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Library providing immutable/persistent collection classes for Java. While collections are immutable they provide methods for adding and removing values by creating new modified copies of themselves. Each copy shares almost all of its structure with other copies to minimize memory consumption.

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// Burton Computer Corporation
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package org.javimmutable.collections;

/**
 * Implemented by objects used to traverse persistent data structures.
 * The iterators themselves must be immutable and always create a new
 * iterator when start() or next() is called.
 *
 * @param 
 */
public interface Cursor
        extends Iterable
{
    /**
     * Thrown by hasValue() and getValue() if the cursor has not been started by calling next() yet.
     */
    public static class NotStartedException
            extends IllegalStateException
    {
    }

    /**
     * Thrown by getValue() if the Cursor's hasValue() method returns false.
     */
    public static class NoValueException
            extends IllegalStateException
    {
    }

    /**
     * All Cursors are created in a pre-start position pointing "before" the first element.  Once traversal has begun a
     * Cursor points to some element in the collection or to end ("after" the last element).  The start() method
     * advances to the first element if traversal has not yet started or does nothing if traversal has
     * already started.  Either next() or start() can be used to initiate a traversal however start() is
     * safer since it can be used safely on already started cursors as well as not-started ones.  This distinction
     * is useful when passing a Cursor as parameter to a method that will traverse from the Cursor's current position
     * forward and using start() prevents it from skipping the current value.
     * 
     * Must always return a non-null Cursor.
     *
     * @return Cursor for first position or this if already started
     */
    Cursor start();

    /**
     * Advances to the next (possibly first) value.  Must always return a non-null Cursor.
     * A newly created Cursor must always point to "before" the first value because next() (or start()) must
     * always be called once before retrieving the first value.  If the Cursor is already at the end
     * of its sequence then it should return a Cursor that will always return false for hasValue().
     *
     * @return Cursor for next position
     */
    Cursor next();

    /**
     * Read-only method with no side effects that determines if the Cursor currently has a value.
     * Users of the Cursor will always call this after calling next() to see if they have reached
     * the end of the sequence.  If hasValue() returns true then next() will be called.  If hasValue()
     * returns false then next() must not be called.
     *
     * @return true iff getValue() can be called
     */
    boolean hasValue();

    /**
     * Return the value at the Cursor's position.  Only valid if a call to hasValue() would return true.
     *
     * @return current value
     * @throws IllegalStateException if getValue() is not allowed for this iterator
     */
    T getValue();
}