How do I sort file names by their extension?

To sort file names by their extension we can use the ExtensionFileComparator class from the Apache Commons IO library. This class provide a couple instances of comparator such as:

Comparator Description
EXTENSION_COMPARATOR Case sensitive extension comparator
EXTENSION_REVERSE Reverse case sensitive extension comparator
EXTENSION_INSENSITIVE_COMPARATOR Case insensitive extension comparator
EXTENSION_INSENSITIVE_REVERSE Reverse case insensitive extension comparator
EXTENSION_SYSTEM_COMPARATOR System sensitive extension comparator
EXTENSION_SYSTEM_REVERSE Reverse system sensitive path comparator

The following snippet show you how to use the first two comparators listed above.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FilenameUtils;
import static org.apache.commons.io.comparator.ExtensionFileComparator.*;

import java.io.File;
import java.util.Arrays;

public class FileSortByExtension {
    public static void main(String[] args) {
        File file = new File(".");

        // Excludes directory in the list
        File[] files = file.listFiles(File::isFile);

        if (files != null) {
            // Sort in ascending order.
            Arrays.sort(files, EXTENSION_COMPARATOR);
            FileSortByExtension.displayFileOrder(files);

            // Sort in descending order.
            Arrays.sort(files, EXTENSION_REVERSE);
            FileSortByExtension.displayFileOrder(files);
        }
    }

    private static void displayFileOrder(File[] files) {
        System.out.printf("%-20s | %s%n", "Name", "Ext");
        System.out.println("--------------------------------");
        for (File file : files) {
            System.out.printf("%-20s | %s%n", file.getName(),
                    FilenameUtils.getExtension(file.getName()));
        }
        System.out.println("");
    }
}

The result of the code snippet:

Name                 | Ext
--------------------------------
README               | 
lipsum.doc           | doc
lipsum.docx          | docx
data.html            | html
contributors.txt     | txt
pom.xml              | xml

Name                 | Ext
--------------------------------
pom.xml              | xml
contributors.txt     | txt
data.html            | html
lipsum.docx          | docx
lipsum.doc           | doc
README               | 

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.5</version>
</dependency>

How do I sort files and directories based on their size?

In this example you will learn how to sort files and directories based on their size. Using the Apache Commons IO we can utilize the SizeFileComparator class. This class provides some instances to sort file size such as:

Comparator Description
SIZE_COMPARATOR Size comparator instance – directories are treated as zero size
SIZE_REVERSE Reverse size comparator instance – directories are treated as zero size
SIZE_SUMDIR_COMPARATOR Size comparator instance which sums the size of a directory’s contents
SIZE_SUMDIR_REVERSE Reverse size comparator instance which sums the size of a directory’s contents

Now let’s jump to the code snippet below:

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.util.Arrays;

import static org.apache.commons.io.comparator.SizeFileComparator.*;

public class FileSortBySize {
    public static void main(String[] args) {
        File dir = new File(".");
        File[] files = dir.listFiles();

        if (files != null) {
            // Sort files in ascending order based on file size.
            System.out.println("Ascending order.");
            Arrays.sort(files, SIZE_COMPARATOR);
            FileSortBySize.displayFileOrder(files, false);

            // Sort files in descending order based on file size
            System.out.println("Descending order.");
            Arrays.sort(files, SIZE_REVERSE);
            FileSortBySize.displayFileOrder(files, false);

            // Sort files in ascending order based on file / directory
            // size
            System.out.println("Ascending order with directories.");
            Arrays.sort(files, SIZE_SUMDIR_COMPARATOR);
            FileSortBySize.displayFileOrder(files, true);

            // Sort files in descending order based on file / directory
            // size
            System.out.println("Descending order with directories.");
            Arrays.sort(files, SIZE_SUMDIR_REVERSE);
            FileSortBySize.displayFileOrder(files, true);
        }
    }

    private static void displayFileOrder(File[] files, boolean displayDirectory) {
        for (File file : files) {
            if (!file.isDirectory()) {
                System.out.printf("%-25s - %s%n", file.getName(),
                        FileUtils.byteCountToDisplaySize(file.length()));
            } else if (displayDirectory) {
                long size = FileUtils.sizeOfDirectory(file);
                String friendlySize = FileUtils.byteCountToDisplaySize(size);
                System.out.printf("%-25s - %s%n", file.getName(),
                        friendlySize);
            }
        }
        System.out.println("------------------------------------");
    }
}

In the code snippet above we use a couple method from the FileUtils class such as the FileUtils.sizeOfDirectory() to calculate the size of a directory and FileUtils.byteCountToDisplaySize() to create human readable file size.

The result of the code snippet:

Ascending order.
.editorconfig             - 389 bytes
kodejava.iml              - 868 bytes
pom.xml                   - 1 KB
------------------------------------
Descending order.
pom.xml                   - 1 KB
kodejava.iml              - 868 bytes
.editorconfig             - 389 bytes
------------------------------------
Ascending order with directories.
.editorconfig             - 389 bytes
src                       - 851 bytes
kodejava.iml              - 868 bytes
pom.xml                   - 1 KB
apache-commons-example    - 8 KB
hibernate-example         - 29 KB
.idea                     - 85 KB
------------------------------------
Descending order with directories.
.idea                     - 85 KB
hibernate-example         - 29 KB
apache-commons-example    - 8 KB
pom.xml                   - 1 KB
kodejava.iml              - 868 bytes
src                       - 851 bytes
.editorconfig             - 389 bytes

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.5</version>
</dependency>

How do I sort string of numbers in ascending order?

In the following example we are going to sort a string containing the following numbers "2, 5, 9, 1, 10, 7, 4, 8" in ascending order so we will get the result of "1, 2, 4, 5, 7, 8, 9, 10".

package org.kodejava.example.util;

import java.util.Arrays;
import java.util.Collections;

public class SortStringNumber {
    public static void main(String[] args) {
        // We have some string numbers separated by comma. First we
        // need to split it so we can get each individual number.
        String data = "2, 5, 9, 1, 10, 7, 4, 8";
        String[] numbers = data.split(",");

        // Convert the string numbers into Integer and placed it into
        // an array of Integer.
        Integer[] intValues = new Integer[numbers.length];
        for (int i = 0; i < numbers.length; i++) {
            intValues[i] = Integer.parseInt(numbers[i].trim());
        }

        // Sort the number in ascending order using the
        // Collections.sort() method.
        Collections.sort(Arrays.asList(intValues));

        // Convert back the sorted number into string using the
        // StringBuilder object. Prints the sorted string numbers.
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < intValues.length; i++) {
            Integer intValue = intValues[i];
            builder.append(intValue);
            if (i < intValues.length - 1) {
                builder.append(", ");
            }
        }
        System.out.println("Before = " + data);
        System.out.println("After  = " + builder.toString());
    }
}

When we run the program we will get the following output:

Before = 2, 5, 9, 1, 10, 7, 4, 8
After  = 1, 2, 4, 5, 7, 8, 9, 10