java - JPanel 未添加到 JFrame

Question

当我尝试将面板添加到框架中时，我没有得到任何结果(因此我尝试将面板的背景设置为红色)。另外，每当我在 Vertex 类中调用 linkedList 时，都会收到一条错误消息:线程“AWT-EventQueue-0”java.lang.NullPointerException 中出现异常。我现在的主要问题是面板。

绘图面板类

@SuppressWarnings("serial")
public class DrawPanel extends JPanel {
Project4Test test = new Project4Test();
Graph graph;
Vertex v;

public DrawPanel() {
    setBackground(Color.red);
    setPreferredSize(new Dimension(500,500));

}

public void paintComponent(Graphics page) {
    Graphics2D page2 = (Graphics2D) page;
    page2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

    page2.setColor(Color.black);

    Iterator iterator = test.hashEdge.entrySet().iterator();
    while(iterator.hasNext()) {
        Map.Entry dualEntry = (Map.Entry)iterator.next();
        Edge e = (Edge) dualEntry.getValue();
        drawEdge(page2, e.vertex1, e.vertex2);
    }

    ListIterator<Vertex> listIterator = v.linkedList.listIterator();
    while(listIterator.hasNext()) {
    drawEdge(page2, v, listIterator.next());
    }
}

public void drawEdge(Graphics2D page2, Vertex vertex1, Vertex vertex2) {
    //x1, y1, x2, y2
    page2.setColor(Color.black);
    page2.draw(new Line2D.Double(vertex1.latitude, vertex1.longitude, vertex2.latitude, vertex2.longitude));
}
}

测试类

@SuppressWarnings("serial")
public class Project4Test {
static int numVertices = 0;
static Map<String, Vertex> hashVertex = new HashMap<String, Vertex>();
static Map<String, Edge> hashEdge = new HashMap<String, Edge>();

//Map with String intersectionID as the key, and Vertexes as the value
@SuppressWarnings({ "resource", "unused" })
public static void main(String[] args) {
    JFrame frame = new JFrame("Map");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setResizable(true);
    DrawPanel drawpanel = new DrawPanel();
    frame.setSize(660, 630);
    frame.add(drawpanel);

    //frame.setExtendedState(Frame.MAXIMIZED_BOTH);



    File inFile = new File(args[0]);
    //frame.setExtendedState(Frame.MAXIMIZED_BOTH);
    //DrawPanel drawpanel;
    //frame.setVisible(true);


    try {
        Scanner scan = new Scanner(inFile);
        Graph newGraph = new Graph(0, false);

        while(scan.hasNext()) {
            String temp = scan.nextLine();
            String[] info = temp.split("\\t", 4);
            String interRoad = info[0];

            //If it is an intersection
            if(info[0].equalsIgnoreCase("i")) {
                String vertexID = info[1];
                double x = Double.parseDouble(info[2]);
                double y = Double.parseDouble(info[3]);
                //System.out.println(interRoad + " " + vertexID + " " + x + " " + y);
                //Insert to vertex here
                Vertex tempVertex = new Vertex(vertexID, numVertices, x , y);
                hashVertex.put(vertexID, tempVertex);
                numVertices++;
            }//end of intersection

            //Update graph to have the number of vertices
            newGraph = new Graph(numVertices, false);

            //If it is a road
            if(info[0].equalsIgnoreCase("r")) {
                String roadID = info[1];
                String vertexID1 = info[2];
                String vertexID2 = info[3];

                Vertex vertex1 = hashVertex.get(vertexID1);
                Vertex vertex2 = hashVertex.get(vertexID2);
                //System.out.println(interRoad + " " + roadID + " " + vertexID1 + " " + vertexID2);
                Edge newEdge = new Edge(roadID, vertex1, vertex2);
                hashEdge.put(roadID, newEdge);

                //Essentially saying linkedList(v1).add(v2) because if it's a road, it's inherently connected
                vertex1.linkedList.add(vertex2);
                vertex2.linkedList.add(vertex1);
                //drawpanel = new DrawPanel(newGraph);
                frame.add(drawpanel);
            }
        }//end of while loop
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

//      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true); 
} //end of main


}

Answer 1

首先，请务必通过在覆盖中调用 super 的方法来调用 JPanel 自己的 paintComponent 方法，因为这将允许 JPanel 绘制其背景颜色并执行任何其他内务图形。换句话说，改变这个:

public void paintComponent(Graphics page) {
    Graphics2D page2 = (Graphics2D) page;

对此:

@Override  // don't forget this
protected void paintComponent(Graphics page) {  // method should be protected
    super.paintComponent(page);  // ****** key to add this and call it first
    Graphics2D page2 = (Graphics2D) page;

对于 NullPointerException，您将需要了解如何调试 NPE (NullPointerException) 的一般概念。您应该仔细检查抛出它的行，异常的堆栈跟踪会告诉您，找出哪个变量为空，然后追溯到您的代码以查看原因。你会一次又一次地遇到这些，相信我。