Develop Reference

How do I connect the dots to get a polygon in JAVA?

I'm making an application for creating convex
polygon.
I imagined it to be so that I first set the vertices of the polygon and then create it.
I was able to make the addition of points (vertices). Now I need help connecting the dots with a line.
This is what it looks like:
It looks like this
And I would like when I click the Draw Polygon button that these points connect and it looks like a polygon, like this:
and it should look like this when I click the button
Here's the code so you can run it yourself:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
import javax.swing.border.EtchedBorder;
public class MyPaint{
public static void main(String[] args){
final PadDraw drawPad = new PadDraw();
JFrame frame = new JFrame("Draw Polygon");
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setBounds(100, 100, 450, 300);
Container contentPane = frame.getContentPane();
((JComponent) contentPane).setBorder(new EmptyBorder(5, 5, 5, 5));
contentPane.setLayout(new BorderLayout(0, 0));
frame.setContentPane(contentPane);
JPanel buttonPanel = new JPanel();
buttonPanel.setBorder(new EtchedBorder(EtchedBorder.LOWERED, null, null));
contentPane.add(buttonPanel, BorderLayout.SOUTH);
JButton buttonDrawPolygon = new JButton("Draw Polygon");
buttonPanel.add(buttonDrawPolygon);
JButton buttonReset = new JButton("Reset");
buttonPanel.add(buttonReset);
contentPane.add(drawPad, BorderLayout.CENTER);
}
}
class PadDraw extends JComponent{
private Image image;
private Graphics2D graphics2D;
private int currentX , currentY , oldX , oldY ;
public PadDraw(){
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
oldX = e.getX();
oldY = e.getY();
}
});
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
currentX = e.getX();
currentY = e.getY();
if(graphics2D != null) {
graphics2D.drawLine(oldX, oldY, currentX, currentY);
repaint();
oldX = currentX;
oldY = currentY;
}
System.out.println(oldX + " " + oldY);
}
});
}
public void paintComponent(Graphics g){
if(image == null){
image = createImage(getSize().width, getSize().height);
graphics2D = (Graphics2D)image.getGraphics();
graphics2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
graphics2D.setStroke(new BasicStroke(5));
}
g.drawImage(image, 0, 0, null);
}
}

Introduction
Oracle has a helpful tutorial, Creating a GUI With Swing. Skip the Learning Swing with the NetBeans IDE section.
Here's your revised GUI
Here's the GUI with four points
Here's the GUI with a polygon
I'm not showing it, but the Reset button clears the drawing area.
Explanation
When I create a Swing GUI, I use the model-view-controller pattern. This pattern allows me to separate my concerns and focus on one part of the Swing application at a time.
A Swing application model consists of one or more plain Java getter/setter classes.
A Swing view consists of a JFrame and one or more JPanels.
Swing controllers are the listeners that are attached to JButtons and drawing JPanels.
Model
For this application, I created a PolygonModel class. The class holds a boolean that tells me whether or not to draw the polygon and a java.util.List of java.awt.Point instances. The Point class holds an X and Y int value.
View
All Swing applications must start with a call to the SwingUtilities invokeLater method. This method ensures that the Swing components are created and executed on the Event Dispatch Thread.
I broke up your main method into a couple of methods. I separate the creation of the JFrame from the creation of the JPanels. This allows me to separate my concerns and focus on one part of the GUI at a time.
The JFrame methods must be called in a specific order. This is the order I use for most of my Swing applications.
I changed your PadDraw class to extend a JPanel. I moved the MouseAdapter code to its own class. Your drawing panel should draw. Period. Nothing else.
The paintComponent method always starts with a call to the super.paintComponent method. This maintains the Swing paint chain and helps to eliminate unwanted drawing artifacts.
The drawing JPanel is cleared before every repaint. Therefore, you have to completely redraw your image each time. That's why we store the List of Point instances in the model.
Controller
I created three controller classes.
The PointListener class extends MouseAdapter. Notice how simple the mousePressed method becomes with an application model.
The two JButtons each have their own ActionListener. Since they are so simple, I made each of them lambdas.
Code
Here's the complete runnable code. I made all the additional classes inner classes so I could make them public and post the code as one block.
import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.border.EtchedBorder;
public class PolygonImage implements Runnable {
public static void main(String[] args) {
SwingUtilities.invokeLater(new PolygonImage());
}
private final PolygonModel model;
private final PadDraw drawPad;
public PolygonImage() {
this.model = new PolygonModel();
this.drawPad = new PadDraw(this, model);
}
#Override
public void run() {
JFrame frame = new JFrame("Draw Polygon");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(drawPad, BorderLayout.CENTER);
frame.add(createButtonPanel(), BorderLayout.SOUTH);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
private JPanel createButtonPanel() {
JPanel buttonPanel = new JPanel();
buttonPanel
.setBorder(new EtchedBorder(EtchedBorder.LOWERED, null, null));
JButton buttonDrawPolygon = new JButton("Draw Polygon");
buttonDrawPolygon.addActionListener(event -> {
model.setConnectPoints(true);
repaint();
});
buttonPanel.add(buttonDrawPolygon);
JButton buttonReset = new JButton("Reset");
buttonReset.addActionListener(event -> {
model.setConnectPoints(false);
model.clearList();
repaint();
});
buttonPanel.add(buttonReset);
return buttonPanel;
}
public void repaint() {
drawPad.repaint();
}
public class PadDraw extends JPanel {
private static final long serialVersionUID = 1L;
private final PolygonModel model;
public PadDraw(PolygonImage view, PolygonModel model) {
this.model = model;
this.addMouseListener(new PointListener(view, model));
this.setPreferredSize(new Dimension(450, 300));
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2d.setColor(Color.black);
// Draw points
for (Point p : model.getPoints()) {
int radius = 6;
int diameter = radius + radius;
g2d.fillOval(p.x - radius, p.y - radius, diameter, diameter);
}
// Draw polygon
if (model.isConnectPoints()) {
g2d.setStroke(new BasicStroke(5));
List<Point> points = model.getPoints();
if (points.size() >= 1) {
Point old = points.get(0);
for (int index = 1; index < points.size(); index++) {
Point p = points.get(index);
g2d.drawLine(old.x, old.y, p.x, p.y);
old = p;
}
Point p = points.get(0);
g2d.drawLine(p.x, p.y, old.x, old.y);
}
}
}
}
public class PointListener extends MouseAdapter {
private final PolygonImage view;
private final PolygonModel model;
public PointListener(PolygonImage view, PolygonModel model) {
this.view = view;
this.model = model;
}
#Override
public void mousePressed(MouseEvent event) {
model.addPoint(event.getPoint());
view.repaint();
}
}
public class PolygonModel {
private boolean connectPoints;
private final List<Point> points;
public PolygonModel() {
this.points = new ArrayList<>();
this.connectPoints = false;
}
public void setConnectPoints(boolean connectPoints) {
this.connectPoints = connectPoints;
}
public boolean isConnectPoints() {
return connectPoints;
}
public void clearList() {
this.points.clear();
}
public void addPoint(Point point) {
this.points.add(point);
}
public List<Point> getPoints() {
return points;
}
}
}

Deleting a line in java when using paint(Graphics p)?

I have drawn a line using the below function:
public void paint(Graphics p) {
super.paint(p);
p.drawLine(600, 200, 580, 250);
}
I am wondering is there a way that I can delete this line?
Then is it possible to call this function in the main() method of a program?

You can use a flag to know whether the line is displaying or not.
As I said before you need to build your GUI towards JPanels and not JFrames. Also overriding paintComponent and not paint method.
For example, the following program displays a line or hides it when you click the JButton, adapt that logic to your own program with your own conditions.
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class LineDrawer {
private JFrame frame;
private JButton button;
public static void main(String[] args) {
SwingUtilities.invokeLater(new LineDrawer()::createAndShowGui); //Put our program on the EDT
}
private void createAndShowGui() {
frame = new JFrame(getClass().getSimpleName());
MyPane pane = new MyPane(); //Create an instance of our custom JPanel class
button = new JButton("Hide/Show");
button.addActionListener(e -> {
pane.setShowLine(!pane.isShowLine()); //Change the state of the flag to its inverse: true -> false / false -> true
});
frame.add(pane);
frame.add(button, BorderLayout.SOUTH);
frame.pack();
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
//Our custom class that handles painting.
#SuppressWarnings("serial")
class MyPane extends JPanel {
private boolean showLine;
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
if (showLine) { //If true, show line
g2d.draw(new Line2D.Double(50, 50, 100, 50));
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(300, 300); //For the size of our JPanel
}
public boolean isShowLine() {
return showLine;
}
public void setShowLine(boolean showLine) {
this.showLine = showLine;
this.repaint(); //Everytime we set a new state to showLine, repaint to make the changes visible
}
}
}
I can't post a GIF right now, but the program itself works. Btw the above code is called a Minimal, Complete and Verifiable Example and on your next questions you're encouraged to post one in order to get specific, faster and better answers to your questions.

Java Swing Scroll through drawing

Im trying to add a JScrollpane to my JPanel. The problem is that the scrollpane doesn't recognize that my drawing is outside the frame. So how do I add the JScrollpane correctly?
Main class:
public MainFrame() extends JFrame{
public MainFrame() {
Container container = getContentPane();
container(new BorderLayout());
container.add(new JScrollPane(new Drawing()));
setSize(1280,720);
setVisible(true);
}
Drawing class:
public class Drawing() extends JPanel {
#Override
protected void paintComponent(Graphics g) {
g.drawLine(10, 100, 30000, 10);
}
}

There are a couple of errors in your code, let's step through each of them:
You're extending JFrame, and you should avoid it, see: Extends JFrame vs. creating it inside the program for more information about it. You're actually not changing its behavior so it's not needed to extend it.
For your JScrollPane to show the whole line, you need to change your window's size to be the same size of your line (as shown in this answer by #MadProgrammer).
Related to point 2, avoid the use of setSize(...) and instead override getPreferredSize(): See Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing? for more information
You forgot to call super.paintComponent(...) method in your paintComponent() method.
Related to points 2, 3, you need to call pack() so Swing calculates the best preferred size for your component.
See this example:
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
public class LongDraw {
private JFrame frame;
private Drawing drawing;
public static void main(String[] args) {
SwingUtilities.invokeLater(new LongDraw()::createAndShowGui);
}
private void createAndShowGui() {
frame = new JFrame(getClass().getSimpleName());
drawing = new Drawing();
JScrollPane scroll = new JScrollPane(drawing);
frame.add(scroll);
frame.pack();
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
class Drawing extends JPanel {
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.drawLine(10, 100, 3000, 10);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(3000, 500);
}
}
}
Which produces something similar to this:

JPanel with JSlider not displaing the Graphics

All i have is a JPanel with a white background and the JSlider on the bottom, not displaying the square, i think i have made some mistake with the JPanel class but i can't figure it out. Just before i made another project with g.fillOval and it worked properly, i checked it out and every line of code seems the same, i am really confused.
public class Main00 {
public static void main(String[] args) {
Gui asd=new Gui();
asd.setVisible(true);
asd.setSize(500,400);
}
}
import java.awt.*;
import javax.swing.*;
import javax.swing.event.*;
public class Gui extends JFrame {
private JSlider slider;
private DrawSquare square;
public Gui() {
super("square modificator");
setDefaultCloseOperation(EXIT_ON_CLOSE);
square = new DrawSquare();
square.setBackground(Color.WHITE);
slider = new JSlider(SwingConstants.HORIZONTAL, 0, 300,
square.getSide());
slider.setMajorTickSpacing(20);
slider.setPaintTicks(true);
add(square);
add(slider, BorderLayout.SOUTH);
slider.addChangeListener(new ChangeListener() {
#Override
public void stateChanged(ChangeEvent e) {
square.setSide(slider.getValue());
}
});
}
}
import java.awt.*;
import javax.swing.*;
public class DrawSquare extends JPanel {
private int side = 10;
public void paintComponents(Graphics g) {
super.paintComponents(g);
g.setColor(Color.RED);
g.fillRect(20, 20, side, side);
}
public void setSide(int side) {
this.side=(side>=0)?side:0;
repaint();
}
public Dimension getPrefferedSize(){
return new Dimension(200,200);
}
public Dimension getMinimumSize(){
return getPrefferedSize();
}
public int getSide(){
return side;
}
}

You're overriding paintComponents rather than the correct paintComponent. These two methods have drastically different effects, and the effect of the second is the one you want.
From the API:
paintComponents API entry: Paints each of the components in this container.
paintComponent API entry: Calls the UI delegate's paint method, if the UI delegate is non-null. We pass the delegate a copy of the Graphics object to protect the rest of the paint code from irrevocable changes
Again, you are interested in painting the component itself via its delegate and not in painting the components held by this component.

Key adapter not working

I have created a KeyAdapter class within my JPanel class, and I tried debugging the click using System.out.println(String par1);
However it doesnt seem to work...
Here is the class:
package net.ryanshah;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.RenderingHints;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import javax.swing.ImageIcon;
import javax.swing.JPanel;
public class Menu extends JPanel
{
private GameFrame gf;
private Image bg = new ImageIcon("res/bg.png").getImage();
public Menu(final GameFrame parent) {
this.gf = parent;
int width = parent.getWidth();
int height = parent.getHeight();
setFocusable(true);
addKeyListener(new MenuOperator());
setBackground(Color.black);
setLayout(null);
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D)g;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
int w = getWidth();
int h = getHeight();
g2d.drawImage(bg, 0, 0, null);
}
public class MenuOperator extends KeyAdapter {
#Override
public void keyPressed(KeyEvent event) {
int keyCode = event.getKeyCode();
if(keyCode == KeyEvent.VK_SPACE) {
System.out.println("LOL");
}
}
}
}
Does anyone know what the problem might be? I have set the key handler in the frame as well as setting the frame to be focusable..
Thanks in advance
~RyanS.

Things not considered right in the code (IMHO) :
The way you accessing the images in your code. Consider ImageIO, over using the constructor of ImageIcon, as the former will notify you, in case something goes wrong. For more info, please refer, Loading Image Icon Exception, for more insight.
KeyListeners are for AWT, Swing on the other hand uses KeyBindings.
Before, setting background on any JComponent prefer to first set the opaque property of the said JComponent to true. Since, opaque property is usually dependent on LookAndFeel used.
Avoid the use of setLayout(null), as much as possible. Please walk through the first paragraph of Absolute Positioning for more information :-)
THe use of ImageObserver as already mentioned in the comments.
This blog Motion Using the Keyboard by #camickr, will again add loads to your knowledge, as to why KeyListeners are not considered a better option and why KeyBindings is more suited for such needs :-)
Please have a look at this example :
import java.awt.*;
import java.awt.event.*;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;
public class TestMenu extends JPanel {
private Image bg;
public TestMenu() {
try {
bg = ImageIO.read(
new URL("http://i.imgur.com/Aoluk8n.gif"));
} catch (Exception e) {
e.printStackTrace();
}
getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "SPACE key");
getActionMap().put("SPACE key", keyAction);
setOpaque(true);
setBackground(Color.black);
}
private Action keyAction = new AbstractAction() {
#Override
public void actionPerformed(ActionEvent ae) {
System.out.println("LOL");
}
};
/*
* Make this a customary habbit to override
* this method for the JPanel/JComponent,
* when ever you extend any of the above classes
*/
#Override
public Dimension getPreferredSize() {
return (new Dimension(200, 200));
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D)g;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
int w = getWidth();
int h = getHeight();
/*
* Here the last part of the drawImage is the
* ImageObserver, here it is a good practise
* to keep the instance of the JPanel on which
* we drawing the image to be notified, till the
* end, till the image loading is not complete
*/
g2d.drawImage(bg, 0, 0, this);
}
private void displayGUI() {
JFrame frame = new JFrame("Painting Example");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setContentPane(this);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
Runnable runnable = new Runnable() {
#Override
public void run() {
new TestMenu().displayGUI();
}
};
EventQueue.invokeLater(runnable);
}
}

Setting your panel focusable is not enough - panel cannot get focus on its own - you have to request the focus on some kind of event. For example you can add a mouse listener to your panel and call panel.requestFocusInWindow() when you click on it.
Key listener will work only if component on which you add it has focus.