Android : How do I find the longest boat (length) by using bubble sort?

I am working on the project with handling a singleton class that is a list of boat. I am working on an activity that will show the longest boat. I want to try using bubble sort to sort the list by its longest boat. The app runs, but then it stop working when I press the button for showing the longest boat. Could anyone help me?

public void showLongBoat(View view)
{
    //Declare references
    BoatList boat_list;
    TextView tv;
    int i;
    int x = 0;
    boolean found;
    int num_items;
    int visit = 0;
    boolean exchange_value;
    int last_item;
    int temp;
    int a = 0;
    int b;

    //Set references. Access the list.
    boat_list = BoatList.getInstance();

    tv = (TextView) findViewById(R.id.text_main);

    //Get the number of items in the list
    num_items = boat_list.size();

    //Find the longest boat
    i = 0;
    found = false;

    while(!found && (i < boat_list.size()))
    {
        //If num_item is 0 or 1, then do not sort. Otherwise, do sorting
        if(num_items > 1)
        {
            //Set the number of values to visit on the first pass
            visit = num_items - 1;
        }

        do
        {
            //No exchange or swapping of item was made, so set the    exchange to false
            exchange_value = false;

            //Set the index for the last item to visit
            last_item = visit - 1;

            for(x = 0; x <= last_item; x++)
            {
                if(boat_list.get(x).getLength() > boat_list.get(x).getLength() + 1)
                {
                    //Swap the item
                    temp = a;
                    a = boat_list.get(x).getLength() + 1;
                    b = temp;

                    exchange_value = true;
                }
            }

            visit--;
        }while(exchange_value && (visit > 0)); //end sort
    }

    if(found)
    {
        boat_list.get(x).getLength();
        tv.append("Longest Boat is: ");
    }
    else
    {
        Toast.makeText(ShowLongBoatActivity.this, "Error: Cannot find the longest boat successfully!",
                Toast.LENGTH_SHORT).show();
    }

} //end of showLongBoat