Android SDK error: Trying instantiate a class that is not a fragment

Question

I am hardly trying to create a simple application with a top menu and a changeable view below (by pressing the buttons in the menu fragment we change the view of the fragment be

Answer 1

You should be using FragmentActivity instead of Activity that is because you are using support Fragments and multiple Fragments in your activity main

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Edit

You can now use the Appcompat support library and extends AppCompatActivity to support toolbar and fragment for lower api.