Collections.sort() using comparator?

Question

import java.util.*;

public class C_2 {
    public static void main(String args[]) {
        String theStrings[] = { "x", "a", "b", "c", "d" };
        List l = Arrays.asList(theStrings);
        Collections.sort(l);                            // line a
        Collections.sort(l, new ThisIsMyThing());       // line b
        System.out.println(l);
    }
}

class ThisIsMyThing implements Comparator {
    public int compare(Object o1, Object o2) {
        String s1 = (String)o1;
        String s2 = (String)o2;

        return -1 * s1.compareTo(s2);
    }
}

I understand that class C_2 does sorting based on two different techniques. One is the standard Collections.sort(l); And the other is Collections.sort(l,Comparator<>()); I am not able to understand this sort method. Can someone please explain it to me?

Solution 1

Collection.sort(l) assumes that the contents of l are Comparable. Collection.sort(1, Comparator) uses a custom comparator to compare the contents of l, this is what you did. The very idea of sorting (including the sort() method) implies the objects MUST be comparable - in this case, with either Comparable or Comparator.

Note that many Java objects are comparable already, including String, Date and Number. For those, you can just use Collection.sort(someList);

Example

Say you have a Circle class

public class Circle {
    double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    public double getArea(){
        return radius * radius * Math.PI;
    }
}

If you created 100 Circle objects:

ArrayList<Circle> circleList = new ArrayList<>();

for (int i = 0; i < 100; i++) {
    // adds a circle with random radius
    circleList.add(new Circle((int)(Math.random() * 100)));
}

// try to sort the list
Collections.sort(circleList);   //compilation error: must be Comparable

You can't sort them because Java has no idea how to compare them. You have to tell this to Java:

public class Circle implements Comparable<Circle> {
    double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    // you MUST override the compareTo method from the Comparable interface
    @Override
    public int compareTo(Circle cirlce){
        if (this.getArea() > circle.getArea())
            return 1;
        else if (this.getArea() == circle.getArea())
            return 0;
        else 
            return -1;
    }

    public double getArea(){
        return radius * radius * Math.PI;
    }
}

With the compareTo() method in the Circle class, Java now knows how to compare them and can sort them.

Now you can do this:

Collections.sort(circleList);
// Yayyy I'm being sorted by the size of my areas!!!!!

Solution 2

Collections.sort which takes comparator sorts the List based on the Comparator provided by you and other follows natural sorting order.That is if you want to follow any custom sorting order then use this method. And nothing much to explain in that.

Use this link for explaination.

Comments

[hakuna12]

Thanks for the explanation. Collections.sort() is possible only on a list ?

[Louis Wasserman]

@peeskillet: ...what? Collections.sort only accepts a List argument.

[Louis Wasserman]

@peeskillet: But even your revised version isn't true. Collections.sort is a static method on the Collections class that only accepts a List argument. Any class can invoke Collections.sort, whether it implements Collection or not, but no matter who's invoking it, only a List argument can be passed to it.