public class DoublyLinkedList
{
    private Element first, last;
    private int size;

    public DoublyLinkedList()
    {
        first = last = null;
        size = 0;
    }

    public int size()
    {
        return size;
    }

    public boolean isEmpty()
    {
        return size == 0;
    }

    public void add( Object content )
    {
        Element e = new Element( content );
        if ( isEmpty() )
        {
            first = last = e;
        }
        else
        {
            last.connectAsSucc( e );
            last = e;
        }
        size++;
    }

    public void addFirst( Object content )
    {
        Element e = new Element( content );
        if ( isEmpty() )
        {
            first = last = e;
        }
        else
        {
            first.connectAsPred( e );
            first = e;
        }
        size++;
    }

    public Object getFirst()
    {
        if ( !isEmpty() )
        {
            return first.getContent();
        }
        else
        {
            throw new RuntimeException();
        }
    }

    public Object get( int index )
    {
        if ( index >= 0 && index < size )
        {
            Element current = first;
            for ( int i = 0; i < index; i++ )
            {
                current = current.getSucc();
            }
            return current.getContent();
        }
        else
        {
            throw new RuntimeException();
        }
    }

    public Object removeFirst()
    {
        if ( !isEmpty() )
        {
            Object result = first.getContent();
            if ( first.hasSucc() )
            {
                first = first.getSucc();
                first.disconnectPred();
            }
            else
            {
                first = last = null;
            }
            size--;
            return result;
        }
        else
        {
            throw new RuntimeException();
        }
    }

    public void showAll()
    {
        Element current = first;
        while ( current != null )
        {
            System.out.print( current.getContent() );  // impliziter Aufruf von toString, falls != null
            if ( current != last )
            {
                System.out.print(", ");
            }
            current = current.getSucc();
        }
        System.out.println();
    }



    // zeigt alle Attribute einer Liste und ihrer Elemente an,
    // insbesondere auch die Werte von Referenzen, so dass die
    // Verkettung kontrolliert werden kann
    public void inspect()
    {
        System.out.println( "list\nsize = " + size() );
        if ( first != null ) {
            System.out.println( "first = " + first.toString().substring(first.toString().indexOf('@')) );
        } else {
            System.out.println( "first = null" );
        }
        if ( last != null ) {
            System.out.println( "last = " + last.toString().substring(last.toString().indexOf('@')) );
        } else {
            System.out.println( "last = null" );
        }
        System.out.println( "\nelements" );
        if (last != null) {
            Element current = first;
            int i = 0;
            while (current != null) {
                inspectElement( current, i );
                current = current.getSucc();
                i++;
            }
        } else {
            System.out.println("none");
        }
        System.out.println();
    }

    // zeigt alle Attribute eines Elements an
    private void inspectElement( Element e, int index )
    {
        String info = "Element " + index + ": "
                      + e.toString().substring(e.toString().indexOf('@'));
        info += " [ content = ";
        if ( e.getContent() != null ) {
            info += e.getContent().toString();
        } else {
            info += "null";
        }
        info += ", pred = ";
        if ( e.getPred() != null ) {
            info += e.getPred().toString().substring(e.getPred().toString().indexOf('@'));
        } else {
            info += "null";
        }
        info += ", succ = ";
        if ( e.getSucc() != null ) {
            info += e.getSucc().toString().substring(e.getSucc().toString().indexOf('@'));
        } else {
            info += "null";
        }

        info += " ]";
        System.out.println( info );
    }

    public void clear()
    {
        while ( !isEmpty() ) removeFirst();
    }

    public Object getLast()
    {
        if ( !isEmpty() )
        {
            return last.getContent();
        }
        else
        {
            throw new IllegalStateException();
        }
    }

    public boolean contains( Object o )
    {
        Element current = first;
        for ( int i = 0; i < size; i++)
        {
            if( current.getContent().equals( o ) ) return true; // über getContent() Element als Object aufrufen!
            if( current.hasSucc() ) current = current.getSucc();
        }
        return false;
    }

    public int count( Object o )
    {
        Element current = first;
        int counter = 0;
        for ( int i = 0; i < size; i++)
        {
            if( current.getContent().equals( o ) ) counter++; // über getContent() Element als Object aufrufen!
            if( current.hasSucc() ) current = current.getSucc();
        }
        return counter;
    }

}