[Q] Changing to another Activity when pressing on a Gridviewpager topic

I am trying to start a new certain Activity based on which page I am clicking on in a Gridview.
I tried to understand the Sample GridViewPager which is coming along with the sdk and trying to adapt the given explanation on stackoverflow (question # 26343337). But I really don't know how to bring these two things together and even where to start.

The first java.file Selection

Code:

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public class Selection extends Activity {
   
        @Override
        protected void onCreate(Bundle savedInstanceState){
            super.onCreate(savedInstanceState);
            setContentView(R.layout.selection_grid);
            final GridViewPager pager = (GridViewPager) findViewById(R.id.pager);

            pager.setAdapter(new Workers(this, getFragmentManager()));
            DotsPageIndicator dotsPageIndicator = (DotsPageIndicator) findViewById(R.id.page_indicator);
            dotsPageIndicator.setPager(pager);
        }
    }

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and the second java.file Users (thats the Adapter):

Code:

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    public class Users extends FragmentGridPagerAdapter {
   
        private static final int TRANSITION_DURATION_MILLIS = 100;
   
        private final Context mContext;
        private List<Row> mRows;
        private ColorDrawable mDefaultBg;
   
        private ColorDrawable mClearBg;
   
        public Users (Context ctx, FragmentManager fm) {
            super(fm);
            mContext = ctx;
   
            mRows = new ArrayList<Workers.Row>();
   
            mRows.add(new Row(cardFragment(R.string.title, R.string.user1)));
            mRows.add(new Row(cardFragment(R.string.title, R.string.user2)));
            mRows.add(new Row(cardFragment(R.string.title, R.string.user3)));
            mRows.add(new Row(cardFragment(R.string.title, R.string.user4)));
   
    //      In case in one row several cardFragments are needed
    //        mRows.add(new Row(
    //                cardFragment(R.string.cards_title, R.string.cards_text),
    //                cardFragment(R.string.expansion_title, R.string.expansion_text)));
   
            mDefaultBg = new ColorDrawable(R.color.dark_grey);
            mClearBg = new ColorDrawable(android.R.color.transparent);
        }
   
   
        LruCache<Integer, Drawable> mRowBackgrounds = new LruCache<Integer, Drawable>(3) {
            @Override
            protected Drawable create(final Integer row) {
                int resid = BG_IMAGES[row % BG_IMAGES.length];
                new DrawableLoadingTask(mContext) {
                    @Override
                    protected void onPostExecute(Drawable result) {
                        TransitionDrawable background = new TransitionDrawable(new Drawable[] {
                                mDefaultBg,
                                result
                        });
                        mRowBackgrounds.put(row, background);
                        notifyRowBackgroundChanged(row);
                        background.startTransition(TRANSITION_DURATION_MILLIS);
                    }
                }.execute(resid);
                return mDefaultBg;
            }
        };
   
   
      private Fragment cardFragment(int titleRes, int textRes) {
            Resources res = mContext.getResources();
            CardFragment fragment =
                    CardFragment.create(res.getText(titleRes), res.getText(textRes));
            // Add some extra bottom margin to leave room for the page indicator
            fragment.setCardMarginBottom(
                    res.getDimensionPixelSize(R.dimen.card_margin_bottom));
            return fragment;
        }
   
   
    static final int[] BG_IMAGES = new int[] {
                R.drawable.user1,
                R.drawable.user2,
                R.drawable.user3,
                R.drawable.user4
        };
   
   
      /** A convenient container for a row of fragments. */
        private class Row {
            final List<Fragment> columns = new ArrayList<Fragment>();
   
            public Row(Fragment... fragments) {
                for (Fragment f : fragments) {
                    add(f);
                }
            }
   
            public void add(Fragment f) {
                columns.add(f);
            }
   
            Fragment getColumn(int i) {
                return columns.get(i);
            }
   
            public int getColumnCount() {
                return columns.size();
            }
        }
   
   
        @Override
        public Fragment getFragment(int row, int col) {
            Row adapterRow = mRows.get(row);
            return adapterRow.getColumn(col);
        }
   
        @Override
        public Drawable getBackgroundForRow(final int row) {
            return mRowBackgrounds.get(row);
        }
   
   
        @Override
        public int getRowCount() {
            return mRows.size();
        }
   
        @Override
        public int getColumnCount(int rowNum) {
            return mRows.get(rowNum).getColumnCount();
        }
   
        class DrawableLoadingTask extends AsyncTask<Integer, Void, Drawable> {
            private static final String TAG = "Loader";
            private Context context;
   
            DrawableLoadingTask(Context context) {
                this.context = context;
            }
   
            @Override
            protected Drawable doInBackground(Integer... params) {
                Log.d(TAG, "Loading asset 0x" + Integer.toHexString(params[0]));
                return context.getResources().getDrawable(params[0]);
            }
        }
    }

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