Java ArrayList how to add elements at the beginning

Question

I need to add elements to an ArrayList queue whatever  but when I call the function to add an element  I want it to add the element at the beginning of the array  so it has the lowest index  and if the array has 10 elements adding a new results in deleting the oldest element  the one with the highest index    Does anyone have any suggestions

User · Answer

Take this example  -  List lt String gt  element1   new ArrayList lt  gt     element1 add  two    element1 add  three    List lt String gt  element2   new ArrayList lt  gt     element2 add  one    element2 addAll element1

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Using Specific Datastructures  There are various data structures which are optimized for adding elements at the first index  Mind though  that if you convert your collection to one of these  the conversation will probably need a time and space complexity of O n    Deque  The JDK includes the Deque structure which offers methods like addFirst e  and offerFirst e   Deque lt String gt  deque   new LinkedList lt  gt     deque add  two    deque add  one    deque addFirst  three      prints  three    two    one    Analysis  Space and time complexity of insertion is with LinkedList constant  O 1    See the Big-O cheatsheet   Reversing the List  A very easy but inefficient method is to use reverse    Collections reverse list    list add elementForTop    Collections reverse list     If you use Java 8 streams  this answer might interest you   Analysis   Time Complexity  O n  Space Complexity  O 1    Looking at the JDK implementation this has a O n  time complexity so only suitable for very small lists

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You can take a look at the add int index  E element       Inserts the specified element at the specified position in this list    Shifts the element currently at that position  if any  and any   subsequent elements to the right  adds one to their indices     Once you add you can then check the size of the ArrayList and remove the ones at the end

User · Answer

I think the implement should be easy  but considering about the efficiency  you should use LinkedList but not ArrayList as the container  You can refer to the following code   import java util LinkedList  import java util List   public class DataContainer        private List lt Integer gt  list       int length   10      public void addDataToArrayList int data           list add 0  data           if list size   gt 10               list remove length                        public static void main String   args            DataContainer comp   new DataContainer            comp list   new LinkedList lt Integer gt              int cycleCount   100000000           for int i   0  i  lt  cycleCount  i                  comp addDataToArrayList i

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You can use  public List lt E gt  addToListStart List lt E gt  list  E obj   list add 0 obj   return  List lt E gt  list       Change E with your datatype  If  deleting the oldest element is necessary then you can add   list remove list size  -1      before return statement  Otherwise list will add your object at beginning and also retain oldest element   This will delete last element in list

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You can use list methods  remove and add  list add lowestIndex  element   list remove highestIndex  element

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import com google common collect Lists   import java util List           author Ciccotta Andrea on 06 11 2020      public class CollectionUtils                   It models the prepend O 1   used against the common append add O n          param head first element of the list         param body rest of the elements of the list         return new list  with different memory-reference  made by  head     body              public static  lt E gt  List lt Object gt  prepend final E head  List lt E gt  final body           return Lists asList head  body toArray                           it models the typed version of prepend E head  List lt E gt  body          param type the array into which the elements of this list are to be stored             public static  lt E gt  List lt E gt  prepend final E head  List lt E gt  body  final E   type           return Lists asList head  body toArray type

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Java LinkedList provides both the addFirst E e  and the push E e  method that add an element to the front of the list   https   docs oracle com javase 7 docs api java util LinkedList html addFirst E

User · Answer

you can use this code  private List myList   new ArrayList    private void addItemToList Object obj       if myList size   lt 10         myList add 0 obj        else        myList add 0 obj         myList remove 10

User · Answer

I had a similar problem  trying to add an element at the beginning of an existing array  shift the existing elements to the right and discard the oldest one  array length-1    My solution might not be very performant but it works for my purposes    Method      updateArray  Element to insert        - for all the elements of the Array        - start from the end and replace with the one on the left        - Array  0   lt - Element   Good luck

User · Answer

You may want to look at Deque  it gives you direct access to both the first and last items in the list

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import java util    public class Logic     List lt String gt  list   new ArrayList lt String gt       public static void main String   args      Scanner input   new Scanner System in       Logic obj   new Logic          for  int i 0 i lt  20 i              String string   input nextLine            obj myLogic string           obj printList                public void myLogic String strObj       if  this list size   gt  10          this list remove this list size  -1        else        list add strObj             public void printList      System out print this list

User · Answer

What you are describing  is an appropriate situation to use Queue   Since you want to add new element  and remove the old one  You can add at the end  and remove from the beginning  That will not make much of a difference   Queue has methods add e  and remove   which adds at the end the new element  and removes from the beginning the old element  respectively   Queue lt Integer gt  queue   new LinkedList lt Integer gt     queue add 5   queue add 6   queue remove        Remove 5   So  every time you add an element to the queue you can back it up with a remove method call     UPDATE  -  And if you want to fix the size of the Queue  then you can take a look at  - ApacheCommons CircularFifoBuffer  From the documentation  -     CircularFifoBuffer is a first in first out buffer with a fixed size   that replaces its oldest element if full    Buffer queue   new CircularFifoBuffer 2      Max size  queue add 5   queue add 6   queue add 7       Automatically removes the first element  5    As you can see  when the maximum size is reached  then adding new element automatically removes the first element inserted

User · Answer

List has the method add int  E   so you can use   list add 0  yourObject     Afterwards you can delete the last element with   if list size    gt  10      list remove list size   - 1     However  you might want to rethink your requirements or use a different data structure  like a Queue  EDIT  Maybe have a look at Apache s CircularFifoQueue      CircularFifoQueue is a first-in first-out queue with a fixed size that replaces its oldest element if full     Just initialize it with you maximum size   CircularFifoQueue queue   new CircularFifoQueue 10

[java] Java ArrayList how to add elements at the beginning

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