Moving items around in an ArrayList

Question

I ve been playing around with ArrayLists  What I m trying to achieve is a method to do something like this   Item 1 Item 2 Item 3 Item 4   I m trying to be able to move items up in the list  unless it is already at the top in which case it will stay the same  For example  if item 3 was moved the list would be   Item 1 Item 3 Item 2 Item 4   From my small understanding at the moment then I would want something along the lines of   IF arrayname index is not equal to 0 THEN move up ELSE do nothing   The part I m struggling with is the  move up  part  Any tips or code samples of how this could be achieved are much appreciated

User · Answer

As Mikkel posted before Collections rotate is a simple way  I m using this method for moving items up- and downward in a List   public static  lt T gt  void moveItem int sourceIndex  int targetIndex  List lt T gt  list        if  sourceIndex  lt   targetIndex            Collections rotate list subList sourceIndex  targetIndex   1   -1         else           Collections rotate list subList targetIndex  sourceIndex   1   1

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Kotlin solution to move an Element from  quot fromPos quot  to  quot toPos quot  and shift all other items accordingly  useful for moving an item in a staggered layout recyclerView in an Android application              if fromPos  lt  toPos                                       val movingElement   myList fromPos                    shifts all elements between fromPos and toPos 1 down                 for i in fromPos 1  toPos                       myList i-1    myList i                                      re-add element that was saved in the beginning                 myList toPos    movingElement                            if fromPos  gt  toPos                   val movingElement   myList fromPos                    shifts elements between toPos and fromPos 1 up                 for i in fromPos downTo toPos 1                       myList i    myList i-1                                                          re-add element that was saved in the beginning                 myList toPos    movingElement

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Applying recursion to reorder items in an arraylist  public class ArrayListUtils               public static  lt T gt  void reArrange List lt T gt  list int from  int to                   if from    to                        if from  gt  to                          reArrange list from -1  to                         else                         reArrange list from  1  to                         Collections swap list  from  to

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To move up  remove and then add   To remove - ArrayList remove and assign the returned object to a variable Then add this object back at the required index  -ArrayList add int index  E element   http   download oracle com javase 6 docs api java util ArrayList html add int  E

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A simple swap is far better for  moving something up  in an ArrayList   if i  gt  0        Item toMove   arrayList get i       arrayList set i  arrayList get i-1        arrayList set i-1  toMove       Because an ArrayList uses an array  if you remove an item from an ArrayList  it has to  shift  all the elements after that item upward to fill in the gap in the array  If you insert an item  it has to shift all the elements after that item to make room to insert it  These shifts can get very expensive if your array is very big  Since you know that you want to end up with the same number of elements in the list  doing a swap like this allows you to  move  an element to another location in the list very efficiently   As Chris Buckler and Michal Kreuzman point out  there is even a handy method in the Collections class to reduce these three lines of code to one   Collections swap arrayList  i  i-1

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To Move item in list simply add      move item to index 0 Object object   ObjectList get index   ObjectList remove index   ObjectList add 0 object     To Swap two items in list simply add      swap item 10 with 20 Collections swap ObjectList 10 20

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I came across this old question in my search for an answer  and I thought I would just post the solution I found in case someone else passes by here looking for the same   For swapping 2 elements  Collections swap is fine  But if we want to move more elements  there is a better solution that involves a creative use of Collections sublist and Collections rotate that I hadn t thought of until I saw it described here   http   docs oracle com javase 6 docs api java util Collections html rotate 28java util List  20int 29  Here s a quote  but go there and read the whole thing for yourself too      Note that this method can usefully be applied to sublists to move one   or more elements within a list while preserving the order of the   remaining elements  For example  the following idiom moves the element   at index j forward to position k  which must be greater than or equal   to j        Collections rotate list subList j  k 1   -1

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you can try this simple code  Collections swap list  i  j  is what you looking for       List lt String gt  list   new ArrayList lt String gt         list add  1        list add  2        list add  3        list add  4         String toMoveUp    3       while  list indexOf toMoveUp     0            int i   list indexOf toMoveUp           Collections swap list  i  i - 1              System out println list

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Moving element with respect to each other is something I needed a lot in a project of mine  So I wrote a small util class that moves an element in an list to a position relative to another element  Feel free to use  and improve upon      import java util List   public class ListMoveUtil       enum Position               BEFORE  AFTER                        Moves element  elementToMove  to be just before or just after  targetElement                  param list         param elementToMove         param targetElement         param pos             public static  lt T gt  void moveElementTo  List lt T gt  list  T elementToMove  T targetElement  Position pos                 if   elementToMove equals  targetElement                           return                    int srcIndex   list indexOf  elementToMove            int targetIndex   list indexOf  targetElement            if   srcIndex  lt  0                         throw new IllegalArgumentException   Element      elementToMove     not in the list                        if   targetIndex  lt  0                         throw new IllegalArgumentException   Element      targetElement     not in the list                        list remove  elementToMove                if the element to move is after the targetelement in the list  just remove it            else the element to move is before the targetelement  When we removed it  the targetindex should be decreased by one         if   srcIndex  lt  targetIndex                         targetIndex -  1                    switch   pos                         case AFTER                  list add  targetIndex   1  elementToMove                    break              case BEFORE                  list add  targetIndex  elementToMove                    break

[java] Moving items around in an ArrayList

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