Monday, September 24, 2007

 

Round Robin List

 

In one of the projects I needed a simple Round Robin list, which would just return the next object in round robin fashion.

Even though it's trivial to come up with one, surprised to find out that I didn't find any hits when I searched for one.

This is what I came up with after half-hour. Use it as you wish.  Download here.

package org.brsanthu.misc;

import java.util.ArrayList;
import java.util.Collection;

/**
 * Simple extension to <code>ArrayList</code> with round robin functionality.
 * <p>
 * Just adds one method {@link #getNext()} which would return the next object
 * in the list and the method starts over at first element after reaching the
 * end of list.
 * <p>
 * This class it NOT designed for multi-threaded access. So necessary synchronization
 * is required before accessing {@link #getNext()} method.
 *
 * @since Sep 24th 2007
 * @author Santhosh Kumar
 */
public class RoundRobinList extends ArrayList {
   
    private static final long serialVersionUID = 1L;
   
    /**
     * Last used index. Initially it would be -1, so that
     * first element returned is 0th element of the list.
     */
    private int lastIndex = -1;
   
    /**
     * Constructs a new round robin list with default capacity.
     */
    public RoundRobinList() {
        super();
    }

    /**
     * Constructs a new round robin list and pre-populates with the
     * objects found in given list.
     * <p>
     * For more information about usage of the c, see the <code>ArrayList</code> constructor
     * page.
     *
     * @param c - collection of objects to be added by default to the new list.
     */
    public RoundRobinList(Collection c) {
        super(c);
    }

    /**
     * Creates the new round robin list with specified capacity.
     *
     * @param initialCapacity - initial capacity.
     */
    public RoundRobinList(int initialCapacity) {
        super(initialCapacity);
    }

    /**
     * Returns the next object in the list.
     * <p>
     * First invocation of this method after creating the list, always returns
     * the 0th element. Starts over the 0th element after reaching the end of
     * list.
     * <p>
     * If there are no objects in the list, returns <code>null</code>.
     *
     * @return - returns the next object in the list. <code>null</code> if list is
     *          empty.
     *
     */
    public Object getNext() {
       
        if (isEmpty()) {
            return null;
        }
       
        lastIndex++;
        if (lastIndex >= size()) {
            lastIndex = 0;
        }
       
        return get(lastIndex);
    }
}
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