[java] how to sort an ArrayList in ascending order using Collections and Comparator

How to sort an ArrayList in ascending order using Comparator? I know how to sort it in descending order using:

Comparator mycomparator = Collections.reverseOrder();

then

Collections.sort(myarrayList,mycomparator);

just want to know how to sort it in ascending order using Collections and comparator? Thanks!

Two ways to get this done:

Collections.sort(myArray)

given elements inside myArray implements Comparable

Second

Collections.sort(myArray, new MyArrayElementComparator());

where MyArrayElementComparator is Comparator for elements inside myArray

Here a complete example :

Suppose we have a Person class like :

public class Person
{
    protected String fname;
    protected String lname;

    public Person()
    {

    }

    public Person(String fname, String lname)
    {
        this.fname = fname;
        this.lname = lname;
    }

    public boolean equals(Object objet)
    {
        if(objet instanceof Person)
        {
            Person p = (Person) objet;
            return (p.getFname().equals(this.fname)) && p.getLname().equals(this.lname));
        }
        else return super.equals(objet);
    }

    @Override
    public String toString()
    {
        return "Person(fname : " + getFname + ", lname : " + getLname + ")";
    }

    /** Getters and Setters **/
}

Now we create a comparator :

import java.util.Comparator;

public class ComparePerson implements Comparator<Person>
{
    @Override
    public int compare(Person p1, Person p2)
    {
        if(p1.getFname().equalsIgnoreCase(p2.getFname()))
        {
            return p1.getLname().compareTo(p2.getLname());
        }
        return p1.getFname().compareTo(p2.getFname());
    }
}

Finally suppose we have a group of persons :

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Group
{
    protected List<Person> listPersons;

    public Group()
    {
        this.listPersons = new ArrayList<Person>();
    }

    public Group(List<Person> listPersons)
    {
        this.listPersons = listPersons;
    }

    public void order(boolean asc)
    {
        Comparator<Person> comp = asc ? new ComparePerson() : Collections.reverseOrder(new ComparePerson());
        Collections.sort(this.listPersons, comp);
    }

    public void display()
    {
        for(Person p : this.listPersons)
        {
            System.out.println(p);
        }
    }

    /** Getters and Setters **/
}

Now we try this :

import java.util.ArrayList;
import java.util.List;

public class App
{
    public static void main(String[] args)
    {
        Group g = new Group();
        List listPersons = new ArrayList<Person>();
        g.setListPersons(listPersons);

        Person p;

        p = new Person("A", "B");
        listPersons.add(p);

        p = new Person("C", "D");
        listPersons.add(p);

        /** you can add Person as many as you want **/

        g.display();

        g.order(true);
        g.display();

        g.order(false);
        g.display();
    }
}

Just throwing this out there...Can't you just do:

Collections.sort(myarrayList);

It's been awhile though...

Sort By Value

  public Map sortByValue(Map map, final boolean ascending) {
            Map result = new LinkedHashMap();
            try {
                List list = new LinkedList(map.entrySet());

                Collections.sort(list, new Comparator() {
                    @Override
                    public int compare(Object object1, Object object2) {
                        if (ascending)
                            return ((Comparable) ((Map.Entry) (object1)).getValue())
                                    .compareTo(((Map.Entry) (object2)).getValue());
                        else
                            return ((Comparable) ((Map.Entry) (object2)).getValue())
                                    .compareTo(((Map.Entry) (object1)).getValue());

                    }
                });

                for (Iterator it = list.iterator(); it.hasNext();) {
                    Map.Entry entry = (Map.Entry) it.next();
                    result.put(entry.getKey(), entry.getValue());
                }

            } catch (Exception e) {
                Log.e("Error", e.getMessage());
            }

            return result;
        }

Use the default version:

Collections.sort(myarrayList);

Of course this requires that your Elements implement Comparable, but the same holds true for the version you mentioned.

BTW: you should use generics in your code, that way you get compile-time errors if your class doesn't implement Comparable. And compile-time errors are much better than the runtime errors you'll get otherwise.

List<MyClass> list = new ArrayList<MyClass>();
// now fill up the list

// compile error here unless MyClass implements Comparable
Collections.sort(list); 

This might work?

Comparator mycomparator = 
    Collections.reverseOrder(Collections.reverseOrder());