Here I am trying to list some of the very basic utilities of commons lang project which can save our application from petty issues and reduce the code size.

1. ArrayUtils: These functions can also be used for primitive types directly. Check the API page (http://commons.apache.org/lang/api-release/index.html)

   ArrayUtils.add(Object[] array, Object element) //adds object to the array
   ArrayUtils.add(Object[] array, index, Object element) //adds object to the array at specified index
   ArrayUtils.addAll(Object[] array1, Object[] array2) //add all objects to the array and create combined array
   ArrayUtils.contains(Object[] array, Object element) //checks if object is part of array or not
   ArrayUtils.indexOf(Object[] array, Object elementToFind) //finds index of object in array
   ArrayUtils.remove(Object[] array, Object elementToremove) //removes object from the array

2. StringUtils: I find this really useful while handling Strings. Go through the API of this class. I am sure you can use this class and save a lot of time & bugs.

   StringUtils.abbreviate(String str, int maxwidth); //use this while showing long strings on UI. it will use  ellipse at the end.
   StringUtils.isBlank(String str); //Checks null and content and returns true in case only blanks are there is string
   StringUtils.isEmpty(String str);//Check null and returns true in case string is empty i.e. no blanks also.
   StringUtils.isNumeric(String str);// Checks if String is numeric
   StringUtils.trim(String str); // Trims the string to only the content and removes blanks
   StringUtils.isAlphnumeric(String str); //Checks if string is alphanumeric
   StringUtils.countMatches(String str, Sting sub);// Count number of matches of sub string in str.

3. StringEscapeUtils: Another possible source of error. Use this to avoid issues and confusions. Keep your string as it is and use these escape functions before using them accordingly.

   StringEscapeUtils.escapeCsv(); // Escapes string for CSV
   StringEscapeUtils.escapeJava();// Escapes string for Java
   StringEscapeUtils.escapeSql();// Escapes string for Sql
   StringEscapeUtils.escapeXml();// Escapes string for XML
   StringEscapeUtils.unescapeCsv(); // Yes you can un-escape also 
   

4. DateUtils : A Utility wrapper on Date and Calendar objects and does provide many functions on it.

   DateUtils.addDays(Date date, int amount); // Add days to the current date, see more in API for similar functions
   DateUtils.isSameDay(Calendar cal1, Calendar cal2); // Checks if cal1 and cal2 are same day
   

5. SystemUtils : Set of functions to find System properties

   SystemUtils.IS_JAVA_1_4;
   SystemUtils.IS_OS_LINUX;
   SystemUtils.USER_TIMEZONE;
   SystemUtils.getJavaHome();
   

I am sure you can save time, code size and a lot of petty issues in code if you use commons. It is widely used by many open source projects.