public class FragmentActivity extends AppCompatActivity {
    private FrameLayout frameLayout;
    private Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fragment);
        frameLayout = findViewById(R.id.fl_content);
        button = findViewById(R.id.btn_click);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Fragment fTwo = new FragmentOne();
                FragmentManager fm = getSupportFragmentManager();
                FragmentTransaction tx = fm.beginTransaction();
                tx.replace(R.id.fl_content, fTwo, "One");
                tx.addToBackStack(null);//添加回退棧
                tx.commit();
                System.out.println("fm.getBackStackEntryCount() = " + fm.getBackStackEntryCount());
            }
        });
    }
}

 @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View mView = inflater.inflate(R.layout.fragment_one, container, false);
        editText = mView.findViewById(R.id.et_input);
        button = mView.findViewById(R.id.btn_next);
        System.out.println("mView.findViewById = "+editText);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Fragment fTwo = new FragmentTwo();
                FragmentManager fm = getActivity().getSupportFragmentManager();
                FragmentTransaction tx = fm.beginTransaction();
                tx.replace(R.id.fl_content, fTwo, "TWO");
                tx.addToBackStack(null);
                tx.commit();
                System.out.println( "fm.getBackStackEntryCount() = "+ fm.getBackStackEntryCount());
            }
        });

        return mView;
    }

    @Override
    public void onStop() {
        super.onStop();
        Log.d("Tina======>", "onStop");
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        Log.d("Tina======>", "onDestroyView"+editText);
    }

    @Override
    public void onDetach() {
        super.onDetach();
        Log.d("Tina======>", "onDetach");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d("Tina======>", "onDestroy");
    }