We Keep Coding

How to make a translucent JPanel within the region Jpanel

I want to make a translucent JPanel for which I can choose the x-coordinate, y-coordinate, width and height. I have found some material that made a translucent JPanel that filled its parent container, but I don't need this effect. I need a translucent JPanel whose position can be specified in the parent container, such that only this area is translucent, while others in the parent container are opaque. I tried this code but it is incorrect. The resultant area is smaller than the area I intend. What is wrong, or how I can get this effect?
public class TranslucentJPanel extends JPanel {
private float transparency;
public TranslucentJPanel(){
}
/**set the transparency
*
* #param transparency:the transparency you want to set
*
* #return void
*/
public void setTransparent(float transparency) {
this.transparency = transparency;
}
#Override
protected void paintComponent(Graphics g){
super.paintComponent(g);
Graphics2D graphics2d = (Graphics2D) g.create();
graphics2d.setComposite(AlphaComposite.SrcOver.derive(transparency));
graphics2d.fill(getBounds());
graphics2d.dispose();
}
}

So, two things come to mind immediately...
First, don't use getBounds, the Grapghics context is already translated to the components x/y position, so you're doubling that up. Instead, simply use 0x0 and provide the width and height, something like...
graphics2d.fillRect(0, 0, getWidth(), getHeight());
Also, remember, most Swing components are opaque by default, this includes the JList, JScrollPane and it's JViewport, you need to set these to be transparent if you want them to be see through at all.
Also, the "default" renderer that the JList uses also renderers it self as opaque, so you need to be able to supply your own if you want the items to be transparent...
import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.DefaultListCellRenderer;
import javax.swing.DefaultListModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.EmptyBorder;
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();
}
DefaultListModel<String> model = new DefaultListModel<>();
model.addElement("Bananas");
model.addElement("Apples");
model.addElement("Pears");
model.addElement("Grapes");
model.addElement("Tim Tams");
JList list = new JList(model);
list.setCellRenderer(new DefaultListCellRenderer() {
#Override
public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
setOpaque(isSelected);
return this;
}
});
list.setOpaque(false);
JScrollPane sp = new JScrollPane(list);
sp.setOpaque(false);
sp.getViewport().setOpaque(false);
TranslucentPane tp = new TranslucentPane();
tp.setLayout(new GridBagLayout());
tp.setLayout(new BorderLayout());
tp.add(sp);
JFrame frame = new JFrame("Testing");
frame.setContentPane(new BackgroundPane());
frame.setLayout(new GridBagLayout());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(tp);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TranslucentPane extends JPanel {
private float alpha = 0.75f;
public TranslucentPane() {
setOpaque(false);
setBorder(new EmptyBorder(5, 5, 5, 5));
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
g2d.setComposite(AlphaComposite.SrcOver.derive(alpha));
g2d.setColor(getBackground());
g2d.fillRect(0, 0, getWidth(), getHeight());
g2d.dispose();
}
}
public class BackgroundPane extends JPanel {
private BufferedImage background;
public BackgroundPane() {
try {
background = ImageIO.read(new File("C:\\Users\\shane\\Dropbox\\MegaTokyo\\thumnails\\megatokyo_omnibus_1_3_cover_by_fredrin-d4oupef.jpg"));
} catch (IOException ex) {
ex.printStackTrace();
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(background.getWidth(), background.getHeight());
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
int x = (getWidth() - background.getWidth()) / 2;
int y = (getHeight() - background.getHeight()) / 2;
g.drawImage(background, x, y, this);
}
}
}

Rectangle is not drawn on top

I have a class "Map" which extends JPanel. I add it to a class that extends a JFrame.
public void paintComponent(Graphics g) {
super.paintComponent(g);
int width = Math.abs(startX - endX);
int height= Math.abs(startY - endY);
g.setColor(Color.RED);
g.fillRect(startX, startY, width, height);
}
My class "Map" also contains a label with an image in it. If the image is smalled than the window, when I draw a rectangle it is seen.
In short, it is under the label.

paintComponent is the "bottom" of the paint chain, so anything painted here will appear below everything else.
A better solution might be to add the Map panel to the label (setting the JLabel's layout manager appropriately).
Or, create a "base" panel, set it's layout manager to use a OverlayLayout manager and add the JLabel and Map panel to it.
This will, of course, all depend on what it is you want to achieve...
Updated with "Panel on Label" example
Basically, this takes a JLabel, sets an icon (as the background image), set it's layout as BorderLayout and then adds a JPanel on to it.
Remember, JPanel is opaque by default, so you need to make it transparent ;)
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class OverlayLabel {
public static void main(String[] args) {
new OverlayLabel();
}
public OverlayLabel() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JLabel background = new JLabel(new ImageIcon(ImageIO.read(new File("/path/to/image"))));
background.setLayout(new BorderLayout());
background.add(new TestPane());
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(background);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
} catch (IOException ex) {
ex.printStackTrace();
}
}
});
}
public class TestPane extends JPanel {
public TestPane() {
setOpaque(false);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics g2d = (Graphics2D) g.create();
int x = (getWidth() - 20) / 2;
int y = (getHeight() - 20) / 2;
g2d.setColor(Color.RED);
g2d.fillRect(x, y, 20, 20);
g2d.dispose();
}
}
}
Updated with example of OverlayLayout
OverlayLayout basically uses the components x/y alignment to make determinations about how best it should place the individual components
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.OverlayLayout;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.LineBorder;
public class OverlayLabel {
public static void main(String[] args) {
new OverlayLabel();
}
public OverlayLabel() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JLabel background = new JLabel(new ImageIcon(ImageIO.read(new File("/path/to/image"))));
background.setAlignmentX(0.5f);
background.setAlignmentY(0.5f);
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new OverlayLayout(frame.getContentPane()));
frame.add(new TestPane());
frame.add(background);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
} catch (IOException ex) {
ex.printStackTrace();
}
}
});
}
public class TestPane extends JPanel {
public TestPane() {
setOpaque(false);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics g2d = (Graphics2D) g.create();
int x = (getWidth() - 20) / 2;
int y = (getHeight() - 20) / 2;
g2d.setColor(Color.RED);
g2d.fillRect(x, y, 20, 20);
g2d.dispose();
}
}
}
And finally, if none of that is working for you, you could use JLayeredPane as the base, which will allow you to determine the z-order of each component...
See How to use layered panes for more details...

How to use AffineTransform.quadrantRotate to rotate a bitmap?

I want to rotate a bitmap about its center point, and then draw it into a larger graphics context.
The bitmap is 40x40 pixels. The graphics context is 500x500 pixels. This is what I'm doing:
BufferedImage bi = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB);
Graphics2D g = bi.createGraphics();
AffineTransform at = new AffineTransform();
at.quadrantRotate(1, -20, -20); // rotate 90 degrees around center point.
at.translate(100, 40); // I want to put its top-left corner at 100,40.
g.drawImage(smallerBitmap, at, null);
...
I'm probably using quadrantRotate() incorrectly - if I remove that line, my image gets drawn at position 100,40 correctly at least.
What am I doing wrong?
Thanks

The order of your transformation matters. Basically your example code is saying "rotate the image by 90 degrees AND then translate it...."
So, using your code (rotate, then translate) produces...
Switching the order (translate then rotate) produces
...
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
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.JSlider;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
public class TestRotation100 {
public static void main(String[] args) {
new TestRotation100();
}
public TestRotation100() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
final TestPane testPane = new TestPane();
final JSlider slider = new JSlider(0, 3);
slider.addChangeListener(new ChangeListener() {
#Override
public void stateChanged(ChangeEvent e) {
testPane.setQuad(slider.getValue());
}
});
slider.setValue(0);
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(testPane);
frame.add(slider, BorderLayout.SOUTH);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
private BufferedImage img;
private int quad = 0;
public TestPane() {
try {
img = ImageIO.read(new File("/Users/swhitehead/Dropbox/MegaTokyo/Rampage_Small.png"));
} catch (IOException ex) {
ex.printStackTrace();
}
}
public void setQuad(int quad) {
this.quad = quad;
repaint();
}
#Override
public Dimension getPreferredSize() {
return new Dimension(400, 400);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
AffineTransform at = new AffineTransform();
at.translate(100, 40);
at.quadrantRotate(quad, img.getWidth() / 2, img.getHeight() / 2);
g2d.drawImage(img, at, this);
g2d.dispose();
}
}
}

Java Swing: Transparent PNG permanently captures original background

I have the following code:
import javax.swing.JWindow;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
public class sutff extends JWindow
{
//Get transparent image that will be use as splash screen image.
Image bi=Toolkit.getDefaultToolkit().getImage("window.png");
ImageIcon ii=new ImageIcon(bi);
public sutff()
{
try
{
setSize(ii.getIconWidth(),ii.getIconHeight());
setLocationRelativeTo(null);
show();
//Thread.sleep(10000);
//dispose();
//JOptionPane.showMessageDialog(null,"This program will exit !!!","<>",JOptionPane.INFORMATION_MESSAGE);
}
catch(Exception exception)
{
exception.printStackTrace();
}
}
//Paint transparent image onto JWindow
public void paint(Graphics g)
{
g.drawImage(bi,0,0,this);
}
public static void main(String[]args)
{
sutff tss=new sutff();
}
}
The purpose is to create a window that is translucent and resembles Windows Aero-style glass. I have the following transparent png that I am using:
http://i.imgur.com/5UNGbsr.png
The problem is that since its transparent, its suppose to show the things behind the window, right? That's what it does when first executed, except whatever window is behind this "transparent window" when it first starts up, the program somehow creates an "image" of that and permanently attaches it with the window. So even if I minimize the windows behind this "transparent window," the image of the first background window remains.
Here is a screenshot:
When I took this screen shot, I had already minimized the command prompt and the IDE which can be seen in the background, yet it still remains in the background of the window.
What am I doing wrong?

Don't override the paint() method of a top level container, especially when you don't invoke super.paint(). This will cause painting problems. If you ever do need to do custom painting then you should override the paintComponent() method of JPanel (or JComponent) and then add the panel to the window/frame. Read the Swing tutorial on Custom Painting. This advice is given daily, I don't know why people still try to override paint()???
However this is only one of your problems. The better solution is to add your image to a JLabel and then add the label to the window. You will also need to make the window background transparent:
import javax.swing.*;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;
import java.awt.*;
import java.awt.Image;
import java.awt.Toolkit;
public class Stuff extends JWindow
{
//Get transparent image that will be use as splash screen image.
Image bi=Toolkit.getDefaultToolkit().getImage("transparent.png");
ImageIcon ii=new ImageIcon(bi);
public Stuff()
{
try
{
setBackground( new Color(0, 0, 0, 0) );
setSize(ii.getIconWidth(),ii.getIconHeight());
setLocationRelativeTo(null);
JLabel label = new JLabel(ii);
add(label);
show();
//Thread.sleep(10000);
//dispose();
//JOptionPane.showMessageDialog(null,"This program will exit !!!","<>",JOptionPane.INFORMATION_MESSAGE);
}
catch(Exception exception)
{
exception.printStackTrace();
}
}
/*
//Paint transparent image onto JWindow
public void paint(Graphics g)
{
super.paint(g);
g.drawImage(bi,0,0,this);
}
*/
public static void main(String[]args)
{
Stuff tss=new Stuff();
}
}

The problem is, you window is actually transparent. Java still thinks that the Window opaque and therefore won't update the graphics in such away as to show what's actually behind.
Creating a transparent window is relatively simple in Java since Java 1.6.10 (I think)
The following is a very simple example, using a semi transparent paint effect that will allow what ever falls below the window to continue to be painted correctly.
import com.sun.awt.AWTUtilities;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Shape;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.RoundRectangle2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class TransaprentBlur {
public static void main(String[] args) {
new TransaprentBlur();
}
public TransaprentBlur() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
}
JFrame frame = new JFrame("Testing");
frame.setUndecorated(true);
frame.setBackground(new Color(0, 0, 0, 0));
// Java 6...
// AWTUtilities.setWindowOpaque(frame, true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
frame.add(new TestPane());
frame.setSize(400, 400);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
public TestPane() {
setOpaque(false);
addMouseListener(new MouseAdapter() {
#Override
public void mouseClicked(MouseEvent e) {
if (e.getClickCount() == 2) {
System.exit(0);
}
}
});
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Shape shape = new RoundRectangle2D.Float(0, 0, getWidth() - 1, getHeight() - 1, 20, 20);
Graphics2D g2d = (Graphics2D) g.create();
g2d.setColor(new Color(225, 225, 225, 128));
g2d.fill(shape);
g2d.setColor(Color.GRAY);
g2d.draw(shape);
g2d.dispose();
}
}
}
Update with image example
Screen shoots showing windows been moved behind the window...
Basically, all you need to do, is place you image rendering code with in the paintComponent method of TestPane
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
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 TransaprentBlur {
public static void main(String[] args) {
new TransaprentBlur();
}
public TransaprentBlur() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
}
JFrame frame = new JFrame("Testing");
frame.setUndecorated(true);
frame.setBackground(new Color(0, 0, 0, 0));
// Java 6...
// AWTUtilities.setWindowOpaque(frame, true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
frame.add(new TestPane());
frame.setSize(400, 400);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class TestPane extends JPanel {
private BufferedImage image;
public TestPane() {
try {
image = ImageIO.read(getClass().getResource("/5UNGbsr.png"));
} catch (IOException ex) {
}
setOpaque(false);
addMouseListener(new MouseAdapter() {
#Override
public void mouseClicked(MouseEvent e) {
if (e.getClickCount() == 2) {
System.exit(0);
}
}
});
}
#Override
public Dimension getPreferredSize() {
return image == null ? super.getPreferredSize() : new Dimension(image.getWidth(), image.getHeight());
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (image != null) {
Graphics2D g2d = (Graphics2D) g.create();
int x = (getWidth() - image.getWidth()) / 2;
int y = (getHeight() - image.getHeight()) / 2;
g2d.drawImage(image, x, y, this);
g2d.dispose();
}
}
}
}

paintComponent draws other components on top of my drawing

I'm trying to build a simple paint tool. The mouseDrag events creates a new ellipse and causes my JPanel to repaint().
This works fine so far.
However, if I press any button (or any other UI component) before firing the mouseDrag event for the first time, the button is painted in the upper left corner of my panel.
I have isolated the code into this test application:
import java.awt.BasicStroke;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Test extends JFrame
{
public Test()
{
final JPanel paintPanel = new JPanel(){
#Override
protected void paintComponent(Graphics g)
{
Graphics2D g2d = (Graphics2D)g;
g2d.setPaintMode();
g2d.setStroke(new BasicStroke(1));
g2d.fillRect(100, 100, 10, 10);
}
};
paintPanel.setPreferredSize(new Dimension(300,300));
paintPanel.addMouseListener(new MouseAdapter() {
#Override
public void mouseClicked(MouseEvent e)
{
paintPanel.repaint();
}
});
this.setLayout(new FlowLayout());
this.add(paintPanel);
this.add(new JButton("Dummy"));
this.pack();
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);
}
public static void main(String... args)
{
new Test();
}
}
A Screenshot for "seeing" the problem in my Main application

+1 to #MadProgrammer's answers.
You should have super.paintComponent(..) as the first call in your overriden paintComponent()
Do not extend JFrame unnecessarily
Create and minipulate Swing components via EDT
Dont call setPrefferedSize() rather override getPrefferedSize()
Here is an example which incorporates my advice's and #MadProgrammer's:
import java.awt.BasicStroke;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class Test {
JFrame frame;
public Test() {
frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
final PaintPanel paintPanel = new PaintPanel();
paintPanel.addMouseListener(new MouseAdapter() {
#Override
public void mouseClicked(MouseEvent e) {
paintPanel.addRect(e.getX(), e.getY());
}
});
frame.setLayout(new FlowLayout());
frame.add(paintPanel);
frame.add(new JButton("Dummy"));
frame.pack();
frame.setVisible(true);
}
public static void main(String... args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new Test();
}
});
}
}
class PaintPanel extends JPanel {
public PaintPanel() {
addRect(100, 100);
}
ArrayList<Rectangle> rects = new ArrayList<>();
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.setPaintMode();
for (Rectangle r : rects) {
g2d.setStroke(new BasicStroke(1));
g2d.fillRect(r.x, r.y, r.width, r.height);
}
}
public void addRect(int x, int y) {
rects.add(new Rectangle(x, y, 10, 10));
repaint();
}
#Override
public Dimension getPreferredSize() {
return new Dimension(300, 300);
}
}

You're not calling super.paintComponent.
The graphics context used for a paint cycle is shared between all the components begin painted, this means if you don't take care to clear it before painting onto, you will end up with what ever was painted before you.
One of the jobs of paintComponent is to prepare the graphics for painting