Adding vertices, edges and ports using JGraph

Question

I want to add new vertices and edges and create a graph using JGraph Library. I always get this java.lang.NullPointer exception. I have created a class function createvertex to create new cells/vertices and drawing edges my connecting to ports. But the port is always shown as null even when I haven't declared it as null. Below is my code. Is anything wrong in my code?

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()

Just because you initialized the port array doesn't mean that you can simply use the first DefaultPort element in the array. You need to populate the array with valid, non-null DefaultPort objects before trying to dereference an item in the array.