[Java] Circular array queue implementation

CSQueue.java

/**
 * Circular array queue implementation
 * @author Jittapan Pluemsumran
 */
public class CSQueue<T> {
    T[] data;
    int first = -1;
    int last = -1;
    int size; // for quick access

    public CSQueue(int size) {
        this.size = size;
        data = (T[])new Object[size];
    }

    public boolean isEmpty() {
        return first == -1 && last == -1;
    }

    public boolean isFull() {
        return (last + 1)%size == first;
    }

    public void enqueue(T el) throws IllegalStateException {
        if(isFull()) {
            throw new IllegalStateException("Queue is full.");
        }
        if(isEmpty()){
            first = last = 0;
        } else {
            last = (last + 1)%size;
        }
        data[last] = el;
    }

    public T dequeue() throws IllegalStateException {
        T tmp;
        if(isEmpty()) {
            throw new IllegalStateException("Queue is full.");
        }
        if(first == last) {
            tmp = data[first];
            first = last = -1;
        } else {
            tmp = data[first];
            first = (first +1)%size;
        }
        return tmp;
    }

    public T getFirst() throws IllegalStateException {
        if(isEmpty()) {
            throw new IllegalStateException("Queue is empty.");
        }
        return data[first];
    }

    public T getLast() throws IllegalStateException {
        if(isEmpty()) {
            throw new IllegalStateException("Queue is empty.");
        }
        return data[last];
    }

    public String toString() {
        int count = (last +size- first)%size + 1;
        StringBuilder output = new StringBuilder("[");
        for(int i = 0; i <count; i++) {
            int index = (first + i) % size;
            output.append(data[index]).append(", ");
        }
        return output.toString().trim() + "]";
    }
}