Example of a NullPointerException

Source:

public class NullPointerExample {
   public static void main(String[] args) {
      String str = "This is a string";
      System.out.println(str);
      System.out.println(str.length());

      str = null;
      System.out.println(str);

      // Should trigger NullPointerException
      System.out.println(str.length());
   }
}

Output:

   $ java NullPointerExample 
   This is a string
   16
   null
   Exception in thread "main" java.lang.NullPointerException
       at Foo.main(Foo.java:11)