Java sorting List

Question

I got a List in java. I get values from a SQL query.

public void ReloadPages() throws Exception {        
    try (Connection conn = Framework.GetDatabaseManager().GetBone().getConnection()) {
        try (ResultSet set = conn.createStatement().executeQuery("SELECT * FROM habbo_shop_pages")) {
            while (set.next()) {
                int Id = set.getInt(1);

                Pages.put(Id, new CatalogPage(set));
            }
        }
    }

    System.out.println("Loaded " + Pages.size() + " Catalog Page(s).");
}

Then I store it all. In another function, I want to retrieve certain pages from a parentid.

public LinkedList<CatalogPage> getSubPages(int parentId) {
    LinkedList<CatalogPage> pages = new LinkedList<>();

    for (CatalogPage page : this.Pages.values()) {
        if (page.getParentId() != parentId) {
            continue;
        }

        pages.add(page);
    }

    return pages;
}

How do I order the list? Now id 4 is above in the shop and 1 at the bottom, but I want it ordered by id. ORDER BY in query doesn't work.

Does CatalogPage implement Comparable interface?

Vishal K
– Vishal K

2013-03-23 12:21:24 +00:00
Commented Mar 23, 2013 at 12:21 — Vishal K
– Vishal K, Commented Mar 23, 2013 at 12:21

mre · Accepted Answer · 2013-03-23 11:28:44Z

21

Have your class implement Comparable and provide said sort ordering in compareTo method. And to sort, simply use Collections#sort, although be aware that it's an inline sort.

answered Mar 23, 2013 at 11:28

mre

44.4k33 gold badges122 silver badges170 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

dkellycollins Over a year ago

You could also just create a new Comparator instead of implementing Comparable.

Pharap Over a year ago

@dkellycollins Comparator is indeed more flexible. Having a single comparison method is fine for values that can only be compared exactly, but for aggregated values you often need more than one way to compare them.

Achintya Jha · Accepted Answer · 2013-03-23 11:28:46Z

14

You want to sort the list in reverse order then try this:

Collections.sort(list,Collections.reverseOrder());

answered Mar 23, 2013 at 11:28

Achintya Jha

12.9k2 gold badges30 silver badges40 bronze badges

1 Comment

user2199080 Over a year ago

I tried using this, now it doesn't display anything in the shop at all.

Community · Accepted Answer · 2017-05-23 12:31:15Z

Like mre writes, you can let the elements in the list implement the Comparable interface. Alternatively the Collections#sort(List, Comparator) method, which allows you to sort on different keys, can be used. This can for example be used handle sorting on multiple fields of the elements in the list, e.g. implement a Comparator to sort on date and a different Comparator for sorting on id.

For more info look at the javadoc for the Comparator interface.

Example

Code:

package com.stackoverflow.questions;

import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;

public class Question15586200 {

    public static void main(String[] args) {
        // Add test data
        LinkedList<CatalogPage> catalogs = new LinkedList<CatalogPage>();
        catalogs.add(new CatalogPage("foo 4", 4));
        catalogs.add(new CatalogPage("bar 1", 1));
        catalogs.add(new CatalogPage("foobar 2", 2));
        catalogs.add(new CatalogPage("barfoo 3", 3));

        // Sort by id
        Collections.sort(catalogs, new Comparator<CatalogPage>() {

            @Override
            public int compare(CatalogPage o1, CatalogPage o2) {
                return Integer.compare(o1.getId(), o2.getId());
            }
        });

        // Print result
        for (CatalogPage page : catalogs) {
            System.err.println(page);
        }
    }

    public static class CatalogPage {
        private String name;
        private int id;

        public CatalogPage(String name, int id) {
            this.name = name;
            this.id = id;
        }

        public int getId() {
            return id;
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return "CatalogPage [name=" + name + ", id=" + id + "]";
        }
    }
}

Ouput:

CatalogPage [name=bar 1, id=1]
CatalogPage [name=foobar 2, id=2]
CatalogPage [name=barfoo 3, id=3]
CatalogPage [name=foo 4, id=4]

Georgios · Accepted Answer · 2014-08-30 21:37:44Z

4

You can do it using Lambda expressions, added in Java version 8.

Comparator<ProductColor> byProductColorName = (color1, color2) -> Integer.compare(
    color1.getId(), color2.getId());

myProductColorList.stream().sorted(byProductColorName)
    .forEach(e -> System.out.println(e));

Also i found this post very usefull.

answered Aug 30, 2014 at 21:37

Georgios

20.2k9 gold badges99 silver badges100 bronze badges

1 Comment

VGR Over a year ago

I would favor Comparator<ProductColor> byProductColorName = Comparator.comparingInt(ProductColor::getId);.

Ankur Shanbhag · Accepted Answer · 2013-03-23 11:29:39Z

1

List maintains insertion order. If you want to order in by ID, then use TreeSet and write a external Comparator, by implementing a compare() method of Comparator interface and pass it to the TreeSet constructor. Values will be sorted and ordered by ID.

answered Mar 23, 2013 at 11:29

Ankur Shanbhag

7,8042 gold badges31 silver badges38 bronze badges

Collectives™ on Stack Overflow

Java sorting List

5 Answers 5

2 Comments

1 Comment

Example

Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

1 Comment

Example

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related