this is my first Java GUI program, and really only my second java program, so take it easy on me :) My program is a result of a lot of googling and reading java docs. My problem is that I have a sprite sheet of 52 cards, and am attempting to save these cards individually to a Buffered Image array using subImage, and just for testing purposes, display all 52 in a window. The File is in the correct directory I made sure of that. I believe that my problem lies with my use of Jlabels, or simply a foolish mistake. Anyways, here is my class that does the sprite sheet splitting
package gui;
import java.awt.GridLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JPanel;
public class crdimgs extends JPanel {/**
*
*/
static final long serialVersionUID = 1L;
public final int width = 10;
public final int height = 20;
public int rows = 13;
public int cols = 5;
public BufferedImage image;
File cardimg = new File("Cards.jpg");
BufferedImage cards[];
public void loadsplit(File loadimage){
try{
image = ImageIO.read(loadimage);
} catch(Exception error){
System.out.print("error");
}
cards = new BufferedImage[cols*rows];
}
public crdimgs() {
loadsplit(cardimg);
setLayout(new GridLayout(rows, cols, 1, 1));
int x = 0;
int y = 0;
int subimg = 0;
for( int i = 0; i < rows; i++)
{
JPanel panel = new JPanel();
cards[subimg] = new BufferedImage(width, height, 5);
cards[subimg] = image.getSubimage(x, y, width, height);
panel.add(new JLabel(new ImageIcon(cards[subimg])));
add(panel);
x+=width;
subimg++;
}
y+=height;
x=0;
}
}
}
And my Main class
package gui;
import javax.swing.JFrame;
import java.awt.Color;
public class cards extends JFrame {
private static final long serialVersionUID = 1L;
public cards(){
setTitle("Poker");
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(1000, 700);
setLocationRelativeTo(null);
this.getContentPane().setBackground(Color.GREEN);
setVisible(true);
setResizable(false);
add(new crdimgs());
}
public static void main(String[] args){
new cards();
}
}
Errors I receive at the moment are:
errorException in thread "main" java.lang.NullPointerException
at gui.crdimgs.<init>(crdimgs.java:53)
at gui.cards.<init>(cards.java:22)
at gui.cards.main(cards.java:28)
Likely your image is null, possibly because you're looking in the wrong place for it, but to find out, check out which line is 53.
Suggestions:
- Start small and then build on. Don't try to do complex image manipulation until you first successfully read and show a single image.
- Check where Java is looking for the image, because likely it isn't where you think it is. Put
System.out.println(System.getProperty("user.dir"));in your code to find out. - Call
setVisible(true)after adding all components to the JFrame. - Improve your exception display code by printing the stack trace:
error.printStackTrace()(thanks Mad!).
For example:
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import javax.imageio.ImageIO;
import javax.swing.*;
public class PlayingCardTest {
public static void main(String[] args) {
String pathToDeck = "http://www.jfitz.com/cards/classic-playing-cards.png";
try {
final List<ImageIcon> cardImgList = CreateCards.createCardIconList(pathToDeck);
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame frame = new JFrame("Moving Cards");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new CardGameTable(cardImgList, frame));
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
} catch (MalformedURLException e) {
e.printStackTrace();
System.exit(-1);
} catch (IOException e) {
e.printStackTrace();
System.exit(-1);
}
}
}
@SuppressWarnings("serial")
class CardGameTable extends JLayeredPane {
private static final int PREF_W = 600;
private static final int PREF_H = 400;
private static final Color BASE_COLOR = new Color(0, 80, 0);
private static final int CARD_COUNT = 20;
private static final int WIDTH_SHOWING = 20;
private JPanel basePane = new JPanel(null);
public CardGameTable(List<ImageIcon> cardImgList, final JFrame frame) {
basePane.setSize(getPreferredSize());
basePane.setBackground(BASE_COLOR);
add(basePane, JLayeredPane.DEFAULT_LAYER);
final MyMouseAdapter myMouseAdapter = new MyMouseAdapter(this, basePane);
addMouseListener(myMouseAdapter);
addMouseMotionListener(myMouseAdapter);
for (int i = 0; i < CARD_COUNT; i++) {
JLabel card = new JLabel(cardImgList.remove(0));
card.setSize(card.getPreferredSize());
int x = (PREF_W / 2) + WIDTH_SHOWING * (CARD_COUNT - 2 * i) / 2 -
card.getPreferredSize().width / 2;
int y = PREF_H - card.getPreferredSize().height - WIDTH_SHOWING * 2;
card.setLocation(x, y);
basePane.add(card);
}
}
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
}
class MyMouseAdapter extends MouseAdapter {
private JLabel selectedCard = null;
private JLayeredPane cardGameTable = null;
private JPanel basePane = null;
private int deltaX = 0;
private int deltaY = 0;
public MyMouseAdapter(JLayeredPane gameTable, JPanel basePane) {
this.cardGameTable = gameTable;
this.basePane = basePane;
}
@Override
public void mousePressed(MouseEvent mEvt) {
Component comp = basePane.getComponentAt(mEvt.getPoint());
if (comp != null && comp instanceof JLabel) {
selectedCard = (JLabel) comp;
basePane.remove(selectedCard);
basePane.revalidate();
basePane.repaint();
cardGameTable.add(selectedCard, JLayeredPane.DRAG_LAYER);
cardGameTable.revalidate();
cardGameTable.repaint();
deltaX = mEvt.getX() - selectedCard.getX();
deltaY = mEvt.getY() - selectedCard.getY();
}
}
@Override
public void mouseReleased(MouseEvent mEvt) {
if (selectedCard != null) {
cardGameTable.remove(selectedCard);
cardGameTable.revalidate();
cardGameTable.repaint();
basePane.add(selectedCard, 0);
basePane.revalidate();
basePane.repaint();
selectedCard = null;
}
}
@Override
public void mouseDragged(MouseEvent mEvt) {
if (selectedCard != null) {
int x = mEvt.getX() - deltaX;
int y = mEvt.getY() - deltaY;
selectedCard.setLocation(x, y);
cardGameTable.revalidate();
cardGameTable.repaint();
}
}
}
class CreateCards {
private static final int SUIT_COUNT = 4;
private static final int RANK_COUNT = 13;
public static List<ImageIcon> createCardIconList(String pathToDeck)
throws MalformedURLException, IOException {
BufferedImage fullDeckImg = ImageIO.read(new URL(pathToDeck));
int width = fullDeckImg.getWidth();
int height = fullDeckImg.getHeight();
List<ImageIcon> iconList = new ArrayList<ImageIcon>();
for (int suit = 0; suit < SUIT_COUNT; suit++) {
for (int rank = 0; rank < RANK_COUNT; rank++) {
int x = (rank * width) / RANK_COUNT;
int y = (suit * height) / SUIT_COUNT;
int w = width / RANK_COUNT;
int h = height / SUIT_COUNT;
BufferedImage cardImg = fullDeckImg.getSubimage(x, y, w, h);
iconList.add(new ImageIcon(cardImg));
}
}
Collections.shuffle(iconList);
return iconList;
}
}
Which shows:
来源：https://stackoverflow.com/questions/18995466/jlabel-wont-display-image-nullpointerexception