How to sort a array of tuple if the tuple is a pre-defined class without implement comparator in Java

Question

I have a pre-defined tuple class which cannot be modified as follows:

public class Tuple {
    protected String name; 
    public Tuple(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

I have another class which defines an array of the above tuple:

public class Stack {
    protected ArrayList<Tuple> tupleLst; 
    public Stack(){
        this.tupleLst=new ArrayList<Tuple>(10);
    }
}

Then I need to sort the array of tuple in another class: for instance:

public class Algo {
    public static int Sort(Relation r){

    }
    public static void main(String[] arg){
    Algo.Sort();
}

The Relation r is the input array of tuples. I have searched online, where I saw many people actually are using Comparators and Collections.sort() to do that. However, after I tried, I noticed that the Tuple class need to implement comparators in order to sort. How can I sort the tuple without changing the first two classes? Thank you in advance!

Answer 1

Although Tuple cannot be modified, you can implement a Comparator of your own and then pass it to the Collections.sort() method (which will do a sort of your collection of tuples based on the comparator rule(s). For example:

Comparator<Tuple> myComparator = new Comparator<Tuple>() {
    public int compare(Tuple t1, Tuple t2) {
        //the comparison rules go here
    }
};
Collections.sort(tupleList, myComparator);

If you use Java8, you can achieve this in a single line:

myList.sort((t1, t2) -> { <comparison rules implementation> });

or (thanks to @MarkoTopolnik)

myList.sort(Comparator.comparing(t -> <<get sort key from t>>) 

Answer 2

I think you're confusing two methods. One way is to make your Tuple class implement the Comparable interface. I think this is the one you want to avoid since it seems you don't want to modify the Tuple class.

The other way is to define a new class, whose purpose is to compare two Tuples. This is what is called a Comparator. If you pass your list of Tuples and an instance of this comparator then you can sort your Tuples any way you like without modifying the Tuple class.