How to sort ArrayList using Comparator? [duplicate]

Question

I have a Class Student that Implements a static method

public static Comparator<Student> getCompByName()

that returns a new comparator object for Student that compares 2 Students objects by the attribute 'name'.

I need to now test this by sorting a students ArrayList by 'name' using my function getCompByName().

Here is my Comparator method in my Student class.

public static Comparator<Student> getCompByName()
{   
 Comparator comp = new Comparator<Student>(){
     @Override
     public int compare(Student s1, Student s2)
     {
         return s1.name.compareTo(s2.name);
     }        
 };
 return comp;
}

And Main where I need to test

public static void main(String[] args)
{
    // TODO code application logic here

    //--------Student Class Test-------------------------------------------
    ArrayList<Student> students = new ArrayList();
    Student s1 = new Student("Mike");
    Student s2 = new Student("Hector");
    Student s3 = new Student("Reggie");
    Student s4 = new Student("zark");
    students.add(s1);
    students.add(s2);
    students.add(s3);
    students.add(S4);

    //Use getCompByName() from Student class to sort students

Can anyone show me how I would use the getCompByName() in my main to actually sort the ArrayList by name? Im new to comparators and having a hard time with their usages. The method returns a comparator, so Im not sure how this will be implemented. I know I need to use getCompByName() to sort, im just not sure how to implement it.

@SotiriosDelimanolis I need to use the getCompByName() to sort, not another algoritm — Reeggiie
– Reeggiie, Commented Mar 11, 2014 at 0:21
I think you misunderstand what a Comparator does. On its own it just compares two elements. You have to actually write (or use the ones in the JDK) the sorting algorith,. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 11, 2014 at 0:22
@user3345200 You may want to have a look at the official tutorial on Object Ordering. It is concise and well-written and does a good job at explaining how to implement and use a Comparator to sort objects. — Jason C
– Jason C, Commented Mar 11, 2014 at 0:24

Kevin Bowersox · Accepted Answer · 2014-03-11 00:33:56Z

16

Use the Collections.sort(List, Comparator) method:

Collections.sort(students, Student.getCompByName());

Also in your code it would be good to use the List interface when declaring the List:

List<Student> students = new ArrayList();

You could also tighten up the code by using a Student[] and passing it to the ArrayList constructor:

public static void main(String[] args) {
    Student[] studentArr = new Student[]{new Student("Mike"),new Student("Hector"), new Student("Reggie"),new Student("zark")};
    List<Student> students = new ArrayList<Student>(Arrays.asList(studentArr));
    Collections.sort(students, Student.getCompByName());

    for(Student student:students){
        System.out.println(student.getName());
    }
}

Here is a Gist of the full source.

edited Mar 11, 2014 at 0:33

answered Mar 11, 2014 at 0:21

Kevin Bowersox

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1 Comment

Costi Ciudatu Over a year ago

That's Collections.sort(*List*, Comparator).

fge · Accepted Answer · 2014-03-11 00:27:47Z

5

Use Collections.sort():

Collections.sort(students, getCompByName());

Note: might be useful to make your comparator a private static final variable.

Note 2: modifies the list in place; does not create a new list.

edited Mar 11, 2014 at 0:27

answered Mar 11, 2014 at 0:21

fge

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