StringIndexOutOfBoundException for unknown reason

StringIndexOutOfBoundException for unknown reason

I have no clue, why I get an StringIndexOutOfBoundExeption at this point:
if (!dataList.get(i).toString().contains("null")){
shortList.add(dataList.get(i).toString()); }
else {shortList.add("");}
if (!dataList.get(i+1).toString().contains("null")){
shortList.add(dataList.get(i + 1).toString());}
else {shortList.add("");}
if (!dataList.get(i+2).toString().contains("null")){
shortList.add(dataList.get(i + 2).toString());}
else {shortList.add("");}
if (!dataList.get(i+3).toString().contains("null")){
shortList.add(dataList.get(i + 3).toString()); }
else {shortList.add("");}
if (!dataList.get(i+4).toString().contains("null")){
shortList.add(dataList.get(i + 4).toString()); }
else {shortList.add("");}
This is inside a loop, which adds the data from this ArrayList:
[null, 00010, 2013, 2.0, null, and so on. As you can see, the first and
fifth element is null.
When I check the first element, for if it contains "null", it throws the
Exception. When I check the fifth element, for if it contains "null", it
doesn't, and displays the " " in the UI.
What is the reason for this?
And if I comment the if-statement for the first element out, my UI shows
me "null". (This is code from my android application, the TextView, which
displays the first element of the ArrayList displays "null")