NullPointerException paint()

Question

我在弄清楚为什么会抛出一个nullpointerexception时遇到了问题。它实际上不会影响任何东西(我认为至少不会)，因为所有的东西都可以正确地绘制出来，但是我不喜欢错误消息。

如您所见，paint()调用this.game.paint(g2)。为了尝试调试，this.game.print中的所有内容都被注释掉了-该函数什么也不做，所以它不应该是问题的一部分。

public class Application extends JFrame {

private Game game;///<stores the rules for the game and facilitates interactions

public Application() throws RoyException {
    this.game = new Game(this, 2);
}


//   @Override
public void paint(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    try{
    this.game.paint(g2);
    }catch(Exception e)
    {
        System.out.println("CAUGHT " + e.getMessage());
        e.printStackTrace();
    }
}

public static void main(String[] args) {
    try {
        Application app = new Application();
    } catch (RoyException e) {
        System.out.println("An error occurred while running the program: " + e.getMessage());
        System.out.print("Stack trace: ");
        e.getStackTrace();
    }
}

}

下面是堆栈跟踪- Application.java:153 is this.game.paint(g2)；

CAUGHT null java.lang.NullPointerException
    at bullshitakemushrooms.Application.paint(Application.java:153)
   Game created.
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:781)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:728)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:677)
    at javax.swing.RepaintManager.access$700(RepaintManager.java:59)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:701)
    at java.awt.EventQueue.access$000(EventQueue.java:102)
    at java.awt.EventQueue$3.run(EventQueue.java:662)
    at java.awt.EventQueue$3.run(EventQueue.java:660)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:671)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

Answer 1

显然，在设置可见性/大小之后设置窗口的图标图像不是一个好主意。

移动此行后

this.app.setIconImage(new ImageIcon("mushy.jpg").getImage());

在此之前

    this.app.setSize(this.maxX, this.maxY);
    this.mmenu.setVisible(true);
    app.setVisible(true);
    app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    app.setAlwaysOnTop(true);

nullpointerexception抛出已停止。经验教训:首先设置窗口图标。