Question

I have a class which holds a list inside a list, and its declared as type String, but i no longer know that its a string.

private ArrayList<List<String>> table;
table = new ArrayList<List<String>>();
table.add(new ArrayList<String>()); //is possible
table.add(new ArrayList<Integer>()); //not possible

But now the nested list could either be string or integer. How can i fix that? I also can’t declare Object must either be string or integer

Also can a single Arraylist hold both ints and strings without declaring as objects?

Edit: It seems you guys are debating the right answer. I will insight why i chose Vikdor answer.

My job is to load a csv, and then user types in command to define each column as string or int. My table class has a private instance variable, and has methods which adds the values. Doing it in Vikdor way makes performing operations on table very easy where i want to merge two table or sort. It will also prevent user from inputting non string or int through an exception.

Answer 1

You might be looking for this:

List<List<? extends Object>> table;
table = new ArrayList<List<? extends Object>>();
table.add(new ArrayList<String>()); 
table.add(new ArrayList<Integer>()); 

This would result in unchecked code when you retrieve the inner lists, as follows (which obviously is not recommended in production code):

List<String> stringList = (List<String>) table.get(0); // Unchecked cast.

From OP’s comment:

Because its my requirement to have it as either int or string. Else you can put more than those types in the list. The nested list holds value of a table in columns and each column holds int or string. Do you have any other dynamic way of reading csv file and storing in table?

It is better to use a proper CSV processor (like SuperCSV) that would parse directly into a POJO with appropriate data types associated with each column.