java - JFrame Paint() 上的 NullPointerException

Question

我无法弄清楚为什么会抛出 nullpointerexception。它并没有真正影响任何东西(至少我不认为)，因为一切仍然被正确地绘制出来，但我不喜欢错误消息。

如您所见，paint() 调用了 this.game.paint(g2)。为了尝试调试，this.game.print 中的所有内容都已被注释掉 - 该函数不执行任何操作，因此它不应该成为问题的一部分。

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();
    }
}

}

这是堆栈跟踪 - 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/大小后设置窗口的图标图像是一个坏主意。

移动此行后

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

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);

空指针异常抛出已停止。经验教训:首先设置窗口图标。