Sorting an ArrayList of Person with java collections

Collections.sort(List<T>) method expects the element of the list it is sorting to be comparable. Either the element type T should implement the Comparable interface, or you should use the overloaded sort() method, that takes a generic Comparator instance.

In the code below, you are satisfying neither of the above conditions. Neither your Person class implements Comparable, nor you are passing any Comparator instance.

ArrayList<Person> nameFromText = new ArrayList<Person>();
fillArrayList(nameFromText, pullFile);
// Sort ArrayList
Collections.sort(nameFromText);  // How to sort?

You should create a Comparator for your Person class to tell the sort() method how to sort it (may be on String stored in Person class)

Here's how you implement a generic comparator:

public class PersonNameComparator implements Comparator<Person> {
    public int compare(Person p1, Person p2) {
        return p1.getName().compareTo(p2.getName());
    }
}

And then your Collections.sort() method invocation should be like: -

Collections.sort(nameFromText, new PersonNameComparator());

Alternatively you can implement Comparable interface directly in Person class and override 'compareTo(Object obj) ' method. In this case you would not need to create new class for comparator. And this behaves like inbuilt sorting.

Try this:

List<String> inputString =  Arrays.asList("Sijan", "Sudeep", "Parasar", "Raj Kumar");
Collections.sort(inputString);
System.out.println(inputString);