LinkedList class

// LinkedList class
//http://www.java-tips.org/java-se-tips/java.lang/linked-list-implementation-in-java.html
// CONSTRUCTION: with no initializer
// Access is via LinkedListIterator class
//
// ******************PUBLIC OPERATIONS*********************
// boolean isEmpty( ) --> Return true if empty; else false
// void makeEmpty( ) --> Remove all items
// LinkedListIterator zeroth( )
// --> Return position to prior to first
// LinkedListIterator first( )
// --> Return first position
// void insert( x, p ) --> Insert x after current iterator position p
// void remove( x ) --> Remove x
// LinkedListIterator find( x )
// --> Return position that views x
// LinkedListIterator findPrevious( x )
// --> Return position prior to x
// ******************ERRORS********************************
// No special errors

/**
* Linked list implementation of the list
* using a header node.
* Access to the list is via LinkedListIterator.
* @author Mark Allen Weiss
* @see LinkedListIterator
*/
public class LinkedList {
/**
* Construct the list
*/
public LinkedList( ) {
header = new ListNode( null );
}

/**
* Test if the list is logically empty.
* @return true if empty, false otherwise.
*/
public boolean isEmpty( ) {
return header.next == null;
}

/**
* Make the list logically empty.
*/
public void makeEmpty( ) {
header.next = null;
}

/**
* Return an iterator representing the header node.
*/
public LinkedListIterator zeroth( ) {
return new LinkedListIterator( header );
}

/**
* Return an iterator representing the first node in the list.
* This operation is valid for empty lists.
*/
public LinkedListIterator first( ) {
return new LinkedListIterator( header.next );
}

/**
* Insert after p.
* @param x the item to insert.
* @param p the position prior to the newly inserted item.
*/
public void insert( Object x, LinkedListIterator p ) {
if( p != null && p.current != null )
p.current.next = new ListNode( x, p.current.next );
}

/**
* Return iterator corresponding to the first node containing an item.
* @param x the item to search for.
* @return an iterator; iterator is not valid if item is not found.
*/
public LinkedListIterator find( Object x ) {
ListNode itr = header.next;

while( itr != null && !itr.element.equals( x ) )
itr = itr.next;

return new LinkedListIterator( itr );
}

/**
* Return iterator prior to the first node containing an item.
* @param x the item to search for.
* @return appropriate iterator if the item is found. Otherwise, the
* iterator corresponding to the last element in the list is returned.
*/
public LinkedListIterator findPrevious( Object x ) {
ListNode itr = header;

while( itr.next != null && !itr.next.element.equals( x ) )
itr = itr.next;

return new LinkedListIterator( itr );
}

/**
* Remove the first occurrence of an item.
* @param x the item to remove.
*/
public void remove( Object x ) {
LinkedListIterator p = findPrevious( x );

if( p.current.next != null )
p.current.next = p.current.next.next; // Bypass deleted node
}

// Simple print method
public static void printList( LinkedList theList ) {
if( theList.isEmpty( ) )
System.out.print( "Empty list" );
else {
LinkedListIterator itr = theList.first( );
for( ; itr.isValid( ); itr.advance( ) )
System.out.print( itr.retrieve( ) + " " );
}

System.out.println( );
}

private ListNode header;

// In this routine, LinkedList and LinkedListIterator are the
// classes written in Section 17.2.
public static int listSize( LinkedList theList ) {
LinkedListIterator itr;
int size = 0;

for( itr = theList.first(); itr.isValid(); itr.advance() )
size++;

return size;
}

public static void main( String [ ] args ) {
LinkedList theList = new LinkedList( );
LinkedListIterator theItr;
int i;

theItr = theList.zeroth( );
printList( theList );

for( i = 0; i < 10; i++ ) {
theList.insert( new Integer( i ), theItr );
printList( theList );
theItr.advance( );
}
System.out.println( "Size was: " + listSize( theList ) );

for( i = 0; i < 10; i += 2 )
theList.remove( new Integer( i ) );

for( i = 0; i < 10; i++ )
if( ( i % 2 == 0 ) == ( theList.find( new Integer( i ) ).isValid( ) ) )
System.out.println( "Find fails!" );

System.out.println( "Finished deletions" );
printList( theList );
}

}


// LinkedListIterator class; maintains "current position"
//
// CONSTRUCTION: Package visible only, with a ListNode
//
// ******************PUBLIC OPERATIONS*********************
// void advance( ) --> Advance
// boolean isValid( ) --> True if at valid position in list
// Object retrieve --> Return item in current position

/**
* Linked list implementation of the list iterator
* using a header node.
* @author Mark Allen Weiss
* @see LinkedList
*/
public class LinkedListIterator {
/**
* Construct the list iterator
* @param theNode any node in the linked list.
*/
LinkedListIterator( ListNode theNode ) {
current = theNode;
}

/**
* Test if the current position is a valid position in the list.
* @return true if the current position is valid.
*/
public boolean isValid( ) {
return current != null;
}

/**
* Return the item stored in the current position.
* @return the stored item or null if the current position
* is not in the list.
*/
public Object retrieve( ) {
return isValid( ) ? current.element : null;
}

/**
* Advance the current position to the next node in the list.
* If the current position is null, then do nothing.
*/
public void advance( ) {
if( isValid( ) )
current = current.next;
}

ListNode current; // Current position
}


// Basic node stored in a linked list
// Note that this class is not accessible outside
// of package weiss.nonstandard

class ListNode {
// Constructors
public ListNode( Object theElement ) {
this( theElement, null );
}

public ListNode( Object theElement, ListNode n ) {
element = theElement;
next = n;
}

public Object element;
public ListNode next;
}