title: Why String concatenate null with + operator and throws NullPointerException with concate() method
body: <p>Here is my class, where i am concatenating two string. 
  String concatenate with <code>null</code> using + operator execute smoothly but throws <code>NullPointerException</code> with <code>concate()</code> method.</p>
  
  <pre><code>public class Test {
  
      public static void main(String[] args) {
  
          String str="abc";
          String strNull=null;
  
          System.out.println(strNull+str);
  
          str.concat(strNull);
      }
  }
  </code></pre>
  
  <p>can anybody tell me the reason behind it ??</p>