Question

This might be a stupid question, but I’m having trouble thinking this through.

I wrote a method that uses a LinkedList to move through loaded MIDI instruments. I want to make a next and a previous button so that every time you click the button you traverse through the LinkedList.

If I hardcode itr.next(); or itr.previous(); multiple times I can traverse through the LinkedList

public void setInsturment(Synthesizer start,MidiChannel currentChannel[])
{
    try
    {
        start.open();

        Soundbank bank = start.getDefaultSoundbank();

        start.loadAllInstruments(bank);

        LinkedList<Instrument> currentInstrument = new LinkedList<Instrument>();

        Instrument instrs[] = start.getLoadedInstruments();

        currentInstrument.add(instrs[0]);
        currentInstrument.add(instrs[1]);
        currentInstrument.add(instrs[2]);
        currentInstrument.add(instrs[3]);
        currentInstrument.add(instrs[4]);

        ListIterator itr = currentInstrument.listIterator();
        itr.next();
        itr.next();
        itr.next();
     // nextInstrument();

        currentChannel[1].programChange(0,itr.nextIndex());

    }

    catch(MidiUnavailableException e)
    {
        System.out.print("error");
    }

}

I’m having a lot of trouble making a button that can traverse through the list. Is there an efficient way to do this? I tried something like this with no success.

public void actionPerformed(ActionEvent e)
{
    if (e.getSource() == nextButton)
    {
        sound.nextInstrument();
    }

public void nextInstrument()
{
    itr.next();
}

Thanks in advance guys!

Answer 1

Well, hmm, a linked list is a List and its items can be accessed by index, this is not the optimal structure to access items by index but I really don’t know if you can have a cursor on that kind of collection but you can store the current index on an instance variable.

If you really want random access, then you should consider using ArrayList instead of linked list.

Example:

class NextPrevList {
    private int index = 0;
    private List currentList; //initialize it with some list

    public Object next() {
        return list.get(++index);
    }
    public Object prev() {
        //maybe add a check for out of bounds
        if (index == 0) return null;
        return list.get(--index);
    }
}

Personally I think it would be more performant with an ArrayList rather than a LinkedList