title: how to convert hashMap to Json file
body: <p>I am leaning Java.</p>
  
  <p>I have to transfer a Hashmap to Server using rpc.</p>
  
  <p>HashMap</p>
  
  <pre><code>Map&lt;String, String&gt; testMap = new HashMap&lt;String, String&gt;();
  testMap .put("1", "abc");
  testMap .put("2", "ezc");
  testMap .put("3", "afc");
  testMap .put("4", "cvc");
  ..
  </code></pre>
  
  <p>how to do that.</p>