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boolean isString(ArrayList<Object> arr, int idx) {
                if (arr == null || idx < 0 || idx >= arr.size()) return false;
                return  arr.get(idx) instanceof String  ? true : false;
    }

Maybe with instanceof operator? Like
if (element instanceof String) {
//do stuff
}

yes, you are partially right. The explanation to this is:

as in ArrayList each element will be stored as its corresponding wrapper class, i.e. int as Integer, float as Float, double as Double, and hence now we may apply:

if(element instanceof yourwrapper_datatype)
{
//do stuff
}

Each non-primitive type (like Integer but not int), inherits from the Object class the method getClass(). this getClass() further supports .getName(), which returns the name of the class of the object.

For example:

String s="blabla";
System.out.println(s.getClass().getName()); //returns java.lang.String

Integer i=1;
System.out.println(s.getClass().getName()); //returns java.lang.Integer

Perform a divison or multiplication if its throws an exception its a string .