如何使用Graphviz从点文件格式生成PNG图像

我有一个实现优先级队列的Java类。然后，我有一个类测试，该类生成如下图：

digraph G {
Milan  (0.0)       ->     Turin  (1.2)       
Milan  (0.0)       ->     Montreal  (7.0)       
Turin  (1.2)       ->     Paris  (5.8)       
Turin  (1.2)       ->     Tokyo  (2.2)       
}

此图保存在名为“队列”的文件中。

现在，我希望使用Graphviz在PNG图像中显示该图。因此，我的测试文件的最后一次调用（在您创建优先级并填充队列之后）是：

queue.toString("queue");

行。toString方法如下：

public void toString(String fileDot){
    try {
        FileOutputStream file = new FileOutputStream(fileDot); 
        PrintStream Output = new PrintStream(file); 
        Output.print(this.printQueue()); 
        Output.close(); 
        File f = new File(fileDot); 
        String arg1 = f.getAbsolutePath(); 
        String arg2 = arg1 + ".png"; 
        String[] c = {"dot", "-Tpng", arg1, "-o", arg2};
        Process p = Runtime.getRuntime().exec(c); 
        int err = p.waitFor(); 
    }
    catch(IOException e1) {
        System.out.println(e1);
    }
    catch(InterruptedException e2) {
        System.out.println(e2);
    }
}

private String printQueue() throws IOException {
    String g = new String("");
    char c = '"';
    g = g.concat("digraph G {\n");
    if(isEmpty()) 
        g = g.concat("    " + "Empty priority queue.");
    else {
        for(int i = 0; i < lastIndex; i++) {
            if(heap[2 * i] != null) { 
                g = g.concat("" + heap[i].elem + "  (" + heap[i].prior + ")   " + "   " + " -> " + "    " + "" + heap[i * 2].elem + "  (" + heap[i * 2].prior + ")       \n" );
                if(heap[2 * i + 1] != null) 
                    g = g.concat("" + heap[i].elem + "  (" + heap[i].prior + ")   " + "   " + " -> " + "    " + "" + heap[i * 2 + 1].elem + "  (" + heap[i * 2 + 1].prior + ")       \n" ); 
            } 
        } //end for
    } //end else  
    g = g.concat("}");
    return g;   
}

为什么不生成图像.png？我哪里错了？当然我安装了Graphviz。谢谢