为什么此代码会引发java.lang.NullPointerException? [英] Why does this code throw a java.lang.NullPointerException?
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问题描述
我找到了源代码,并将其添加到框架中只是为了测试它是否使用Java2D. 但是它引发了一个例外.我不明白为什么.
I have found a source code and i added it to my frame just for testing which it uses Java2D. But it thows an exception. I don't understand why.
我的课:
package ClientGUI;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.MediaTracker;
import java.awt.RenderingHints;
import java.awt.geom.CubicCurve2D;
import java.awt.geom.PathIterator;
import java.awt.geom.Point2D;
import java.awt.image.BufferedImage;
/**
*
* @author ICC
*/
public class SignInFrame extends javax.swing.JFrame implements Runnable {
private static int iw, ih, iw2, ih2;
private static Image img;
private static final int FORWARD = 0;
private static final int BACK = 1;
// the points of the curve
private Point2D pts[];
// initializes direction of movement forward, or left-to-right
private int direction = FORWARD;
private int pNum;
private int x, y;
private Thread thread;
private BufferedImage bimg;
/** Creates new form SignInFrame */
public SignInFrame() {
initComponents();
img = getToolkit().getImage(Image.class.getResource("Yahoo-Messanger.jpg"));
try {
MediaTracker tracker = new MediaTracker(this);
tracker.addImage(img, 0);
tracker.waitForID(0);
} catch (Exception e) {
}
iw = img.getWidth(this);
ih = img.getHeight(this);
iw2 = iw / 2;
ih2 = ih / 2;
}
public void reset(int w, int h) {
pNum = 0;
direction = FORWARD;
// initializes the cubic curve
CubicCurve2D cc = new CubicCurve2D.Float(
w * .2f, h * .5f, w * .4f, 0, w * .6f, h, w * .8f, h * .5f);
// creates an iterator to define the boundary of the flattened curve
PathIterator pi = cc.getPathIterator(null, 0.1);
Point2D tmp[] = new Point2D[200];
int i = 0;
// while pi is iterating the curve, adds points to tmp array
while (!pi.isDone()) {
float[] coords = new float[6];
switch (pi.currentSegment(coords)) {
case PathIterator.SEG_MOVETO:
case PathIterator.SEG_LINETO:
tmp[i] = new Point2D.Float(coords[0], coords[1]);
}
i++;
pi.next();
}
pts = new Point2D[i];
// copies points from tmp to pts
System.arraycopy(tmp, 0, pts, 0, i);
}
public void step(int w, int h) {
if (pts == null) {
return;
}
x = (int) pts[pNum].getX();
y = (int) pts[pNum].getY();
if (direction == FORWARD) {
if (++pNum == pts.length) {
direction = BACK;
}
}
if (direction == BACK) {
if (--pNum == 0) {
direction = FORWARD;
}
}
}
public void drawDemo(int w, int h, Graphics2D g2) {
g2.drawImage(img,
0, 0, x, y,
0, 0, iw2, ih2,
this);
g2.drawImage(img,
x, 0, w, y,
iw2, 0, iw, ih2,
this);
g2.drawImage(img,
0, y, x, h,
0, ih2, iw2, ih,
this);
g2.drawImage(img,
x, y, w, h,
iw2, ih2, iw, ih,
this);
}
public Graphics2D createGraphics2D(int w, int h) {
Graphics2D g2 = null;
if (bimg == null || bimg.getWidth() != w || bimg.getHeight() != h) {
bimg = (BufferedImage) createImage(w, h);
reset(w, h);
}
g2 = bimg.createGraphics();
g2.setBackground(getBackground());
g2.clearRect(0, 0, w, h);
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2.setRenderingHint(RenderingHints.KEY_RENDERING,
RenderingHints.VALUE_RENDER_QUALITY);
return g2;
}
@Override
public void paint(Graphics g) {
Dimension d = getSize();
step(d.width, d.height);
Graphics2D g2 = createGraphics2D(d.width, d.height);
drawDemo(d.width, d.height, g2);
g2.dispose();
g.drawImage(bimg, 0, 0, this);
}
public void start() {
thread = new Thread(this);
thread.setPriority(Thread.MIN_PRIORITY);
thread.start();
}
public synchronized void stop() {
thread = null;
}
public static void main(String argv[]) {
SignInFrame f = new SignInFrame();
f.start();
}
public void run() {
Thread me = Thread.currentThread();
while (thread == me) {
repaint();
try {
Thread.sleep(10);
} catch (InterruptedException e) {
break;
}
}
thread = null;
}}
例外:
init:
deps-jar:
Compiling 1 source file to C:\Users\ICC\Documents\NetBeansProjects\YahooServer\build\classes
compile-single:
run-single:
Uncaught error fetching image:
java.lang.NullPointerException
at sun.awt.image.URLImageSource.getConnection(URLImageSource.java:97)
at sun.awt.image.URLImageSource.getDecoder(URLImageSource.java:107)
at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:240)
at sun.awt.image.ImageFetcher.fetchloop(ImageFetcher.java:172)
at sun.awt.image.ImageFetcher.run(ImageFetcher.java:136)
推荐答案
违规行在这里
img = getToolkit().getImage(Image.class.getResource("Yahoo-Messanger.jpg"));
确保该文件存在,请参考此文档以查看如何加载资源的顺序
Java用于getResource的文档
The offending line is here
img = getToolkit().getImage(Image.class.getResource("Yahoo-Messanger.jpg"));
Make sure that the file exists refer to this doc to see the order on how the resources are loaded
Java Doc for getResource
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