old, highlight);
repaint();
}
}
public Color getHighlight() {
return highlight;
}
#Override
protected void paintComponent(Graphics g) {
Color highlight = getHighlight();
if (highlight != null) {
BufferedImage img = createCompatibleImage(getWidth(), getHeight(), Transparency.TRANSLUCENT);
Graphics2D g2d = img.createGraphics();
super.paintComponent(g2d);
g2d.dispose();
BufferedImage mask = generateMask(img, getHighlight(), 1f);
g.drawImage(img, 0, 0, this);
g.drawImage(mask, 0, 0, this);
} else {
super.paintComponent(g);
}
}
}
public static BufferedImage createCompatibleImage(int width, int height, int transparency) {
BufferedImage image = getGraphicsConfiguration().createCompatibleImage(width, height, transparency);
image.coerceData(true);
return image;
}
public static GraphicsConfiguration getGraphicsConfiguration() {
return GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDefaultConfiguration();
}
public static BufferedImage generateMask(BufferedImage imgSource, Color color, float alpha) {
int imgWidth = imgSource.getWidth();
int imgHeight = imgSource.getHeight();
BufferedImage imgMask = createCompatibleImage(imgWidth, imgHeight, Transparency.TRANSLUCENT);
Graphics2D g2 = imgMask.createGraphics();
applyQualityRenderingHints(g2);
g2.drawImage(imgSource, 0, 0, null);
g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_IN, alpha));
g2.setColor(color);
g2.fillRect(0, 0, imgSource.getWidth(), imgSource.getHeight());
g2.dispose();
return imgMask;
}
public static void applyQualityRenderingHints(Graphics2D g2d) {
g2d.setRenderingHint(RenderingHints.KEY_ALPHA_INTERPOLATION, RenderingHints.VALUE_ALPHA_INTERPOLATION_QUALITY);
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2d.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
g2d.setRenderingHint(RenderingHints.KEY_DITHERING, RenderingHints.VALUE_DITHER_ENABLE);
g2d.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
g2d.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE);
}
}
Basically, what this does, is paints a "masked" image, colored in the highlighted color OVER the top of the button. This is important to remember. The higher the alpha value of the color becomes, the less likely you are to see the text.
I've not tested this on windows, so I can't guarantee the results.
The content filling is performed by the look and feel delegate and generally ignores the color properties of the class (yeah, I know, awesome), so if you want to try and do something which was a little more robust, you'd need to define your own look and feel delegate and take over the painting process, no simple task.
Of course, you could just dispense with the content filling and borders used by the look and feel delegate and paint your own (overriding the paintComponent method), but this will not take advantage of the look and feel settings, so that's a balancing act you need to decide on

Setting the origin on a canvas

I am trying to create a simple graphing program, and I want/need the origin to be at the bottom left corner, so I am using the following custom canvas:
public class GraphingCanvas extends Canvas {
public GraphingCanvas() {
}
public void paint(Graphics g) {
((Graphics2D) g).translate(this.getWidth(), this.getHeight());
g.translate(10, 10);
g.setColor(Color.BLACK);
g.drawLine(0, 0, 10, 10);
}
}
However, when I use this canvas, like so:
private void initialize() {
frame = new JFrame();
frame.setBounds(100, 100, 450, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().setLayout(null);
GraphingCanvas canvas = new GraphingCanvas();
canvas.setBackground(Color.WHITE);
canvas.setBounds(10, 10, 414, 241);
frame.getContentPane().add(canvas);
}
The origin appears to remain at the default. Am I doing something wrong?

This may or may not do what you want, but basically I used scale(1, -1) to flip the orientation through the y-axis (and translated the context)
The magic basically happens in the paintComponent method using...
g2d.scale(1, -1);
g2d.translate(0, -getHeight());
Runnable example...
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Test {
public static void main(String[] args) {
new Test();
}
public Test() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
private Rectangle box = new Rectangle(10, 10, 20, 20);
public TestPane() {
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
draw(g2d, Color.RED, Color.MAGENTA);
g2d.dispose();
g2d = (Graphics2D) g.create();
g2d.scale(1, -1);
g2d.translate(0, -getHeight());
draw(g2d, Color.BLUE, Color.GREEN);
g2d.dispose();
}
protected void draw(Graphics2D g2d, Color boxColor, Color lineColor) {
g2d.setColor(boxColor);
g2d.fill(box);
g2d.setColor(lineColor);
g2d.drawLine(0, 0, getWidth(), getHeight());
}
}
}
Just to be sure, I created a simple method...
protected void draw(Graphics2D g2d, Color boxColor, Color lineColor) {
g2d.setColor(boxColor);
g2d.fill(box);
g2d.setColor(lineColor);
g2d.drawLine(0, 0, getWidth(), getHeight());
}
which is called with the "normal" orientation and then called again with the transformed orientation, so it's the same code been used to paint the output, the only thing that's changed is the orientation

Creating a JLabel with a Gradient

I'm new to Java and I'm trying to create a heading using a JLabel and for its fill to be a gradient. I cannot get it to work and I've been trying for a while. I've been grabbing bits of come from here and other websites and cannot seem to make this work, nor make sense of other peoples more complex code that does work. Here are my two classes so far:
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.BorderFactory;
import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EtchedBorder;
public class Test {
public static void main(String[] args) {
new Test().setupGUI();
}
public void setupGUI() {
//set up frames and buttons etc.
JFrame theFrame = new JFrame ("Crystal Ball");
theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel background = new JPanel();
background.setBackground(Color.BLUE);
background.setLayout(new BoxLayout(background, BoxLayout.PAGE_AXIS));
theFrame.setSize(500,1000);
DLabel heading = new DLabel("Guess a Number");
heading.setText("GUESS A NUMBER");
heading.setPreferredSize(new Dimension(theFrame.getWidth(),100));
heading.setFont(new Font("Serif", Font.PLAIN, 40));
heading.setAlignmentX(Component.CENTER_ALIGNMENT);
//heading.setBackground(Color.YELLOW);
heading.setBorder(BorderFactory.createEtchedBorder(EtchedBorder.RAISED));
background.add(heading);
theFrame.getContentPane().add(background);
theFrame.pack();
theFrame.setVisible(true);
//startGame();
}
}
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.BorderFactory;
import javax.swing.JLabel;
public class DLabel extends JLabel
{
Dimension size = new Dimension(70, 80);
public DLabel(String name)
{
this.setPreferredSize(size);
this.setText(name);
this.setBorder(BorderFactory.createBevelBorder(TOP, Color.white, Color.black));
this.setOpaque(true);
}
public void paintComponent(Graphics g) {
// super.paintComponent(g); // *** commented
Graphics2D g2d = (Graphics2D) g;
Color color1 = Color.YELLOW;
Color color2 = color1.brighter();
int w = getWidth();
int h = getHeight();
GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
g2d.setPaint(gp);
g2d.fillRect(0, 0, w, h);
super.paintComponent(g); // *** added
}
}

There is one little "trick" you can actually do, by leaving the label transparent, you can actually paint under the text by painting BEFORE you call super.paintComponent, for example...
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.LinearGradientPaint;
import java.awt.Point;
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class TestLabel101 {
public static void main(String[] args) {
new TestLabel101();
}
public TestLabel101() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JLabel {
public TestPane() {
setText("Happy, Happy");
setForeground(Color.WHITE);
setHorizontalAlignment(CENTER);
}
#Override
protected void paintComponent(Graphics g) {
Graphics2D g2d = (Graphics2D) g.create();
LinearGradientPaint lgp = new LinearGradientPaint(
new Point(0, 0),
new Point(0, getHeight()),
new float[]{0.142f, 0.284f, 0.426f, 0.568f, 0.71f, 0.852f, 1f},
new Color[]{Color.PINK, Color.MAGENTA, Color.BLUE, Color.GREEN, Color.YELLOW, Color.ORANGE, Color.RED});
g2d.setPaint(lgp);
g2d.fill(new Rectangle(0, 0, getWidth(), getHeight()));
g2d.dispose();
super.paintComponent(g);
}
}
}
nb: I should point out that this process is inefficient, as the RepaintManager will still want to paint under the component
There is another trick, but my two year old daughter wants to check to see if Santa is here ;)
Updated
The other trick involves understanding how the paint process actually works. When you call super.paintComponent, it calls the update method on the ComponentUI (look and feel delegate), this is actually the method that fills the background if the component is opaque, this method then calls the look and feels delegate's paint method, which actually does the base painting...
We can circumvent the process slightly and instead of calling super.paintComponent, we can call the look and feels delegate's paint method directly...
public class TestPane extends JLabel {
public TestPane() {
setText("Happy, Happy");
setForeground(Color.WHITE);
setHorizontalAlignment(CENTER);
setOpaque(true);
}
#Override
protected void paintComponent(Graphics g) {
Graphics2D g2d = (Graphics2D) g.create();
LinearGradientPaint lgp = new LinearGradientPaint(
new Point(0, 0),
new Point(0, getHeight()),
new float[]{0.142f, 0.284f, 0.426f, 0.568f, 0.71f, 0.852f, 1f},
new Color[]{Color.PINK, Color.MAGENTA, Color.BLUE, Color.GREEN, Color.YELLOW, Color.ORANGE, Color.RED});
g2d.setPaint(lgp);
g2d.fill(new Rectangle(0, 0, getWidth(), getHeight()));
g2d.dispose();
getUI().paint(g, this);
}
}
This is more efficient then the previous example, as it doesn't require the RepaintManager to paint the area underneath this component, but it might not work with all look and feels

The problem is that you made the JLabel opaque, which means it will paint the background. So the call to super.paintComponent(...) over paints the gradient background. So get rid of:
//this.setOpaque(true);
Another problem with the code is that color1.brighter() doesn't work. Try something like:
Color color2 = Color.RED;
Also, You should not change the Graphics object, since that object is also used to paint other Swing components. Instead you should use g.create() to get a copy of the Graphics object. Make your changes to that object, do the painting and then dispose() the object.
So the (untested) code would be something like:
Graphics2D g2d = (Graphics2D)g.create();
...
g2d.fillRect(...);
g2d.dispose();
super.paintCompoenent(g);

You should set your gradientPaint object to your Graphics and then pass that into super.paintComponent
try this:
public void paintComponent(Graphics g) {
Graphics2D g2d = (Graphics2D) g;
Color color1 = Color.YELLOW;
Color color2 = color1.brighter();
int w = getWidth();
int h = getHeight();
GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
g2d.setPaint(gp);
super.paintComponent(g2d);
}

How to disable java.awt.Graphics.fillRect(int x, int y, int width, int height)'s effect?

It's the original image:
I use java.awt.Graphics.fillRect(int x, int y, int width, int height) to add a coloured rectangle on the image.
Graphics imageGraphics = image.createGraphics();
Color color = new Color(0,0,0,100);
imageGraphics.setColor(color);
imageGraphics.fillRect(0, 0, 800, 600);
So the image has been inverted and looks like this:
After that,I want to clear the black transparent rectangle partly and show the original image.
imageGraphics.clearRect(100,100,100,100);
But the effect is like this:
What my requirement is:
I want to know why it doesn't work and is there any other way to realize it?

Remember, painting is destructive. It might be possible to use AlphaComposite to achieve this result, but a simpler solution might be to simple constructive a compound shape and paint that instead.
The following example creates two Rectangles, one been the area we want to fill and one been the area we want to show, the second is then subtracted from the first (to create the window) and then the result is painted on top of the image
import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Area;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class ShowArea {
public static void main(String[] args) {
new ShowArea();
}
public ShowArea() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
private BufferedImage img;
public TestPane() {
try {
img = ImageIO.read(new File("sample.png"));
Rectangle bounds = new Rectangle(0, 0, img.getWidth(), img.getHeight());
Rectangle clip = new Rectangle(150, 10, 100, 100);
Area area = new Area(bounds);
area.subtract(new Area(clip));
Graphics2D g2d = img.createGraphics();
g2d.setColor(Color.BLACK);
g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
g2d.fill(area);
g2d.dispose();
} catch (IOException ex) {
ex.printStackTrace();
}
}
#Override
public Dimension getPreferredSize() {
return img == null ? new Dimension(200, 200) : new Dimension(img.getWidth(), img.getHeight());
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
int x = (getWidth() - img.getWidth()) / 2;
int y = (getHeight() - img.getHeight()) / 2;
g2d.drawImage(img, x, y, this);
g2d.dispose();
}
}
}
If I were doing some kind of paint program, I would do this all within the paintComponent method or in some way that it didn't effect the original image, otherwise you've basic destroyed the image until you re-load it
Another solution might be to take a copy of the original area you want to keep and repaint it back on top after, for example...
img = ImageIO.read(new File("sample.png"));
// This is the portion of the image we want to save...
BufferedImage cutout = img.getSubimage(150, 10, 100, 100);
// This is the area we want to paint over...
Rectangle bounds = new Rectangle(0, 0, img.getWidth(), img.getHeight());
Graphics2D g2d = img.createGraphics();
g2d.setColor(Color.BLACK);
// Save the current Composite so we can reset it...
Composite comp = g2d.getComposite();
// Apply the composite and fill the area...
g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
g2d.fill(area);
// Reset the composite
g2d.setComposite(comp);
// Draw the part of the image we saved previously...
g2d.drawImage(cutout, 150, 10, this);
g2d.dispose();

You cant do that the way you're trying to.
It can be done by creating a BufferedImage filled with Color(0,0,0,200), then in that image draw rectangle with color Color(0,0,0,200) and then apply it on image.
Remember, that drawing filledRectange is not a "undo operation"- it is written on pixels and it can't be undone.
EDIT
Icon imageIcon = new javax.swing.ImageIcon("image.jpg");
BufferedImage mask = new BufferedImage(imageIcon.getIconWidth(), imageIcon.getIconHeight(), BufferedImage.TYPE_4BYTE_ABGR);
BufferedImage image = new BufferedImage(imageIcon.getIconWidth(), imageIcon.getIconHeight(), BufferedImage.TYPE_4BYTE_ABGR);
imageIcon.paintIcon(null, image.getGraphics(), 0, 0);
Graphics maskGraphics = mask.getGraphics();
//drawing grey background
maskGraphics.setColor(new Color(0, 0, 0, 120));
maskGraphics.fillRect(0, 0, mask.getWidth(), mask.getHeight());
//drawing black frame
maskGraphics.setColor(new Color(0, 0, 0, 255));
maskGraphics.drawRect(99, 99, 301, 301);
//drawing original image window
maskGraphics.drawImage(image, 100, 100, 400, 400, 100, 100, 400, 400, null);
//apply mask on image
new ImageIcon(mask).paintIcon(null, image.getGraphics(), 0, 0);
//result presentation
label.setIcon(new ImageIcon(image));

Yes this is easily doable. The problem is that any draw operation is destructive, what you see is what you get, the information that was painted upon is lost.
Something like this where you store a subimage, do your paint operation then draw the subimage back on top.
public class Q23709070 {
public static void main(String[] args) {
JFrame frame = new JFrame();
Panel p = new Panel();
frame.getContentPane().add(p);
frame.pack();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
static class Panel extends JPanel {
int w = 400;
int h = 400;
int x = 100;
int y = 100;
BufferedImage img;
BufferedImage subImg;
BufferedImage save;
public Panel() {
try {
img = ImageIO.read(getClass().getResourceAsStream("0qzCf.jpg"));
} catch (IOException e) {
}
subImg = img.getSubimage(x, y, w, h);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
Color g2dColor = g2d.getColor();
Color fillColor = new Color(0, 0, 0, 100);
g2d.drawImage(img, 0, 0, null);
g2d.setColor(fillColor);
g2d.fillRect(0, 0, img.getWidth(), img.getHeight());
g2d.drawImage(subImg, x, y, null);
g2d.setColor(g2dColor);
if (save == null) {
save = new BufferedImage(img.getWidth(), img.getHeight(),
img.getType());
this.paint(save.getGraphics());
try {
ImageIO.write(save, "jpg", new File("save.jpg"));
} catch (IOException e) {
}
}
}
#Override
#Transient
public Dimension getPreferredSize() {
return new Dimension(img.getWidth(), img.getHeight());
}
}
}
Rendering

JPanel gradient background

I googled but could find no correct answer. I have a JPanel and I want it to have a gradient that comes from top to bottom. I'm just going to use two colors. How can I achieve this?

Here you go:
import java.awt.Color;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class TestPanel extends JPanel {
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
int w = getWidth(), h = getHeight();
Color color1 = Color.RED;
Color color2 = Color.GREEN;
GradientPaint gp = new GradientPaint(0, 0, color1, w, h, color2);
g2d.setPaint(gp);
g2d.fillRect(0, 0, w, h);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
JFrame frame = new JFrame();
TestPanel panel = new TestPanel();
frame.add(panel);
frame.setSize(200, 200);
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
});
}
}

You can use this :
JPanel contentPane = new JPanel() {
#Override
protected void paintComponent(Graphics grphcs) {
super.paintComponent(grphcs);
Graphics2D g2d = (Graphics2D) grphcs;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
GradientPaint gp = new GradientPaint(0, 0,
getBackground().brighter().brighter(), 0, getHeight(),
getBackground().darker().darker());
g2d.setPaint(gp);
g2d.fillRect(0, 0, getWidth(), getHeight());
}
};
hope that help;
you can also back to this artical for more help:
Gradient background to any jcomponent