Java Arrays 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?

Answer 1

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.

Answer 2

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<Integer> queue = new LinkedList<Integer>();
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.

Answer 3

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] <- Element

Good luck

Answer 4

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<String> deque = new LinkedList<>();
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.

Answer 5

you can use this code

private List myList = new ArrayList();
private void addItemToList(Object obj){
    if(myList.size()<10){
      myList.add(0,obj);
    }else{
      myList.add(0,obj);
      myList.remove(10);
    }
}

Answer 6

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