一、当只有addRequestProperty的时候

 URL url = new URL("http://localhost:8080/net/listnets.jsp");
                URLConnection connection = url.openConnection();
                connection.addRequestProperty("name", "asad");
                connection.addRequestProperty("name", "komal");
                connection.addRequestProperty("class", "10th");
                connection.addRequestProperty("Address", "Delhi 17");

                Map map = connection.getRequestProperties();
                Set set = map.entrySet();

                Iterator iterator = set.iterator();
                while (iterator.hasNext()) {
                    System.out.println(iterator.next());
                }

输出结果：name=[komal, asad]
Address=[Delhi 17]
class=[10th]

二、当只有setRequestProperty的时候

             URL url = new URL("http://localhost:8080/net/listnets.jsp");
                URLConnection connection = url.openConnection();
                connection.setRequestProperty("name", "komal");
                connection.setRequestProperty("name", "asad");
                connection.setRequestProperty("class", "10th");
                connection.setRequestProperty("Address", "Delhi 17");

                Map map = connection.getRequestProperties();
                Set set = map.entrySet();

                Iterator iterator = set.iterator();
                while (iterator.hasNext()) {
                    System.out.println(iterator.next());
                }

输出结果：

name=[asad]
Address=[Delhi 17]
class=[10th]

注意name的设置，会发生覆盖的作用。

三、当先set后add的时候

URL url = new URL("http://localhost:8080/net/listnets.jsp");
        URLConnection connection = url.openConnection();
        connection.setRequestProperty("name", "asad");
        connection.addRequestProperty("name", "komal");
        connection.addRequestProperty("class", "10th");
        connection.addRequestProperty("Address", "Delhi 17");

        Map map = connection.getRequestProperties();
        Set set = map.entrySet();

        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

输出结果

name=[komal, asad]
Address=[Delhi 17]
class=[10th]

四、当先add后set的时候

URL url = new URL("http://localhost:8080/net/listnets.jsp");
        URLConnection connection = url.openConnection();
        connection.addRequestProperty("name", "komal");
        connection.setRequestProperty("name", "asad");
        connection.addRequestProperty("class", "10th");
        connection.addRequestProperty("Address", "Delhi 17");

        Map map = connection.getRequestProperties();
        Set set = map.entrySet();

        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

name=[asad]
Address=[Delhi 17]
class=[10th]

五、结

setRequestProperty方法，如果key存在，则覆盖；不存在，直接添加。

addRequestProperty方法，不管key存在不存在，直接添加。