How to keep the last 100 objects inserted in an Arraylist?

Question

How to keep the last 100 objects inserted in an Arraylist? This will be constantly fed but I want to keep only the last 100.

Answer 1

The simplest way to solve this problem is to apply the concept of FIFO (First in, first out).

Fifo refers to a row-type data structure. Lists are widely used in programming to implement queues. In a FIFO type queue the elements are placed in the queue and removed (or processed) in order of arrival. The basic idea of the queue is that we can only insert a new element at the end of the queue and we can only remove the element from the beginning.

You can see more here

In addition to this concept it will be necessary to limit the volume of records you want to store, in this way use the Apache Commons Collections

If you are using Maven you can import:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.1</version>
</dependency>

If you are using Gradle

`compile group: 'org.apache.commons', name: 'commons-collections4', version: '4.1'`

Basically you just need to add the library commons-collections4 in their dependencies, for example I used summer 4.1.

After doing import you can create your previously limited lists for example:

 new CircularFifoQueue<String>(100)
 new CircularFifoQueue<Integer(100)

among others.

To illustrate better I made an example by creating random string and storing in the queue, at the end I display the remaining values:

public static void main(String args[]) {
    Collection<String> fifo = new CircularFifoQueue<String>(100);
    for (int i = 1; i <= 500; i++) {
        String id = UUID.randomUUID().toString();
        System.out.println(i + " - " + id);
        fifo.add(id);
    }

    System.out.println("----> Result Fifo <----");
    int i = 1;
    for (String id : fifo) {
        System.out.println(i++ + " - " + id);
    }
}

Answer 2

I got what I wanted with while..

public static void main(String[] args) {
    // TODO Auto-generated method stub

    ArrayList<String> arr = new ArrayList<>();

    for(int i = 0;i < 150 ; i++){
        arr.add(Integer.toString(i));
    }

    while(arr.size() >= 100){
        arr.remove(0);
    }

    for(String x : arr){
        System.out.println(x);
    }

}