simplified and hopefully fixed

gistfile1.txt

 const int last = pics_array.length - 1;
 
// this method gets called when the forward button got clicked (for testing)
    public void go (View view) {
        index++;
        if(index > last)//out of bounds
            index = 0;//jump to beginning
        default_Pic.setImageResource(pics_array[index]);
    }
    
// this method gets called when the back button got clicked (for testing)
    public void back (View view) {
        index--;
        if(index < 0)//out of bounds
            index = last;//jump to end
        default_Pic.setImageResource(pics_array[index]);
    }