for http://stackoverflow.com/q/12900044/116639

CircularQueue.java

public class CircularQueue<T> {
    
    private T[] queue = newArray(5);
    private int front = 0;
    private int rear = 0;
    
    public void enqueue(T element) {
        if (front == (rear + 1) % queue.length) {
            expandCapacity();
        }
        queue[rear] = element;
        rear = (rear + 1) % queue.length;
    }
    
    public void expandCapacity() {
        T[] larger = newArray(queue.length * 2);
        
        for (int scan = 0; scan < queue.length; scan++) {
            larger[scan] = queue[front];
            front = (front + 1) % queue.length;
        }
        front = 0;
        rear = queue.length - 1;
        queue = larger;
    }
    
    private static <T> T[] newArray(int length) {
        @SuppressWarnings("unchecked")
        T[] larger = ((T[]) (new Object[length]));
        return larger;
    }
    
    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = 0; i < queue.length; i++) {
            T element = queue[i];
            sb.append(i == front ? ">" : " ");
            sb.append(element != null ? element : " ");
            sb.append(i == rear ? "<" : " ");
            sb.append("|");
        }
        sb.setLength(sb.length() - 3);
        sb.append("]");
        if (rear == queue.length - 1) sb.append("<");
        return sb.toString();
    }
    
    public static void main(String[] args) {
        CircularQueue<Integer> cq = new CircularQueue<Integer>();
        System.out.println(cq);
        for (int i = 0; i < 10; i++) {
            cq.enqueue(i);
            System.out.println(cq);
        }
    }
}