Android-java- How to sort a list of objects by a certain value within the object

Question

Im trying to sort through an arraylist of objects by a particular value within the object. What would be the best approach to do such a thing. Should I use Collections.sort() with some kind of comparator?

Im trying to sort a list of objects by a float value they hold in one of the variables.

EDIT: This is what I have so far:

public class CustomComparator implements Comparator<Marker> {
    @Override
    public int compare(Mark o1, Mark o2) {
        return o1.getDistance().compareTo(o2.getDistance());
    }
}

the error states: Cannot invoke compareTo(double) on the primitive type double.

Is it because a comparator cant return anything other than a certain type?

Solution 1

You should use Comparable instead of a Comparator if a default sort is what your looking for.

See here, this may be of some help - When should a class be Comparable and/or Comparator?

Try this -

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

public class TestSort {

    public static void main(String args[]){

        ToSort toSort1 = new ToSort(new Float(3), "3");
        ToSort toSort2 = new ToSort(new Float(6), "6");
        ToSort toSort3 = new ToSort(new Float(9), "9");
        ToSort toSort4 = new ToSort(new Float(1), "1");
        ToSort toSort5 = new ToSort(new Float(5), "5");
        ToSort toSort6 = new ToSort(new Float(0), "0");
        ToSort toSort7 = new ToSort(new Float(3), "3");
        ToSort toSort8 = new ToSort(new Float(-3), "-3");

        List<ToSort> sortList = new ArrayList<ToSort>();
        sortList.add(toSort1);
        sortList.add(toSort2);
        sortList.add(toSort3);
        sortList.add(toSort4);
        sortList.add(toSort5);
        sortList.add(toSort6);
        sortList.add(toSort7);
        sortList.add(toSort8);

        Collections.sort(sortList);

        for(ToSort toSort : sortList){
            System.out.println(toSort.toString());
        }
    }

}

public class ToSort implements Comparable<ToSort> {

    private Float val;
    private String id;

    public ToSort(Float val, String id){
        this.val = val;
        this.id = id;
    }

    @Override
    public int compareTo(ToSort f) {

        if (val.floatValue() > f.val.floatValue()) {
            return 1;
        }
        else if (val.floatValue() <  f.val.floatValue()) {
            return -1;
        }
        else {
            return 0;
        }

    }

    @Override
    public String toString(){
        return this.id;
    }
}

Solution 2

Follow this code to sort any ArrayList

Collections.sort(myList, new Comparator<EmployeeClass>(){
    public int compare(EmployeeClass obj1, EmployeeClass obj2) {
        // ## Ascending order
        return obj1.firstName.compareToIgnoreCase(obj2.firstName); // To compare string values
        // return Integer.valueOf(obj1.empId).compareTo(Integer.valueOf(obj2.empId)); // To compare integer values

        // ## Descending order
        // return obj2.firstName.compareToIgnoreCase(obj1.firstName); // To compare string values
        // return Integer.valueOf(obj2.empId).compareTo(Integer.valueOf(obj1.empId)); // To compare integer values
        }
    });

Solution 3

I think this will help you better

Person p = new Person("Bruce", "Willis");
Person p1  = new Person("Tom", "Hanks");
Person p2 = new Person("Nicolas", "Cage");
Person p3 = new Person("John", "Travolta");

ArrayList<Person> list = new ArrayList<Person>();
list.add(p);
list.add(p1);
list.add(p2);
list.add(p3);

Collections.sort(list, new Comparator() {
    @Override
    public int compare(Object o1, Object o2) {
        Person p1 = (Person) o1;
        Person p2 = (Person) o2;
        return p1.getFirstName().compareToIgnoreCase(p2.getFirstName());
    }
});

Solution 4

Now no need to Boxing (i.e no need to Creating OBJECT using new Operator use valueOf insted with compareTo of Collections.Sort..)

1)For Ascending order

Collections.sort(temp, new Comparator<XYZBean>() 
{
     @Override
     public int compare(XYZBean lhs, XYZBean rhs) {

       return Integer.valueOf(lhs.getDistance()).compareTo(rhs.getDistance());
      }
 });

1)For Deascending order

Collections.sort(temp, new Comparator<XYZBean>() 
{
     @Override
     public int compare(XYZBean lhs, XYZBean rhs) {

       return Integer.valueOf(rhs.getDistance()).compareTo(lhs.getDistance());
      }
 });

Solution 5

It's very easy for Kotlin!

listToBeSorted.sortBy { it.distance }

View more solutions

Comments

[James andresakis]

But how do I make it sort by a value within the object. Thats what Im stuck on.

[James andresakis]

I went ahead and implemented Comparable in my class but and created a method to call sort on the list when i need to have it sorted but how do I sort it by a value with in the object?

[Jave]

The compareTo()-method is where you do the comparison. A little googling gave several detailed examples on how to use it, here is one of them: javadeveloper.co.in/java-example/java-comparable-example.htm‌​l

[James andresakis]

I set up a method similar to the example however I get an error stating I cant use compareTo on a cast double. It seems like for what ever reason, what Im doing it doesnt like. Cannot invoke compareTo(double) on the primitive type double. Ill add my code up above to show what I mean

[James andresakis]

Hey thanks for the link :) I go back and forth from one language to the next so Im always missing something :p you know how it goes......jack of all trades but a master of none lol

[John Smith]

This should be the top answer!

[Sadashiv]

simple & great answer, kudos bro:)

[Ambran]

This example works for comparing strings. For sorting by comparing integers look at Pranav's answer below.

[Meet Vora]

Simple and small.. Thanks!

[Anurag]

This seems really clean. thanks. And One plus for provided tips.

[Sohail Yasin]

Best and simple answer. saved my time

[Innocent]

great answers are always from great people, thank you so much you

[Vaclovas Rekašius Jr.]

This should really be the real answer, quick, simple, clear. Thanks.

[Atif Mahmood]

@Themelis it's java function

[Ali Zarei]

int compare can be replaced by: return Integer.compare(obj1.empId, obj2.empId);

[tenTen]

Perfectly Working! Thanks bro

[toto_tata]

Stackoverflow should add a 'donate' button next to such wonderful answers :)

[User]

this places names starting from numbers (i.e 1gbfilename) on bottom. how they can be placed at top?

[Snigdhajyoti]

same answer is already given. You can also see different different approaches too.