Question

I would like to know what kind of data structure (queue) I should use if I have the following problem:

• The queue must have a dynamically assigned length (say, 512).
• Every new value is saved at the end of the queue.
• When a new value is added, the first is dropped if the queue is already full. If I add 30 new values to a full queue, the 30 first are automatically dropped.
• The kind of data stored be arrays or some other simple object.
• I need to be able to quickly retrieve the values using a loop, always in order (no random access).

The purpose of this is to have a fixed width data source that a graph will scan to draw its curve.

EDIT: This graph is meant to be shown on an Android custom View. Is there a specific length I could use that would make the looping thru this faster?

EDIT2: Added “When a new value is added, the first is dropped if the queue is already full. If I add 30 new values to a full queue, the 30 first are automatically dropped.”

Answer 1

If the capacity of the stack is never going to change, I would use an array. I would keep track of where the “first” node is and keep wrapping it around.

I’d also like to point out that you’re behavior seems more like a queue than a stack. Are you sure the stack is what you want?

class RotatingQueue<E> {
    Object[] data; // can't do E[]
    final int maxSize;
    int size = 0; // starts empty
    int first = 0; // starts at the front, why not?

    RotatingQueue(int size) {
        this.maxSize = size;
        data = new Object[size];
    }

    E add(E e) {
        E old = (E)(data[first]);
        old[first++] = e;
        if(size < maxSize) size++;
        return old;
    }

}