How do I remove objects from an array in Java

Question

Given an array of n Objects  let s say it is an array of strings  and it has the following values   foo 0     a   foo 1     cc   foo 2     a   foo 3     dd     What do I have to do to delete remove all the strings objects equal to  a  in the array

User · Answer

If it doesn t matter the order of the elements  you can swap between the elements foo x  and foo 0   then call foo drop 1     foo drop n  removes  n  first elements from the array   I guess this is the simplest and resource efficient way to do   PS  indexOf can be implemented in many ways  this is my version   Integer indexOf String   arr  String value       for Integer i   0   i  lt  arr length  i             if arr i     value              return i             return the index of the element     return -1                    otherwise -1    while  true       Integer i     i   indexOf foo  a      if  i    -1  break     foo i    foo 0                preserve foo 0     foo drop 1

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You can use external library   org apache commons lang ArrayUtils remove java lang Object   array  int index    It is in project Apache Commons Lang http   commons apache org lang

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to remove   only the first   of several equal entries with a lambda boolean   done    false   String   arr   Arrays stream  foo   filter  e - gt         done 0   amp  amp  Objects equals  e  item    amp  amp   done 0    true          toArray String    new    can remove null entries

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EDIT   The point with the nulls in the array has been cleared  Sorry for my comments    Original   Ehm    the line  array   list toArray array     replaces all gaps in the array where the removed element has been with null  This might be dangerous  because the elements are removed  but the length of the array remains the same   If you want to avoid this  use a new Array as parameter for toArray    If you don t want to use removeAll  a Set would be an alternative           String   array   new String      a    bc    dc    a    ef              System out println Arrays toString array             Set lt String gt  asSet   new HashSet lt String gt  Arrays asList array            asSet remove  a            array   asSet toArray new String                 System out println Arrays toString array      Gives    a  bc  dc  a  ef   dc  ef  bc    Where as the current accepted answer from Chris Yester Young outputs    a  bc  dc  a  ef   bc  dc  ef  null  ef    with the code      String   array   new String      a    bc    dc    a    ef          System out println Arrays toString array         List lt String gt  list   new ArrayList lt String gt  Arrays asList array        list removeAll Arrays asList  a         array   list toArray array                System out println Arrays toString array      without any null values left behind

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Something about the make a list of it then remove then back to an array strikes me as wrong   Haven t tested  but I think the following will perform better   Yes I m probably unduly pre-optimizing   boolean    deleteItem   new boolean arr length   int size 0  for int i 0 i lt arr length i        if arr i  equals  a           deleteItem i  true          else        deleteItem i  false        size           String   newArr new String size   int index 0  for int i 0 i lt arr length i        if  deleteItem i          newArr index    arr i

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Will copy all elements except the one with index i   if i    0                   System arraycopy edges  1  copyEdge  0  edges length -1                 else                  System arraycopy edges  0  copyEdge  0  i                    System arraycopy edges  i 1  copyEdge  i  edges length -  i 1

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If you want some ready-to-use code  please scroll to my  Edit3   after the cut   The rest is here for posterity    To flesh out Dustman s idea   List lt String gt  list   new ArrayList lt String gt  Arrays asList array    list removeAll Arrays asList  a     array   list toArray array     Edit  I m now using Arrays asList instead of Collections singleton  singleton is limited to one entry  whereas the asList approach allows you to add other strings to filter out later  Arrays asList  a    b    c     Edit2  The above approach retains the same array  so the array is still the same length   the element after the last is set to null  If you want a new array sized exactly as required  use this instead   array   list toArray new String 0        Edit3  If you use this code on a frequent basis in the same class  you may wish to consider adding this to your class   private static final String   EMPTY STRING ARRAY   new String 0     Then the function becomes   List lt String gt  list   new ArrayList lt  gt     Collections addAll list  array   list removeAll Arrays asList  a     array   list toArray EMPTY STRING ARRAY     This will then stop littering your heap with useless empty string arrays that would otherwise be newed each time your function is called   cynicalman s suggestion  see comments  will also help with the heap littering  and for fairness I should mention it   array   list toArray new String list size        I prefer my approach  because it may be easier to get the explicit size wrong  e g   calling size   on the wrong list

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See code below  ArrayList lt String gt  a   new ArrayList lt  gt  Arrays asList strings    a remove i   strings   new String a size     a toArray strings

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I realise this is a very old post  but some of the answers here helped me out  so here s my tuppence  ha penny s worth   I struggled getting this to work for quite a while before before twigging that the array that I m writing back into needed to be resized  unless the changes made to the ArrayList leave the list size unchanged   If the ArrayList that you re modifying ends up with greater or fewer elements than it started with  the line List toArray   will cause an exception  so you need something like List toArray new String       or List toArray new String 0   in order to create an array with the new  correct  size   Sounds obvious now that I know it   Not so obvious to an Android Java newbie who s getting to grips with new and unfamiliar code constructs and not obvious from some of the earlier posts here  so just wanted to make this point really clear for anybody else scratching their heads for hours like I was

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In an array of Strings like  String name    a b c d e a f b d e     could be like String name    aa bb c d e aa f bb d e   I build the following class  class clearname  def parts def tv public def str      String name clearname String name       this name   name     this parts   this name split          this tv   this parts size     public String cleared             int i         int k         int j 0             for i 0 i lt tv i             for k 0 k lt tv k                 if this parts k     this parts i   amp  amp  k  i                  this parts k                        j                                     def str          for i 0 i lt tv i             if this parts i                   this str    this parts i  trim                  return this str           return new clearname name  cleared     getting this result  a b c d e f  hope this code help anyone  Regards

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Make a List out of the array with Arrays asList    and call remove   on all the appropriate elements  Then call toArray   on the  List  to make back into an array again   Not terribly performant  but if you encapsulate it properly  you can always do something quicker later on

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It depends on what you mean by  remove   An array is a fixed size construct - you can t change the number of elements in it  So you can either a  create a new  shorter  array without the elements you don t want or b  assign the entries you don t want to something that indicates their  empty  status  usually null if you are not working with primitives   In the first case create a List from the array  remove the elements  and create a new array from the list  If performance is important iterate over the array assigning any elements that shouldn t be removed to a list  and then create a new array from the list  In the second case simply go through and assign null to the array entries

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You can always do   int i  j  for  i   j   0  j  lt  foo length    j    if    a  equals foo j    foo i      foo j   foo   Arrays copyOf foo  i

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Arrgh  I can t get the code to show up correctly  Sorry  I got it working  Sorry again  I don t think I read the question properly   String  foo       a   cc   a   dd    remove    a   boolean gaps     new boolean foo length   int newlength   0   for  int c   0  c lt foo length  c          if  foo c  equals remove                 gaps c    true          newlength              else          gaps c    false       System out println foo c       String newString     new String newlength    System out println       for  int c1 0  c2 0  c1 lt foo length  c1          if   gaps c1                 newString c2    foo c1           System out println newString c2            c2

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If you need to remove multiple elements from array without converting it to List nor creating additional array  you may do it in O n  not dependent on count of items to remove   Here  a is initial array  int    r are distinct ordered indices  positions  of elements to remove   public int removeItems Object   a  int    r        int shift   0                                   for  int i   0  i  lt  a length  i                     if  shift  lt  r length  amp  amp  i    r shift       i-th item needs to be removed             shift                                  increment  shift          else              a i - shift    a i                     move i-th item  shift  positions left           for  int i   a length - shift  i  lt  a length  i            a i    null                                replace remaining items by nulls      return a length - shift                        return new  length        Small testing   String   a     0    1    2    3    4    removeItems a  0  3  4                          remove 0-th  3-rd and 4-th items System out println Arrays asList a               1  2  null  null  null    In your task  you can first scan array to collect positions of  a   then call removeItems

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An alternative in Java 8    String   filteredArray   Arrays stream array       filter e - gt   e equals foo   toArray String    new

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Use    list removeAll         post what char you need in the     section

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Initial array      int   array    5 6 51 4 3 2     if you want remove 51 that is index 2  use following   for int i   2  i  lt  array length -1  i         array i    array i   1

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There are a lot of answers here--the problem as I see it is that you didn t say WHY you are using an array instead of a collection  so let me suggest a couple reasons and which solutions would apply  Most of the solutions have already been answered in other questions here  so I won t go into too much detail    reason      You didn t know the collection package existed or didn t trust it  solution    Use a collection     If you plan on adding deleting from the middle  use a LinkedList   If you are really worried about size or often index right into the middle of the collection use an ArrayList   Both of these should have delete operations   reason     You are concerned about size or want control over memory allocation  solution   Use an ArrayList with a specific initial size   An ArrayList is simply an array that can expand itself  but it doesn t always need to do so   It will be very smart about adding removing items  but again if you are inserting removing a LOT from the middle  use a LinkedList   reason     You have an array coming in and an array going out--so you want to operate on an array  solution   Convert it to an ArrayList  delete the item and convert it back  reason    You think you can write better code if you do it yourself  solution  you can t  use an Array or Linked list   reason  this is a class assignment and you are not allowed or you do not have access to the collection apis for some reason  assumption  You need the new array to be the correct  size   solution  Scan the array for matching items and count them   Create a new array of the correct size  original size - number of matches    use System arraycopy repeatedly to copy each group of items you wish to retain into  your new Array   If this is a class assignment and you can t use System arraycopy  just copy them one at a time by hand in a loop but don t ever do this in production code because it s much slower    These solutions are both detailed in other answers   reason      you need to run bare metal  assumption  you MUST not allocate space unnecessarily or take too long  assumption  You are tracking the size used in the array  length  separately because otherwise you d have to reallocate your array for deletes inserts   An example of why you might want to do this  a single array of primitives  Let s say int values  is taking a significant chunk of your ram--like 50    An ArrayList would force these into a list of pointers to Integer objects which would use a few times that amount of memory   solution    Iterate over your array and whenever you find an element to remove  let s call it element n    use System arraycopy to copy the tail of the array over the  deleted  element  Source and Destination are same array --it is smart enough to do the copy in the correct direction so the memory doesn t overwrite itself     System arraycopy ary  n 1  ary  n  length-n    length--    You ll probably want to be smarter than this if you are deleting more than one element at a time   You would only move the area between one  match  and the next rather than the entire tail and as always  avoid moving any chunk twice   In this last case  you absolutely must do the work yourself  and using System arraycopy is really the only way to do it since it s going to choose the best possibly way to move memory for your computer architecture--it should be many times faster than any code you could reasonably write yourself

User · Answer

Assign null to the array locations

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My little contribution to this problem   public class DeleteElementFromArray   public static String foo       a   cc   a   dd    public static String search    a     public static void main String   args        long stop   0      long time   0      long start   0      System out println  Searched value in Array is    search       System out println  foo length before is    foo length       for int i 0 i lt foo length i     System out println  foo   i        foo i         System out println                                                                        start   System nanoTime        foo   removeElementfromArray search  foo       stop   System nanoTime        time   stop - start      System out println  Equal search took in nano seconds     time       System out println                                                                    for int i 0 i lt foo length i     System out println  foo   i        foo i       public static String   removeElementfromArray  String toSearchfor  String arr           int i   0       int t   0       String tmp1     new String arr length                 for  i lt arr length i                   if arr i     toSearchfor                      i                                 tmp1 t    arr i                t                   String tmp2     new String arr length-t           System arraycopy tmp1  0  tmp2  0  tmp2 length        arr   tmp2  tmp1   null  tmp2   null      return arr

[java] How do I remove objects from an array in Java?

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