Custom comparator in Java

Hi friends,

Let’s look into the code how to make a custom comparator that sorts a list, according to the field we pass, as a parameter. Both the field and the list are passed as parameters.

Initially, we create two comparators – one for sorting the list by “id” field and another for sorting by “name” field.

Next, we write a static sort method, which accepts a list and a field of type String. The logic inside this method decides how to sort the list.

Let’s move onto a sample code. Actually I’ve found this logic on another website while i was digging on internet.

import crit.pojo.UtilPojo;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class CustomComparator {

    private static Comparator<UtilPojo> idComparator = new Comparator<UtilPojo>() {

        public int compare(final UtilPojo pojo1, final UtilPojo pojo2) {
            return Long.valueOf(pojo1.getId()).compareTo(Long.valueOf(pojo2.getId()));
        }
    };
    private static Comparator<UtilPojo> nameComparator = new Comparator<UtilPojo>() {

        public int compare(final UtilPojo pojo1, final UtilPojo pojo2) {
            return pojo1.getName().compareTo(pojo2.getName());
        }
    };

    public static List<UtilPojo> sort(final List<UtilPojo> list, final String field) throws Exception {
        try {
            final Comparator<UtilPojo> comparator;
            if (field.equals("id")) {
                comparator = idComparator;
            } else if (field.equals("name")) {
                comparator = nameComparator;
            } else {
                throw new IllegalArgumentException("Comparator not found for the field, " + field);
            }
            Collections.sort(list, comparator);
            return list;

        } catch (Exception e) {
            throw new Exception(e.getMessage());
        }
    }
}

The POJO, type of the list created is given below.

public class UtilPojo {

    private String name;
    private long id;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public UtilPojo(long id, String name) {
        this.id = id;
        this.name = name;
    }

    public UtilPojo() {
    }
}

Let’s see an example using this “Custom Comparator” in the next post.

Thanks.