/**
 * A table used to store and look up elements.
 * The periodic table organizes elements by their atomic numbers.
 * Internally, an array representation is used to allow for
 * speedy access.  We are assuming that the number and order of
 * elements is not going to change.
 * @author Paul Gestwicki
 */
public class PeriodicTable {

    /**
     * The internal representation of the periodic table.
     * The array is indexed by atomic number.
     */
    private Element[] table;

    /**
     * Construct a new periodic table.
     * @param numberOfElements the number of elements that can be
     *   stored in the periodic table
     */
    public PeriodicTable(int numberOfElements) {
	table = new Element[numberOfElements];
    }

    /**
     * Add an element to the periodic table.
     * @param e the element to add
     */
    public void addElement(Element e) { table[e.getAtomicNumber()] = e; }

    /**
     * Fetch an element from the table by its atomic number.
     * @param atomicNumber the atomic number of the element that
     *  is being retrieved from the table.
     * @return the element with the given atomic number, or null if it
     *   has not been added yet (see {@link #addElement(Element)}).
     */
    public Element getElement(int atomicNumber) { return table[atomicNumber]; }
}