使用JGraph添加頂點，邊和端口

Question

我想添加新的頂點和邊，並使用JGraph庫創建一個圖形。 我總是收到此java.lang.NullPointer異常。 我創建了一個類函數createvertex，以創建新的單元格/頂點並繪制連接端口的邊緣。 但是即使我尚未將端口聲明為null，該端口也始終顯示為null。 下面是我的代碼。 我的代碼有什么問題嗎？

public class HelloWorld {

public static void main(String[] args) {

    // Construct Model and Graph
    GraphModel model = new DefaultGraphModel();
    GraphLayoutCache view= new GraphLayoutCache(model,new DefaultCellViewFactory());

    JGraph graph = new JGraph(model,view);

    // Control-drag should clone selection
    graph.setCloneable(true);

    // Enable edit without final RETURN keystroke
    graph.setInvokesStopCellEditing(true);

    // When over a cell, jump to its default port (we only have one, anyway)
    graph.setJumpToDefaultPort(true);

    // Insert all three cells in one call, so we need an array to store them
    DefaultGraphCell[] cells = new DefaultGraphCell[5];
    DefaultPort[] port = new DefaultPort[4];

    // Create Hello Vertex
    cells[0] = createVertex("Hello", 20, 20, 40, 20, Color.BLACK, true);

    port[0].setParent(cells[0]);

    // Create World Vertex
    cells[1] = createVertex("World", 140, 140, 40, 20, Color.ORANGE, true);
    cells[1].add(port[1]);
    cells[1].add(port[2]);
    port[1].setParent(cells[1]);
    port[2].setParent(cells[1]);

    cells[3]=  createVertex("Optical Cards",150,150,20,40,Color.GREEN, true);
    cells[3].add(port[3]);
    port[3].setParent(cells[3]);


    // Create Edge
    DefaultEdge[] edge = new DefaultEdge[2];

    // Fetch the ports from the new vertices, and connect them with the edge
    edge[0].setSource(cells[0].getChildAt(0));
    edge[0].setTarget(cells[1].getChildAt(0));

    cells[2] = edge[0];

    edge[1].setSource(cells[1].getChildAt(1));
    edge[1].setTarget(cells[3].getChildAt(0));

    cells[4]=edge[1];

    // Set Arrow Style for edge
    int arrow = GraphConstants.ARROW_CLASSIC;
    GraphConstants.setLineEnd(edge[0].getAttributes(), arrow);
    GraphConstants.setEndFill(edge[0].getAttributes(), true);

    GraphConstants.setLineEnd(edge[1].getAttributes(), arrow);
    GraphConstants.setEndFill(edge[1].getAttributes(), true);

    // Insert the cells via the cache, so they get selected
    graph.getGraphLayoutCache().insert(cells);

    // Show in Frame
    JFrame frame = new JFrame();
    frame.getContentPane().add(new JScrollPane(graph));
    //frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
}

public static DefaultGraphCell createVertex(String name, double x,double y,double w,double h, Color bg, boolean raised) {

    // Create vertex with the given name
    DefaultGraphCell cell = new DefaultGraphCell(name);

    // Set bounds
    GraphConstants.setBounds(cell.getAttributes(), new Rectangle2D.Double(
            x, y, w, h));

    // Set fill color

        GraphConstants.setGradientColor(cell.getAttributes(), Color.orange);
        GraphConstants.setOpaque(cell.getAttributes(), true);

    // Set raised border
    if (raised)
        GraphConstants.setBorder(cell.getAttributes(), BorderFactory
                .createRaisedBevelBorder());
    else
        // Set black border
        GraphConstants.setBorderColor(cell.getAttributes(), Color.black);

    // Add a Port
    DefaultPort port = new DefaultPort();
    cell.add(port);

    return cell;
}
}

Answer 1

DefaultPort[] port = new DefaultPort[4];




// Create Hello Vertex
cells[0] = createVertex("Hello", 20, 20, 40, 20, Color.BLACK, true);

port[0].setParent(cells[0]);  //   <---------- port[0] is null here, so you 
                              //               cannot call setParent()

僅僅因為初始化端口數組並不意味着您可以簡單地使用數組中的第一個DefaultPort元素。 在嘗試取消引用數組中的項目之前，需要使用有效的非null DefaultPort對象填充數組。