How to switch between fragments during onclick?

Not sure what's the minimal fix to get your code working, but have you looked at using a Navigation Drawer to switch between the fragments? It looks to me like the example in the official docs matches pretty much exactly what you want to achieve.

A key is to have some kind of container for the currently displayed fragment (instead of using <fragment> like in your XML). For example:

 <FrameLayout
        android:id="@+id/content_frame"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

Then, switching fragments goes something like this:

Fragment fragment = new Fragment2();
// Insert the fragment by replacing any existing fragment
FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
               .replace(R.id.content_frame, fragment)
               .commit();

Further reading: Add a Fragment to an Activity at Runtime in Android developer docs.

private void openFragment (int number) {

        Fragment fragment = new Fragment();

        switch (number) {
            case FRAGMENT_1:
                fragment = new *Fragment1*();
                break;

            case FRAGMENT_2:
                fragment = new *Fragment2*();
                break;
        }

        getSupportFragmentManager().beginTransaction()
                .replace(R.id.fragment_frame, fragment)
                .commit();
    }