buffer

public class Buffer<T> {

    List<T> items;
    private int capacity;

    public Buffer(int capacity){
        this.items = new ArrayList<>(capacity);
        this.capacity = capacity;
    }

    public T getItem() {
        T item = null;

        if(items.size() > 0) {
            item = items.remove(items.size() - 1);
        }

        return item;
    }

    public void addItem(T item) {
        if(items.size() == capacity)
            return;

        items.add(item);
    }

}