FIFO class in Java

Try ArrayDeque or LinkedList, which both implement the Queue interface.

http://docs.oracle.com/javase/6/docs/api/java/util/ArrayDeque.html

You're looking for any class that implements the Queue interface, excluding PriorityQueue and PriorityBlockingQueue, which do not use a FIFO algorithm.

Probably a LinkedList using add (adds one to the end) and removeFirst (removes one from the front and returns it) is the easiest one to use.

For example, here's a program that uses a LinkedList to queue and retrieve the digits of PI:

import java.util.LinkedList;

class Test {
    public static void main(String args[]) {
        char arr[] = {3,1,4,1,5,9,2,6,5,3,5,8,9};
        LinkedList<Integer> fifo = new LinkedList<Integer>();

        for (int i = 0; i < arr.length; i++)
            fifo.add (new Integer (arr[i]));

        System.out.print (fifo.removeFirst() + ".");
        while (! fifo.isEmpty())
            System.out.print (fifo.removeFirst());
        System.out.println();
    }
} 

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Alternatively, if you know you only want to treat it as a queue (without the extra features of a linked list), you can just use the Queue interface itself:

import java.util.LinkedList;
import java.util.Queue;

class Test {
    public static void main(String args[]) {
        char arr[] = {3,1,4,1,5,9,2,6,5,3,5,8,9};
        Queue<Integer> fifo = new LinkedList<Integer>();

        for (int i = 0; i < arr.length; i++)
            fifo.add (new Integer (arr[i]));

        System.out.print (fifo.remove() + ".");
        while (! fifo.isEmpty())
            System.out.print (fifo.remove());
        System.out.println();
    }
}

This has the advantage of allowing you to replace the underlying concrete class with any class that provides the Queue interface, without having to change the code too much.

The basic changes are to change the type of fifo to a Queue and to use remove() instead of removeFirst(), the latter being unavailable for the Queue interface.

Calling isEmpty() is still okay since that belongs to the Collection interface of which Queue is a derivative.