Use equal for string literal, rather than for an object

When you wanna compare String object with string literal, we often disregard what we compare with what, however there is one and safe way to do it.

It's much better to use equals() and equalsIgnoreCase() for a string literal, instead of Object, because it helps to avoid possible NullPointerException.

Here is an example:

 String a = null;  
 System.out.print("123".equals(a));     // false  
 System.out.print(a.equals("123"));     // java.lang.NullPointerException