title: What is the difference between "using" namespace and declaring namespace?
body: <p>Could somebody tell me what is the difference between</p>
  
  <pre><code>using namespace android;
      .... 
  </code></pre>
  
  <p>and </p>
  
  <pre><code>namespace android {
      ....
  }
  </code></pre>
  
  <p>I found that almost all .cpp files in Android source code use the second one.<br>
  Also, If I want to include some files that use the type of second one in my own project, do I need to use <code>namespace android{...}</code> too?<br>
  Because if I do not, compiler would report error when I call methods of included files. Or do I need to add any prefix before method call?</p>