Best way to list files in Java sorted by Date Modified

Question

I want to get a list of files in a directory  but I want to sort it such that the oldest files are first   My solution was to call File listFiles and just resort the list based on File lastModified  but I was wondering if there was a better way   Edit  My current solution  as suggested  is to use an anonymous Comparator   File   files   directory listFiles     Arrays sort files  new Comparator lt File gt         public int compare File f1  File f2                return Long valueOf f1 lastModified    compareTo f2 lastModified

User · Answer

You might also look at apache commons IO  it has a built in last modified comparator and many other nice utilities for working with files

User · Answer

You can try guava Ordering   Function lt File  Long gt  getLastModified   new Function lt File  Long gt          public Long apply File file            return file lastModified              List lt File gt  orderedFiles   Ordering natural   onResultOf getLastModified                             sortedCopy files

User · Answer

If the files you are sorting can be modified or updated at the same time the sort is being performed     Java 8   private static List lt Path gt  listFilesOldestFirst final String directoryPath  throws IOException       try  final Stream lt Path gt  fileStream   Files list Paths get directoryPath              return fileStream              map Path  toFile               collect Collectors toMap Function identity    File  lastModified                entrySet                stream                sorted Map Entry comparingByValue                   sorted Collections reverseOrder Map Entry comparingByValue         replace the previous line with this line if you would prefer files listed newest first              map Map Entry  getKey               map File  toPath      remove this line if you would rather work with a List lt File gt  instead of List lt Path gt               collect Collectors toList               Java 7  private static List lt File gt  listFilesOldestFirst final String directoryPath  throws IOException       final List lt File gt  files   Arrays asList new File directoryPath  listFiles         final Map lt File  Long gt  constantLastModifiedTimes   new HashMap lt File Long gt         for  final File f   files            constantLastModifiedTimes put f  f lastModified               Collections sort files  new Comparator lt File gt               Override         public int compare final File f1  final File f2                return constantLastModifiedTimes get f1  compareTo constantLastModifiedTimes get f2                          return files       Both of these solutions create a temporary map data structure to save off a constant last modified time for each file in the directory  The reason we need to do this is that if your files are being updated or modified while your sort is being performed then your comparator will be violating the transitivity requirement of the comparator interface s general contract because the last modified times may be changing during the comparison   If  on the other hand  you know the files will not be updated or modified during your sort  you can get away with pretty much any other answer submitted to this question  of which I m partial to   Java 8   No concurrent modifications during sort   private static List lt Path gt  listFilesOldestFirst final String directoryPath  throws IOException       try  final Stream lt Path gt  fileStream   Files list Paths get directoryPath              return fileStream              map Path  toFile               sorted Comparator comparing File  lastModified                map File  toPath      remove this line if you would rather work with a List lt File gt  instead of List lt Path gt               collect Collectors toList               Note  I know you can avoid the translation to and from File objects in the above example by using Files  getLastModifiedTime api in the sorted stream operation  however  then you need to deal with checked IO exceptions inside your lambda which is always a pain  I d say if performance is critical enough that the translation is unacceptable then I d either deal with the checked IOException in the lambda by propagating it as an UncheckedIOException or I d forego the Files api altogether and deal only with File objects   final List lt File gt  sorted   Arrays asList new File directoryPathString  listFiles     sorted sort Comparator comparing File  lastModified

User · Answer

Imports    org apache commons io comparator LastModifiedFileComparator   Apache Commons  Code    public static void main String   args  throws IOException           File directory   new File                  get just files  not directories         File   files   directory listFiles  FileFilter  FileFileFilter FILE            System out println  Default order            displayFiles files            Arrays sort files  LastModifiedFileComparator LASTMODIFIED COMPARATOR           System out println   nLast Modified Ascending Order  LASTMODIFIED COMPARATOR             displayFiles files            Arrays sort files  LastModifiedFileComparator LASTMODIFIED REVERSE           System out println   nLast Modified Descending Order  LASTMODIFIED REVERSE             displayFiles files

User · Answer

I came to this post when i was searching for the same issue but in android  I don t say this is the best way to get sorted files by last modified date  but its the easiest way I found yet   Below code may be helpful to someone-  File downloadDir   new File  mypath        File   list   downloadDir listFiles        for  int i   list length-1  i  gt  0   i--              use list getName to get the name of the file         Thanks

User · Answer

You might also look at apache commons IO  it has a built in last modified comparator and many other nice utilities for working with files

User · Answer

I came to this post when i was searching for the same issue but in android  I don t say this is the best way to get sorted files by last modified date  but its the easiest way I found yet   Below code may be helpful to someone-  File downloadDir   new File  mypath        File   list   downloadDir listFiles        for  int i   list length-1  i  gt  0   i--              use list getName to get the name of the file         Thanks

User · Answer

A slightly more modernized version of the answer of  jason-orendorf  Note  this implementation keeps the original array untouched  and returns a new array  This might or might not be desirable  files   Arrays stream files           map FileWithLastModified  ofFile           sorted comparingLong FileWithLastModified  lastModified            map FileWithLastModified  file           toArray File    new    private static class FileWithLastModified       private final File file      private final long lastModified       private FileWithLastModified File file  long lastModified            this file   file          this lastModified   lastModified             public static FileWithLastModified ofFile File file            return new FileWithLastModified file  file lastModified                public File file             return file             public long lastModified             return lastModified           But again  all credits to  jason-orendorf for the idea

User · Answer

You can use Apache LastModifiedFileComparator  library   import org apache commons io comparator LastModifiedFileComparator      File   files   directory listFiles            Arrays sort files  LastModifiedFileComparator LASTMODIFIED COMPARATOR           for  File file   files                Date lastMod   new Date file lastModified                 System out println  File      file getName        Date      lastMod

User · Answer

You might also look at apache commons IO  it has a built in last modified comparator and many other nice utilities for working with files

User · Answer

Collections sort listFiles  new Comparator lt File gt              public int compare File f1  File f2                return Long compare f1 lastModified    f2 lastModified                         where listFiles is the collection of all files in ArrayList

User · Answer

I think your solution is the only sensible way   The only way to get the list of files is to use File listFiles   and the documentation states that this makes no guarantees about the order of the files returned   Therefore you need to write a Comparator that uses File lastModified   and pass this  along with the array of files  to Arrays sort

User · Answer

In Java 8   Arrays sort files   a  b  - gt  Long compare a lastModified    b lastModified

User · Answer

There is also a completely different way which may be even easier  as we do not deal with large numbers    Instead of sorting the whole array after you retrieved all filenames and lastModified dates  you can just insert every single filename just after you retrieved it at the right position of the list   You can do it like this   list add 1  object1  list add 2  object3  list add 2  object2    After you add object2 to position 2  it will move object3 to position 3

User · Answer

What s about similar approach  but without boxing to the Long objects   File   files   directory listFiles     Arrays sort files  new Comparator lt File gt          public int compare File f1  File f2            return Long compare f1 lastModified    f2 lastModified

User · Answer

This might be faster if you have many files  This uses the decorate-sort-undecorate pattern so that the last-modified date of each file is fetched only once rather than every time the sort algorithm compares two files  This potentially reduces the number of I O calls from O n log n  to O n    It s more code  though  so this should only be used if you re mainly concerned with speed and it is measurably faster in practice  which I haven t checked    class Pair implements Comparable       public long t      public File f       public Pair File file            f   file          t   file lastModified               public int compareTo Object o            long u     Pair  o  t          return t  lt  u   -1   t    u   0   1               Obtain the array of  file  timestamp  pairs  File   files   directory listFiles    Pair   pairs   new Pair files length   for  int i   0  i  lt  files length  i        pairs i    new Pair files i        Sort them by timestamp  Arrays sort pairs       Take the sorted pairs and extract only the file part  discarding the timestamp  for  int i   0  i  lt  files length  i        files i    pairs i  f

User · Answer

public String   getDirectoryList String path        String   dirListing   null      File dir   new File path       dirListing   dir list         Arrays sort dirListing  0  dirListing length       return dirListing

User · Answer

Collections sort listFiles  new Comparator lt File gt              public int compare File f1  File f2                return Long compare f1 lastModified    f2 lastModified                         where listFiles is the collection of all files in ArrayList

User · Answer

I think your solution is the only sensible way   The only way to get the list of files is to use File listFiles   and the documentation states that this makes no guarantees about the order of the files returned   Therefore you need to write a Comparator that uses File lastModified   and pass this  along with the array of files  to Arrays sort

User · Answer

This might be faster if you have many files  This uses the decorate-sort-undecorate pattern so that the last-modified date of each file is fetched only once rather than every time the sort algorithm compares two files  This potentially reduces the number of I O calls from O n log n  to O n    It s more code  though  so this should only be used if you re mainly concerned with speed and it is measurably faster in practice  which I haven t checked    class Pair implements Comparable       public long t      public File f       public Pair File file            f   file          t   file lastModified               public int compareTo Object o            long u     Pair  o  t          return t  lt  u   -1   t    u   0   1               Obtain the array of  file  timestamp  pairs  File   files   directory listFiles    Pair   pairs   new Pair files length   for  int i   0  i  lt  files length  i        pairs i    new Pair files i        Sort them by timestamp  Arrays sort pairs       Take the sorted pairs and extract only the file part  discarding the timestamp  for  int i   0  i  lt  files length  i        files i    pairs i  f

User · Answer

There is also a completely different way which may be even easier  as we do not deal with large numbers    Instead of sorting the whole array after you retrieved all filenames and lastModified dates  you can just insert every single filename just after you retrieved it at the right position of the list   You can do it like this   list add 1  object1  list add 2  object3  list add 2  object2    After you add object2 to position 2  it will move object3 to position 3

User · Answer

If the files you are sorting can be modified or updated at the same time the sort is being performed     Java 8   private static List lt Path gt  listFilesOldestFirst final String directoryPath  throws IOException       try  final Stream lt Path gt  fileStream   Files list Paths get directoryPath              return fileStream              map Path  toFile               collect Collectors toMap Function identity    File  lastModified                entrySet                stream                sorted Map Entry comparingByValue                   sorted Collections reverseOrder Map Entry comparingByValue         replace the previous line with this line if you would prefer files listed newest first              map Map Entry  getKey               map File  toPath      remove this line if you would rather work with a List lt File gt  instead of List lt Path gt               collect Collectors toList               Java 7  private static List lt File gt  listFilesOldestFirst final String directoryPath  throws IOException       final List lt File gt  files   Arrays asList new File directoryPath  listFiles         final Map lt File  Long gt  constantLastModifiedTimes   new HashMap lt File Long gt         for  final File f   files            constantLastModifiedTimes put f  f lastModified               Collections sort files  new Comparator lt File gt               Override         public int compare final File f1  final File f2                return constantLastModifiedTimes get f1  compareTo constantLastModifiedTimes get f2                          return files       Both of these solutions create a temporary map data structure to save off a constant last modified time for each file in the directory  The reason we need to do this is that if your files are being updated or modified while your sort is being performed then your comparator will be violating the transitivity requirement of the comparator interface s general contract because the last modified times may be changing during the comparison   If  on the other hand  you know the files will not be updated or modified during your sort  you can get away with pretty much any other answer submitted to this question  of which I m partial to   Java 8   No concurrent modifications during sort   private static List lt Path gt  listFilesOldestFirst final String directoryPath  throws IOException       try  final Stream lt Path gt  fileStream   Files list Paths get directoryPath              return fileStream              map Path  toFile               sorted Comparator comparing File  lastModified                map File  toPath      remove this line if you would rather work with a List lt File gt  instead of List lt Path gt               collect Collectors toList               Note  I know you can avoid the translation to and from File objects in the above example by using Files  getLastModifiedTime api in the sorted stream operation  however  then you need to deal with checked IO exceptions inside your lambda which is always a pain  I d say if performance is critical enough that the translation is unacceptable then I d either deal with the checked IOException in the lambda by propagating it as an UncheckedIOException or I d forego the Files api altogether and deal only with File objects   final List lt File gt  sorted   Arrays asList new File directoryPathString  listFiles     sorted sort Comparator comparing File  lastModified

User · Answer

You might also look at apache commons IO  it has a built in last modified comparator and many other nice utilities for working with files

User · Answer

Elegant solution since Java 8   File   files   directory listFiles    Arrays sort files  Comparator comparingLong File  lastModified        Or  if you want it in descending order  just reverse it   File   files   directory listFiles    Arrays sort files  Comparator comparingLong File  lastModified  reversed

User · Answer

A slightly more modernized version of the answer of  jason-orendorf  Note  this implementation keeps the original array untouched  and returns a new array  This might or might not be desirable  files   Arrays stream files           map FileWithLastModified  ofFile           sorted comparingLong FileWithLastModified  lastModified            map FileWithLastModified  file           toArray File    new    private static class FileWithLastModified       private final File file      private final long lastModified       private FileWithLastModified File file  long lastModified            this file   file          this lastModified   lastModified             public static FileWithLastModified ofFile File file            return new FileWithLastModified file  file lastModified                public File file             return file             public long lastModified             return lastModified           But again  all credits to  jason-orendorf for the idea

User · Answer

private static List lt File gt  sortByLastModified String dirPath        List lt File gt  files   listFilesRec dirPath       Collections sort files  new Comparator lt File gt              public int compare File o1  File o2                return Long compare o1 lastModified    o2 lastModified                           return files

User · Answer

Elegant solution since Java 8   File   files   directory listFiles    Arrays sort files  Comparator comparingLong File  lastModified        Or  if you want it in descending order  just reverse it   File   files   directory listFiles    Arrays sort files  Comparator comparingLong File  lastModified  reversed

User · Answer

There is a very easy and convenient way to handle the problem without any extra comparator  Just code the modified date into the String with the filename  sort it  and later strip it off again   Use a String of fixed length 20  put the modified date  long  into it  and fill up with leading zeros  Then just append the filename to this string   String modified 20 digits     00000000000000000000  concat Long toString temp lastModified      substring Long toString temp lastModified    length       result filenames add modified 20 digits temp getAbsoluteFile   toString       What happens is this here   Filename1  C  data file1 html Last Modified 1532914451455 Last Modified 20 Digits 00000001532914451455  Filename1  C  data file2 html Last Modified 1532918086822 Last Modified 20 Digits 00000001532918086822  transforms filnames to   Filename1  00000001532914451455C  data file1 html  Filename2  00000001532918086822C  data file2 html  You can then just sort this list   All you need to do is to strip the 20 characters again later  in Java 8  you can strip it for the whole Array with just one line using the  replaceAll function

User · Answer

public String   getDirectoryList String path        String   dirListing   null      File dir   new File path       dirListing   dir list         Arrays sort dirListing  0  dirListing length       return dirListing

User · Answer

What s about similar approach  but without boxing to the Long objects   File   files   directory listFiles     Arrays sort files  new Comparator lt File gt          public int compare File f1  File f2            return Long compare f1 lastModified    f2 lastModified

User · Answer

Imports    org apache commons io comparator LastModifiedFileComparator   Apache Commons  Code    public static void main String   args  throws IOException           File directory   new File                  get just files  not directories         File   files   directory listFiles  FileFilter  FileFileFilter FILE            System out println  Default order            displayFiles files            Arrays sort files  LastModifiedFileComparator LASTMODIFIED COMPARATOR           System out println   nLast Modified Ascending Order  LASTMODIFIED COMPARATOR             displayFiles files            Arrays sort files  LastModifiedFileComparator LASTMODIFIED REVERSE           System out println   nLast Modified Descending Order  LASTMODIFIED REVERSE             displayFiles files

User · Answer

I think your solution is the only sensible way   The only way to get the list of files is to use File listFiles   and the documentation states that this makes no guarantees about the order of the files returned   Therefore you need to write a Comparator that uses File lastModified   and pass this  along with the array of files  to Arrays sort

User · Answer

You can try guava Ordering   Function lt File  Long gt  getLastModified   new Function lt File  Long gt          public Long apply File file            return file lastModified              List lt File gt  orderedFiles   Ordering natural   onResultOf getLastModified                             sortedCopy files

User · Answer

private static List lt File gt  sortByLastModified String dirPath        List lt File gt  files   listFilesRec dirPath       Collections sort files  new Comparator lt File gt              public int compare File o1  File o2                return Long compare o1 lastModified    o2 lastModified                           return files

User · Answer

You can use Apache LastModifiedFileComparator  library   import org apache commons io comparator LastModifiedFileComparator      File   files   directory listFiles            Arrays sort files  LastModifiedFileComparator LASTMODIFIED COMPARATOR           for  File file   files                Date lastMod   new Date file lastModified                 System out println  File      file getName        Date      lastMod

User · Answer

There is a very easy and convenient way to handle the problem without any extra comparator  Just code the modified date into the String with the filename  sort it  and later strip it off again   Use a String of fixed length 20  put the modified date  long  into it  and fill up with leading zeros  Then just append the filename to this string   String modified 20 digits     00000000000000000000  concat Long toString temp lastModified      substring Long toString temp lastModified    length       result filenames add modified 20 digits temp getAbsoluteFile   toString       What happens is this here   Filename1  C  data file1 html Last Modified 1532914451455 Last Modified 20 Digits 00000001532914451455  Filename1  C  data file2 html Last Modified 1532918086822 Last Modified 20 Digits 00000001532918086822  transforms filnames to   Filename1  00000001532914451455C  data file1 html  Filename2  00000001532918086822C  data file2 html  You can then just sort this list   All you need to do is to strip the 20 characters again later  in Java 8  you can strip it for the whole Array with just one line using the  replaceAll function

User · Answer

I think your solution is the only sensible way   The only way to get the list of files is to use File listFiles   and the documentation states that this makes no guarantees about the order of the files returned   Therefore you need to write a Comparator that uses File lastModified   and pass this  along with the array of files  to Arrays sort

User · Answer

In Java 8   Arrays sort files   a  b  - gt  Long compare a lastModified    b lastModified

[java] Best way to list files in Java, sorted by Date Modified?

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