Android ArrayAdapter Indexout of bounds exception

I'm using Android ArrayAdapter in my code, sometimes i see this error cropping up

java.lang.ArrayIndexOutOfBoundsException: index=-1 length=12
at java.util.ArrayList.get(ArrayList.java:310)
at com.sample.search.AutoCompleteAdapter.getItem(AutoCompleteAdapter.java:29)

Why would the index ever be -1?

I populate the ArrayAdapter using

@Override
    public Filter getFilter() {
        Filter myFilter = new Filter() {
            @Override
            protected FilterResults performFiltering(CharSequence constraint) {
                FilterResults filterResults = new FilterResults();
                if(constraint != null) {
                    // A class that queries a web API, parses the data and returns an ArrayList<Symbol>
                    SymbolLookUp symLookUp = SymbolLookUp.getInstance();
                    try {
                        mData = symLookUp.getMatches(getContext(),constraint.toString());
                    }
                    catch(Exception e) {}
                    // Now assign the values and count to the FilterResults object
                    filterResults.values = mData;
                    filterResults.count = mData.size();
                }
                return filterResults;
            }

            @Override
            protected void publishResults(CharSequence contraint, FilterResults results) {
                if(results != null && results.count > 0) {
                    notifyDataSetChanged();
                }
                else {
                    notifyDataSetInvalidated();
                }
            }
        };
        return myFilter;
    }

And my getItem method is simply

@Override
    public String getItem(int index) {     开发者_如何学Go 
        return mData.get(index).getSuggestedText();     
    }

@Override
    public String getItem(int index) {   
        if(mData != null &&  mData.get(index) != null) {

        return mData.get(index).getSuggestedText();     
}
    }

EDIT

You have not put your All code so i can not exactly say that you may face problem in mData.get(index) or not ..

But if you face problem of mData.get(index) is index out of bound exception then try below code

@Override
        public String getItem(int index) {   
            if(mData != null &&  mData.size() >= index) {

            return mData.get(index).getSuggestedText();     
            }
        }