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Java Collections - Comparator.compare() Examples

Java Collections Java Java API 

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Interface:

java.util.Comparator

java.util.ComparatorComparatorLogicBig

Method:

int compare(T o1, T o2)

This method returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. It is used to compare two objects for sorting/ordering.

Note that Comparator is a functional interface and int compare(T o1, T o2) is the only abstract method. Other declared methods are either default or static.


Examples


package com.logicbig.example.comparator;

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

public class CompareExample {

    public static void main(String... args) {

        //Comparator is a functional interface
        Comparator<Integer> c = (o1, o2) -> {
            if (o1 == o2) {
                return 0;
            } else if (o1 == null) {
                return -1;
            } else if (o2 == null) {
                return 1;
            } else {
                return Integer.compare(o1, o2);
            }
        };

        int result = c.compare(1, 4);
        System.out.println(result);

        result = c.compare(4, 1);
        System.out.println(result);

        result = c.compare(4, 4);
        System.out.println(result);

        //Alternatively
        result = Integer.compare(1, 4);
        System.out.println(result);

        //sorting a list
        List<Integer> list = Arrays.asList(1, 3, 2, 5, 4, 7);
        Collections.sort(list, c);
        System.out.println(list);

        //or
        list = Arrays.asList(1, 3, 2, 5, 4, 7);
        Collections.sort(list, Integer::compareTo);
        System.out.println(list);
    }
}

Output

-1
1
0
-1
[1, 2, 3, 4, 5, 7]
[1, 2, 3, 4, 5, 7]




In this example class Page implements Comparable interface. The difference between interfaces, Comparable and Comparator is; Comparable is implemented by a class to specify how this object should be ordered when sorted as an item in a collection, whereas, a Comparator specifies how to compare two objects to order them in a collection. 

package com.logicbig.example.comparator;

import java.util.*;

public class CompareExample2 {

    public static void main(String... args) {
        List<Page> list = createExamplePages();

        System.out.printf("before sort: %s%n", list);
        Collections.sort(list, Page::compare);
        System.out.printf("after sort: %s%n", list);

        SortedSet<Page> set = new TreeSet<>(createExamplePages());
        System.out.printf("sorted set: %s%n", set);
    }

    private static List<Page> createExamplePages() {
        return Arrays.asList(
                Page.create(3, "content 3"),
                Page.create(2, "content 2"),
                Page.create(1, "content 1"));
    }

    private static class Page implements Comparable<Page> {
        private int index;
        private String content;

        public Page(int index, String content) {
            this.index = index;
            this.content = content;
        }

        @Override
        public int compareTo(Page other) {
            return compare(this, other);
        }


        public static int compare(Page o1, Page o2) {
            if (o1 == o2) {
                return 0;
            } else if (o1 == null) {
                return -1;
            } else if (o2 == null) {
                return 1;
            } else {
                return Integer.compare(o1.index, o2.index);
            }
        }


        @Override
        public String toString() {
            return "Page{" +
                    "index=" + index +
                    ", content='" + content + '\'' +
                    '}';
        }

        public static Page create(int index, String content) {
            return new Page(index, content);
        }
    }
}

Output

before sort: [Page{index=3, content='content 3'}, Page{index=2, content='content 2'}, Page{index=1, content='content 1'}]
after sort: [Page{index=1, content='content 1'}, Page{index=2, content='content 2'}, Page{index=3, content='content 3'}]
sorted set: [Page{index=1, content='content 1'}, Page{index=2, content='content 2'}, Page{index=3, content='content 3'}]

See Also