XML : IllegalStateException: Could not find method showPopup(View)

Here's my card view activity where three dots are there as in toolbar and i want to overflow it with menu but getting this error

java.lang.IllegalStateException: Could not find method showPopup(View) in a parent or ancestor Context for android:onClick attribute defined on view class android.support.v7.widget.AppCompatImageButton with id 'img_menu'

here's XML

  xmlns:card_view="http://schemas.android.com/apk/res-auto"    android:orientation="vertical"    android:layout_width="match_parent"    android:layout_height="wrap_content">    <android.support.v7.widget.CardView        android:id="@+id/card_view"        android:layout_gravity="center"        android:layout_width="fill_parent"        android:layout_height="80dp"        android:layout_margin="5dp"        card_view:cardCornerRadius="2dp"        card_view:contentPadding="10dp"        android:foreground="?android:attr/selectableItemBackground">        <RelativeLayout            android:layout_width="fill_parent"            android:layout_height="fill_parent">            <TextView                android:id="@+id/textView"                android:layout_width="wrap_content"                android:layout_height="wrap_content"                android:textStyle="bold"                android:layout_alignParentTop="true"/>            <TextView                android:id="@+id/textView2"                android:layout_width="wrap_content"                android:layout_height="wrap_content"                android:layout_marginTop="10dp"                android:layout_below="@+id/textView"/>            <ImageButton                android:id="@+id/img_menu"                android:layout_width="wrap_content"                android:layout_height="wrap_content"                android:background="@drawable/ic_action_navigation_more_vert"                android:layout_alignParentRight="true"                android:layout_marginTop="12dp"                android:onClick="showPopup"/>        </RelativeLayout>    </android.support.v7.widget.CardView>    

Here's activity

  public class CardViewActivity extends AppCompatActivity {    private RecyclerView mRecyclerView;    private RecyclerView.Adapter mAdapter;    private RecyclerView.LayoutManager mLayoutManager;    private static String LOG_TAG = "CardViewActivity";    ImageButton overflowMenu;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_card_view);        overflowMenu = (ImageButton) findViewById(R.id.img_menu);        mRecyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);        mRecyclerView.setHasFixedSize(true);        mLayoutManager = new LinearLayoutManager(this);        mRecyclerView.setLayoutManager(mLayoutManager);        mAdapter = new MyRecyclerViewAdapter(getDataSet());        mRecyclerView.setAdapter(mAdapter);        if (getSupportActionBar() != null) {      getSupportActionBar().setHomeAsUpIndicator(R.drawable.abc_ic_ab_back_mtrl_am_alpha);            getSupportActionBar().setDisplayHomeAsUpEnabled(true);            getSupportActionBar().setHomeButtonEnabled(true);          }          overflowMenu.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                PopupMenu popup = new PopupMenu(CardViewActivity.this,   overflowMenu);                popup.getMenuInflater().inflate(R.menu.card_overflow_menu, popup.getMenu());                  popup.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {                    public boolean onMenuItemClick(MenuItem item) {                        Toast.makeText(CardViewActivity.this,                                "You Clicked : " + item.getTitle(),                                Toast.LENGTH_SHORT).show();                        return true;                    }                });                popup.show();              }        });    }    public boolean onOptionsItemSelected(MenuItem item) {        switch (item.getItemId()) {            case android.R.id.home:                onBackPressed();                return true;            case R.id.img_menu:                Toast.makeText(getApplicationContext(), "You clicked img_menu",   Toast.LENGTH_SHORT).show();                return true;            default:                return super.onOptionsItemSelected(item);        }    }    public void showPopup(View v) {        PopupMenu popup = new PopupMenu(CardViewActivity.this, v);        MenuInflater inflater = popup.getMenuInflater();        inflater.inflate(R.menu.card_overflow_menu,popup.getMenu());        popup.setOnMenuItemClickListener((PopupMenu.OnMenuItemClickListener) {        public boolean onMenuClick (MenuItem item){            switch (item.getItemId()) {                case R.id.new_game:                    Toast.makeText(getApplicationContext(), "You clicked item   1", Toast.LENGTH_SHORT).show();                    return true;                case R.id.help:                    Toast.makeText(getApplicationContext(), "You clicked item 2", Toast.LENGTH_SHORT).show();                    return true;                default:                    return false;            }        }}    }          popup.show();    }*/      /*public void showMenu(View v) {        PopupMenu popup = new PopupMenu(this, v);          // This activity implements OnMenuItemClickListener        popup.setOnMenuItemClickListener((PopupMenu.OnMenuItemClickListener) this);        popup.inflate(R.menu.card_overflow_menu);        popup.show();    }*/      @Override    protected void onResume() {        super.onResume();        ((MyRecyclerViewAdapter) mAdapter).setOnItemClickListener(new   MyRecyclerViewAdapter              .MyClickListener() {            @Override            public void onItemClick(int position, View v) {                Log.i(LOG_TAG, " Clicked on Item " + position);            }        });    }    private ArrayList<DataObject> getDataSet() {        ArrayList results = new ArrayList<DataObject>();        for (int index = 0; index < 20; index++) {            DataObject obj = new DataObject("Test " + index,                    "Doc number " + index);            results.add(index, obj);        }        return results;    }