[英]ERROR : Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException
[英]Reason for “AWT-EventQueue-0” java.lang.NullPointerException error
我正在用Java創建一個視頻游戲,我正在嘗試檢索一個坐標點列表並將它們繪制到地圖上,但每次運行我的應用程序時,我都會收到一個“AWT-EventQueue-0”java.lang.NullPointerException,一直無法弄清楚為什么它指向null。 這是我收到的錯誤報告:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at game.PathPoints.drawObjectPath(PathPoints.java:23)
at game.TestPanel.paintComponent(TestPanel.java:64)
at javax.swing.JComponent.paint(JComponent.java:1029)
at javax.swing.JComponent.paintChildren(JComponent.java:866)
at javax.swing.JComponent.paint(JComponent.java:1038)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:581)
at javax.swing.JComponent.paintChildren(JComponent.java:866)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5145)
at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:302)
at javax.swing.RepaintManager.paint(RepaintManager.java:1216)
at javax.swing.JComponent.paint(JComponent.java:1015)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:78)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:115)
at java.awt.Container.paint(Container.java:1784)
at javax.swing.RepaintManager$3.run(RepaintManager.java:818)
at javax.swing.RepaintManager$3.run(RepaintManager.java:795)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:795)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:764)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:706)
at javax.swing.RepaintManager.access$1000(RepaintManager.java:61)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:226)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:673)
at java.awt.EventQueue.access$300(EventQueue.java:96)
at java.awt.EventQueue$2.run(EventQueue.java:634)
at java.awt.EventQueue$2.run(EventQueue.java:632)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:643)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)
PathPoints類的錯誤代碼如下:
package game;
import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class PathPoints {
public PathPoints (Scanner s){
int numberCoord;
List<Coordinates> path = new ArrayList<Coordinates>();
numberCoord = s.nextInt();
for(int x = 0; x < numberCoord; x++){
Coordinates points = new Coordinates(s.nextInt(), s.nextInt());
path.add(points);
}
}
public void drawObjectPath(Graphics g){
g.setColor(Color.GREEN);
for(int n = 0; n < path.size()- 1; n++){
g.drawLine(((Coordinates)path.get(n)).x, ((Coordinates)path.get(n)).y, ((Coordinates)path.get(n+1)).x, ((Coordinates)path.get(n+1)).y);
g.drawLine(((Coordinates)path.get(n)).x, ((Coordinates)path.get(n)).y+1, ((Coordinates)path.get(n+1)).x, ((Coordinates)path.get(n+1)).y+1);
}
}
List path;
}
TestPanel類的代碼在這里:
public class TestPanel extends JPanel implements MouseListener
{
private static final long serialVersionUID = 1L; // Ignore this - It's just to get rid of a warning.
// Instance variable(s).
int x,y;
private Image backdrop;
PathPoints objectPath;
/**
* Constructor - loads a background image
*/
public TestPanel ()
{
try
{
ClassLoader myLoader = this.getClass().getClassLoader();
InputStream imageStream = myLoader.getResourceAsStream("resources/path_1.jpg");
backdrop = ImageIO.read(imageStream);
InputStream pointStream = myLoader.getResourceAsStream("resources/path_1.txt");
Scanner s = new Scanner (pointStream);
objectPath = new PathPoints(s);
}
catch (IOException e)
{
System.out.println ("Could not load: " + e);
}
addMouseListener(this);
}
/**
* This paint meethod draws the background image anchored
* in the upper-left corner of the panel.
*/
public void paintComponent (Graphics g)
{
g.drawImage(backdrop, 0, 0, null);
objectPath.drawObjectPath(g);
//Coordinates();
}
我已經測試了PathPoints的構造函數,看它是否正確地將值添加到ArrayList並且正確地添加它們,那么我應該在下一步解決這個問題呢?
public PathPoints (Scanner s){
int numberCoord;
List<Coordinates> path = new ArrayList<Coordinates>();
numberCoord = s.nextInt();
for(int x = 0; x < numberCoord; x++){
Coordinates points = new Coordinates(s.nextInt(), s.nextInt());
path.add(points);
}
在這些行中,“path”不是指類的屬性,而是指區域設置變量。 因此,您的屬性“path”永遠不會被初始化。
btw,2個推薦:
您的屬性應該是私有的:
私人列表路徑;
你正在遮蔽變量path
。 具有相同名稱的局部變量在構造函數PathPoints
的范圍內實例化,但類成員變量path
在方法drawObjectPath
為null
。 更換
List<Coordinates> path = new ArrayList<Coordinates>();
同
path = new ArrayList<Coordinates>();
在PathPoints
類構造函數中,您使用局部變量隱藏成員變量path
:
int numberCoord;
List<Coordinates> path = new ArrayList<Coordinates>();
numberCoord = s.nextInt();
這意味着永遠不會為成員變量賦值,並且在drawObjectPath
方法時將為null
。
要解決這個問題,請不要隱藏成員變量; 使用:
int numberCoord;
path = new ArrayList<Coordinates>();
numberCoord = s.nextInt();
public class StationDatabase1 {
File file = new File("lrt.txt");
public ArrayList<Station> getAll() throws SQLException, ClassNotFoundException, FileNotFoundException {
ArrayList<Station> station = new ArrayList<>();
Scanner input = new Scanner(file);
try {
while (input.hasNextLine()) {
String id = input.nextLine();
String name = input.nextLine();
Station temp = new Station(id, name);
station.add(temp);
}
} catch (Exception ex) {
JOptionPane.showMessageDialog(null, ex);
}
Set newSet = new LinkedHashSet(station);
station.clear();
station.addAll(newSet);
return station;
}
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