which exception to throw when user tries to add to a full container

Question

I am trying to write a fixed sized queue for generic type elements.The user can call the constructor, providing a size and an internal array is created for that size.(see below)

class FixedSizeQ<E>{
    private E[] q;
    private int N;

    public FixedSizeQ(int max) {
        super();
        q = (E[]) new Object[max];
        N = 0;
    }
..
}

I thought of defining an isEmpty() and isFull() methods and an insert() method for the above.If the client tries to add an element to the already full array,I want to throw an exception.Going through the javadocs,I thought IllegalStateException would be the correct exception to throw.

public boolean isFull(){
    return N == q.length;
}
public void insert(Item item){
    if(isFull()){
        throw new IllegalStateException("queue full");
    }
    ...     
}

I want to know if my understanding is correct..Someone suggested that IllegalArgumentException is more appropriate.Can someone advise?

Answer 1

I think you should make the insert method return boolean, and return true if the object was inserted and false if the queue is full. Adding objects to a full queue does not seem like an exceptional condition to me.

In any case, if no predefined exception in the Java API is a good match, you can create an exception type of your own to best suit the situation:

public class QueueFullException extends RuntimeException {
    // or "extends Exception" if you want it checked
    ...
}